Tumor suppressor serine/threonine-protein kinase that controls the activity of AMP-activated protein kinase (AMPK) family members, thereby playing a role in various processes such as cell metabolism, cell polarity, apoptosis and DNA damage response. Acts by phosphorylating the T-loop of AMPK family proteins, leading to promote their activity: phosphorylates PRKAA1, PRKAA2, BRSK1, BRSK2, MARK1, MARK2, MARK3, MARK4, NUAK1, NUAK2, SIK1, SIK2, SIK3 and SNRK but not MELK. Also phosphorylates non-AMPK family proteins such as STRADA and possibly p53/TP53. Acts as a key upstream regulator of AMPK by mediating phosphorylation and activation of AMPK catalytic subunits PRKAA1 and PRKAA2: it thereby regulates inhibition of signaling pathways that promote cell growth and proliferation when energy levels are low, glucose homeostasis in liver, activation of autophagy when cells undergo nutrient deprivation, B-cell differentiation in the germinal center in response to DNA damage. Also acts as a regulator of cellular polarity by remodeling the actin cytoskeleton. Required for cortical neurons polarization by mediating phosphorylation and activation of BRSK1 and BRSK2, leading to axon initiation and specification. Involved in DNA damage response: interacts with p53/TP53 and recruited to the CDKN1A/WAF1 promoter to participate in transcription activation. Able to phosphorylate p53/TP53; the relevance of such result in vivo is however unclear and phosphorylation may be indirect and mediated by downstream STK11/LKB1 kinase NUAK1 Also acts as a mediator p53/TP53-dependent apoptosis via interaction with p53/TP53: translocates to mitochondrion during apoptosis and regulates p53/TP53-dependent apoptosis pathways.
The LKB1 gene encodes a serine/threonine kinase that is mutated in the Peutz-Jeghers cancer syndrome. LKB1 is homologous to the Par-4 polarity genes in C. elegans and D. melanogaster. We have previously reported the identification and characterization of an LKB1-specific adaptor protein, STRAD, which activates LKB1 and translocates it from nucleus to cytoplasm. We have now constructed intestinal epithelial cell lines in which inducible STRAD activates LKB1. Upon LKB1 activation, single cells rapidly remodel their actin cytoskeleton to form an apical brush border. The junctional proteins ZO-1 and p120 redistribute in a dotted circle peripheral to the brush border, in the absence of cell-cell contacts. Apical and basolateral markers sort to their respective membrane domains. We conclude that LKB1 can induce complete polarity in intestinal epithelial cells. In contrast to current thinking on polarization of simple epithelia, these cells can fully polarize in the absence of junctional cell-cell contacts.
We recently demonstrated that the LKB1 tumour suppressor kinase, in complex with the pseudokinase STRAD and the scaffolding protein MO25, phosphorylates and activates AMP-activated protein kinase (AMPK). A total of 12 human kinases (NUAK1, NUAK2, BRSK1, BRSK2, QIK, QSK, SIK, MARK1, MARK2, MARK3, MARK4 and MELK) are related to AMPK. Here we demonstrate that LKB1 can phosphorylate the T-loop of all the members of this subfamily, apart from MELK, increasing their activity >50-fold. LKB1 catalytic activity and the presence of MO25 and STRAD are required for activation. Mutation of the T-loop Thr phosphorylated by LKB1 to Ala prevented activation, while mutation to glutamate produced active forms of many of the AMPK-related kinases. Activities of endogenous NUAK2, QIK, QSK, SIK, MARK1, MARK2/3 and MARK4 were markedly reduced in LKB1-deficient cells. Neither LKB1 activity nor that of AMPK-related kinases was stimulated by phenformin or AICAR, which activate AMPK. Our results show that LKB1 functions as a master upstream protein kinase, regulating AMPK-related kinases as well as AMPK. Between them, these kinases may mediate the physiological effects of LKB1, including its tumour suppressor function.
The LKB1 gene encodes a serine/threonine kinase mutated in Peutz-Jeghers cancer syndrome. Despite several proposed models for LKB1 function in development and in tumour suppression, the detailed molecular action of LKB1 remains undefined. Here, we report the identification and characterization of an LKB1-specific adaptor protein and substrate, STRAD (STe20 Related ADaptor). STRAD consists of a STE20- like kinase domain, but lacks several residues that are indispensable for intrinsic catalytic activity. Endogenous LKB1 and STRAD form a complex in which STRAD activates LKB1, resulting in phosphorylation of both partners. STRAD determines the subcellular localization of wild-type, but not mutant LKB1, translocating it from nucleus to cytoplasm. One LKB1 mutation previously identified in a Peutz-Jeghers family that does not compromise its kinase activity is shown here to interfere with LKB1 binding to STRAD, and hence with STRAD-dependent regulation. Removal of endogenous STRAD by siRNA abrogates the LKB1-induced G(1) arrest. Our results imply that STRAD plays a key role in regulating the tumour suppressor activities of LKB1.
Mutations in the LKB1 protein kinase result in the inherited Peutz Jeghers cancer syndrome. LKB1 has been implicated in regulating cell proliferation and polarity although little is known about how this enzyme is regulated. We recently showed that LKB1 is activated through its interaction with STRADalpha, a catalytically deficient pseudokinase. Here we show that endogenous LKB1-STRADalpha complex is associated with a protein of unknown function, termed MO25alpha, through the interaction of MO25alpha with the last three residues of STRADalpha. MO25alpha and STRADalpha anchor LKB1 in the cytoplasm, excluding it from the nucleus. Moreover, MO25alpha enhances the formation of the LKB1-STRADalpha complex in vivo, stimulating the catalytic activity of LKB1 approximately 10-fold. We demonstrate that the related STRADbeta and MO25beta isoforms are also able to stabilize LKB1 in an active complex and that it is possible to isolate complexes of LKB1 bound to STRAD and MO25 isoforms, in which the subunits are present in equimolar amounts. Our results indicate that MO25 may function as a scaffolding component of the LKB1-STRAD complex and plays a crucial role in regulating LKB1 activity and cellular localization.
Here, we investigate the mechanism and function of LKB1, a Ser/Thr kinase mutated in Peutz-Jegher syndrome (PJS). We demonstrate that LKB1 physically associates with p53 and regulates specific p53-dependent apoptosis pathways. LKB1 protein is present in both the cytoplasm and nucleus of living cells and translocates to mitochondria during apoptosis. In vivo, LKB1 is highly upregulated in pyknotic intestinal epithelial cells. In contrast, polyps arising in Peutz-Jegher patients are devoid of LKB1 staining and have reduced numbers of apoptotic cells. We propose that a deficiency in apoptosis is a key factor in the formation of multiple benign intestinal polyps in PJS patients, and possibly for the subsequent development of malignant tumors in these patients.
The tumor suppressor LKB1 is an evolutionarily conserved serine/threonine kinase. In humans, LKB1 can be inactivated either by germ-line mutations resulting in Peutz-Jeghers syndrome or by somatic mutations causing predisposition to multiple sporadic cancers. LKB1 has wide-ranging functions involved in tumor suppression and cell homeostasis, including establishing cell polarity, setting energy metabolic balance (via phosphorylation of AMP-dependent kinase), regulating the cell cycle, and promoting apoptosis. LKB1 function was previously linked to the tumor suppressor p53 and shown to activate the p53 target gene p21/WAF1. In this study, we further investigated LKB1 activation of the p21/WAF1 gene and addressed whether LKB1 is directly involved at the gene promoter. We find that, consistent with previous studies, LKB1 stabilizes p53 in vivo, correlating with activation of p21/WAF1. We show that LKB1 physically associates with p53 in the nucleus and directly or indirectly phosphorylates p53 Ser15 (previously shown to be phosphorylated by AMP-dependent kinase) and p53 Ser392. Further, these two p53 residues are required for LKB1-dependent cell cycle G(1) arrest. Chromatin immunoprecipitation analyses show that LKB1 is recruited directly to the p21/WAF1 promoter, as well as to other p53 activated promoters, in a p53-dependent fashion. Finally, a genetic fusion of LKB1 to defective p53, deleted for its activation domains, promotes activation of p21/WAF1. These results indicate that LKB1 has a direct role in activation of p21/WAF1 gene.
Recent work has shown that the LKB1 tumour suppressor protein kinase phosphorylates and activates protein kinases belonging to the AMP activated kinase (AMPK) subfamily. In this study, we identify the sucrose non-fermenting protein (SNF1)-related kinase (SNRK), a largely unstudied AMPK subfamily member, as a novel substrate for LKB1. We demonstrate that LKB1 activates SNRK by phosphorylating the T-loop residue (Thr173), and that the LKB1 regulatory subunits STRAD and MO25 are required for LKB1 to activate SNRK. We find that SNRK is not active when expressed in HeLa cells that lack expression of LKB1, and its activity is restored by expression of wild type LKB1, but not catalytically deficient LKB1. We also present evidence that two other AMPK-related kinases more distantly related to AMPK than SNRK, namely NIM1 and testis-specific serine/threonine kinase-1 (TSSK1) are not substrates for LKB1. Tissue distribution analysis indicates that SNRK protein is mainly expressed in testis, similar to TSSK isoforms, whereas NIM1 is more widely expressed. These results provide evidence that SNRK could mediate some of the physiological effects of LKB1.
It has been suggested that adenosine monophosphate-activated protein kinase (AMPK) and 12 AMPK-related kinases (ARK), including novel (nua) kinase family 1 (NUAK1), are activated by master kinase LKB1, a major tumor suppressor. Apart from evidence to suggest that NUAK1 participates in induction of tumor survival, invasion and p53-independent cellular senescence, its detailed biological functions remain unclear. Here we showed that in the presence of wild-type LKB1, NUAK1 directly interacts with and phosphorylates p53 in vitro and in vivo. The phosphorylation of p53 induced by LKB1 required the kinase activity of NUAK1 and phosphorylation of NUAK1 at Thr211 by LKB1 was essential for its kinase activity, which leads to the conclusion that LKB1 activates NUAK1 and regulates phosphorylation of p53 through the NUAK1 kinase, at least partially. LKB1/NUAK1 activation leads to cell cycle arrest at the G(1)/S border by inducing expression of p21/WAF1. Under the regulation of LKB1, NUAK1 interacts with p53 in the nucleus and binds to the p53-responsive element of p21/WAF1 promoter. These findings have highlighted a novel role for NUAK1 in LKB1-related signaling pathways; NUAK1 can regulate cell proliferation and exert tumor suppression through direct interaction with p53.
The LKB1 gene encodes a serine/threonine kinase mutated in Peutz-Jeghers cancer syndrome. Despite several proposed models for LKB1 function in development and in tumour suppression, the detailed molecular action of LKB1 remains undefined. Here, we report the identification and characterization of an LKB1-specific adaptor protein and substrate, STRAD (STe20 Related ADaptor). STRAD consists of a STE20- like kinase domain, but lacks several residues that are indispensable for intrinsic catalytic activity. Endogenous LKB1 and STRAD form a complex in which STRAD activates LKB1, resulting in phosphorylation of both partners. STRAD determines the subcellular localization of wild-type, but not mutant LKB1, translocating it from nucleus to cytoplasm. One LKB1 mutation previously identified in a Peutz-Jeghers family that does not compromise its kinase activity is shown here to interfere with LKB1 binding to STRAD, and hence with STRAD-dependent regulation. Removal of endogenous STRAD by siRNA abrogates the LKB1-induced G(1) arrest. Our results imply that STRAD plays a key role in regulating the tumour suppressor activities of LKB1.
Autophagy, the process by which proteins and organelles are sequestered in autophagosomal vesicles and delivered to the lysosome/vacuole for degradation, provides a primary route for turnover of stable and defective cellular proteins. Defects in this system are linked with numerous human diseases. Although conserved protein kinase, lipid kinase and ubiquitin-like protein conjugation subnetworks controlling autophagosome formation and cargo recruitment have been defined, our understanding of the global organization of this system is limited. Here we report a proteomic analysis of the autophagy interaction network in human cells under conditions of ongoing (basal) autophagy, revealing a network of 751 interactions among 409 candidate interacting proteins with extensive connectivity among subnetworks. Many new autophagy interaction network components have roles in vesicle trafficking, protein or lipid phosphorylation and protein ubiquitination, and affect autophagosome number or flux when depleted by RNA interference. The six ATG8 orthologues in humans (MAP1LC3/GABARAP proteins) interact with a cohort of 67 proteins, with extensive binding partner overlap between family members, and frequent involvement of a conserved surface on ATG8 proteins known to interact with LC3-interacting regions in partner proteins. These studies provide a global view of the mammalian autophagy interaction landscape and a resource for mechanistic analysis of this critical protein homeostasis pathway.
Evidence
2:
Inferred from Physical InteractionIntAct
The polarization of eukaryotic cells is controlled by the concerted activities of asymmetrically localized proteins. The PAR proteins, first identified in Caenorhabditis elegans, are common regulators of cell polarity conserved from nematode and flies to man. However, little is known about the molecular mechanisms by which these proteins and protein complexes establish cell polarity in mammals. We have mapped multiprotein complexes formed around the putative human Par orthologs MARK4 (microtubule-associated protein/microtubule affinity-regulating kinase 4) (Par-1), Par-3, LKB1 (Par-4), 14-3-3zeta and eta (Par-5), Par-6a, -b, -c, and PKClambda (PKC3). We employed a proteomic approach comprising tandem affinity purification (TAP) of protein complexes from cultured cells and protein sequencing by tandem mass spectrometry. From these data we constructed a highly interconnected protein network consisting of three core complex "modules" formed around MARK4 (Par-1), Par-3.Par-6, and LKB1 (Par-4). The network confirms most previously reported interactions. In addition we identified more than 50 novel interactors, some of which, like the 14-3-3 phospho-protein scaffolds, occur in more than one distinct complex. We demonstrate that the complex formation between LKB1.Par-4, PAPK, and Mo25 results in the translocation of LKB1 from the nucleus to the cytoplasm and to tight junctions and show that the LKB1 complex may activate MARKs, which are known to introduce 14-3-3 binding sites into several substrates. Our findings suggest co-regulation and/or signaling events between the distinct Par complexes and provide a basis for further elucidation of the molecular mechanisms that govern cell polarity.
Evidence
3:
Inferred from Physical InteractionIntAct
The LKB1 gene encodes a serine/threonine kinase mutated in Peutz-Jeghers cancer syndrome. Despite several proposed models for LKB1 function in development and in tumour suppression, the detailed molecular action of LKB1 remains undefined. Here, we report the identification and characterization of an LKB1-specific adaptor protein and substrate, STRAD (STe20 Related ADaptor). STRAD consists of a STE20- like kinase domain, but lacks several residues that are indispensable for intrinsic catalytic activity. Endogenous LKB1 and STRAD form a complex in which STRAD activates LKB1, resulting in phosphorylation of both partners. STRAD determines the subcellular localization of wild-type, but not mutant LKB1, translocating it from nucleus to cytoplasm. One LKB1 mutation previously identified in a Peutz-Jeghers family that does not compromise its kinase activity is shown here to interfere with LKB1 binding to STRAD, and hence with STRAD-dependent regulation. Removal of endogenous STRAD by siRNA abrogates the LKB1-induced G(1) arrest. Our results imply that STRAD plays a key role in regulating the tumour suppressor activities of LKB1.
The LKB1 gene encodes a serine/threonine kinase mutated in Peutz-Jeghers cancer syndrome. Despite several proposed models for LKB1 function in development and in tumour suppression, the detailed molecular action of LKB1 remains undefined. Here, we report the identification and characterization of an LKB1-specific adaptor protein and substrate, STRAD (STe20 Related ADaptor). STRAD consists of a STE20- like kinase domain, but lacks several residues that are indispensable for intrinsic catalytic activity. Endogenous LKB1 and STRAD form a complex in which STRAD activates LKB1, resulting in phosphorylation of both partners. STRAD determines the subcellular localization of wild-type, but not mutant LKB1, translocating it from nucleus to cytoplasm. One LKB1 mutation previously identified in a Peutz-Jeghers family that does not compromise its kinase activity is shown here to interfere with LKB1 binding to STRAD, and hence with STRAD-dependent regulation. Removal of endogenous STRAD by siRNA abrogates the LKB1-induced G(1) arrest. Our results imply that STRAD plays a key role in regulating the tumour suppressor activities of LKB1.
Here, we investigate the mechanism and function of LKB1, a Ser/Thr kinase mutated in Peutz-Jegher syndrome (PJS). We demonstrate that LKB1 physically associates with p53 and regulates specific p53-dependent apoptosis pathways. LKB1 protein is present in both the cytoplasm and nucleus of living cells and translocates to mitochondria during apoptosis. In vivo, LKB1 is highly upregulated in pyknotic intestinal epithelial cells. In contrast, polyps arising in Peutz-Jegher patients are devoid of LKB1 staining and have reduced numbers of apoptotic cells. We propose that a deficiency in apoptosis is a key factor in the formation of multiple benign intestinal polyps in PJS patients, and possibly for the subsequent development of malignant tumors in these patients.
Interacting selectively and non-covalently with any protein or protein complex (a complex of two or more proteins that may include other nonprotein molecules).
Evidence
1:
Inferred from Physical InteractionIntAct
The LKB1 gene encodes a serine/threonine kinase mutated in Peutz-Jeghers cancer syndrome. Despite several proposed models for LKB1 function in development and in tumour suppression, the detailed molecular action of LKB1 remains undefined. Here, we report the identification and characterization of an LKB1-specific adaptor protein and substrate, STRAD (STe20 Related ADaptor). STRAD consists of a STE20- like kinase domain, but lacks several residues that are indispensable for intrinsic catalytic activity. Endogenous LKB1 and STRAD form a complex in which STRAD activates LKB1, resulting in phosphorylation of both partners. STRAD determines the subcellular localization of wild-type, but not mutant LKB1, translocating it from nucleus to cytoplasm. One LKB1 mutation previously identified in a Peutz-Jeghers family that does not compromise its kinase activity is shown here to interfere with LKB1 binding to STRAD, and hence with STRAD-dependent regulation. Removal of endogenous STRAD by siRNA abrogates the LKB1-induced G(1) arrest. Our results imply that STRAD plays a key role in regulating the tumour suppressor activities of LKB1.
Evidence
2:
Inferred from Physical InteractionIntAct
Autophagy, the process by which proteins and organelles are sequestered in autophagosomal vesicles and delivered to the lysosome/vacuole for degradation, provides a primary route for turnover of stable and defective cellular proteins. Defects in this system are linked with numerous human diseases. Although conserved protein kinase, lipid kinase and ubiquitin-like protein conjugation subnetworks controlling autophagosome formation and cargo recruitment have been defined, our understanding of the global organization of this system is limited. Here we report a proteomic analysis of the autophagy interaction network in human cells under conditions of ongoing (basal) autophagy, revealing a network of 751 interactions among 409 candidate interacting proteins with extensive connectivity among subnetworks. Many new autophagy interaction network components have roles in vesicle trafficking, protein or lipid phosphorylation and protein ubiquitination, and affect autophagosome number or flux when depleted by RNA interference. The six ATG8 orthologues in humans (MAP1LC3/GABARAP proteins) interact with a cohort of 67 proteins, with extensive binding partner overlap between family members, and frequent involvement of a conserved surface on ATG8 proteins known to interact with LC3-interacting regions in partner proteins. These studies provide a global view of the mammalian autophagy interaction landscape and a resource for mechanistic analysis of this critical protein homeostasis pathway.
Evidence
3:
Inferred from Physical InteractionIntAct
The Peutz-Jeghers syndrome (PJS) is a hereditary disorder that predisposes an individual to benign and malignant tumors in multiple organ systems. Recently, the locus responsible for PJS was mapped genetically to the LKB1 gene, with a subsequent investigation proving that it is responsible for most cases of PJS. LKB1 encodes a nuclear serine/threonine protein kinase, and potential tumor-suppressing activity has been attributed to LKB1 kinase. However, how LKB1 exerts its tumor-suppressing function remains to be determined. In this report, we describe the identification of a putative human LKB1-interacting protein, FLIP1, using the yeast two-hybrid system. Two regions of the LKB1 sequence have been determined to be crucial for the interaction with FLIP1. FLIP1 encodes a protein of 429 amino acids with a predicted molecular weight of 47 kd. In contrast to LKB1, which is mainly nuclear, FLIP1 is a cytoplasmic protein, and its expression is ubiquitous in all human tissues examined to date. Interestingly, deletion of the 195 N- terminal amino acids allows FLIP1 to enter the nucleus, suggesting the presence of a regulatory mechanism through its N-terminus for nuclear entry. In addition, we found that ectopic expression of FLIP1 selectively blocks cytokine-induced NF-kappaB activation. The involvement of FLIP1 in the regulation of NF-kappaB activity may shed new light on the role of LKB1 in tumor suppression.
Evidence
4:
Inferred from Physical InteractionUniProtKB
Germline mutations of the serine/threonine kinase LKB1 (also known as STK11) lead to Peutz-Jeghers syndrome (PJS) that is associated with increased incidence of malignant cancers. However, the tumor suppressor function of LKB1 has not been fully elucidated. We applied yeast two-hybrid screening and identified that a novel WD-repeat protein WDR6 was able to interact with LKB1. Immunofluorescence staining revealed that WDR6 was localized in cytoplasm, similar to the localization of LKB1. Expression of LKB1 was able to inhibit colony formation of Hela cells. Interestingly, coexpression of WDR6 with LKB1 enhanced the inhibitory effect of LKB1 on Hela cell proliferation. Consistently, WDR6 was able to synergize with LKB1 in cell cycle G1 arrest in Hela cells. Coexpression of WDR6 and LKB1 was able to induce a cyclin-dependent kinase (CDK) inhibitor p27(Kip1). Furthermore, the stimulatory effect of LKB1 on p27(Kip1) promoter activity was significantly elevated by coexpression with WDR6. Collectively, these results provided initial evidence that WDR6 is implicated in the cell growth inhibitory pathway of LKB1 via regulation of p27(Kip1).
Evidence
5:
Inferred from Physical InteractionBHF-UCL
The LKB1 tumor suppressor is a protein kinase that controls the activity of adenosine monophosphate-activated protein kinase (AMPK). LKB1 activity is regulated by the pseudokinase STRADalpha and the scaffolding protein MO25alpha through an unknown, phosphorylation-independent, mechanism. We describe the structure of the core heterotrimeric LKB1-STRADalpha-MO25alpha complex, revealing an unusual allosteric mechanism of LKB1 activation. STRADalpha adopts a closed conformation typical of active protein kinases and binds LKB1 as a pseudosubstrate. STRADalpha and MO25alpha promote the active conformation of LKB1, which is stabilized by MO25alpha interacting with the LKB1 activation loop. This previously undescribed mechanism of kinase activation may be relevant to understanding the evolution of other pseudokinases. The structure also reveals how mutations found in Peutz-Jeghers syndrome and in various sporadic cancers impair LKB1 function.
Evidence
6:
Inferred from Physical InteractionIntAct
HSP90 is a molecular chaperone that associates with numerous substrate proteins called clients. It plays many important roles in human biology and medicine, but determinants of client recognition by HSP90 have remained frustratingly elusive. We systematically and quantitatively surveyed most human kinases, transcription factors, and E3 ligases for interaction with HSP90 and its cochaperone CDC37. Unexpectedly, many more kinases than transcription factors bound HSP90. CDC37 interacted with kinases, but not with transcription factors or E3 ligases. HSP90::kinase interactions varied continuously over a 100-fold range and provided a platform to study client protein recognition. In wild-type clients, HSP90 did not bind particular sequence motifs, but rather associated with intrinsically unstable kinases. Stabilization of the kinase in either its active or inactive conformation with diverse small molecules decreased HSP90 association. Our results establish HSP90 client recognition as a combinatorial process: CDC37 provides recognition of the kinase family, whereas thermodynamic parameters determine client binding within the family.
Evidence
7:
Inferred from Physical InteractionIntAct
The human ABIN-2 was originally identified as an A20-associating cytosolic protein to block NF-kappaB activation induced by various stimuli. Here we report that ABIN-2 has the potential to enter the nucleus and plays a role in mediating transcriptional activation in both yeast and mammalian cells. The Gal4BD-ABIN-2 fusion protein is able to drive the expression of the GAL4-responsive reporter gene in yeast efficiently without the need of the Gal4p activation domain, suggesting that ABIN-2 functions as a transcriptional coactivator and facilitates transcription in yeast. In contrast to the activity in yeast, however, only the C-terminal fragment of ABIN-2 exerts the transactivating activity in mammalian cells but not the full-length ABIN-2 protein. This observation has led to the identification of the N-terminal 195 amino acids of ABIN-2 as a regulatory domain, which retains the full-length ABIN-2 in the cytoplasm of mammalian cells and thus cannot transactivate. We have also found that BAF60a, a component of chromatin-remodeling complex, interacts with ABIN-2 by the yeast two-hybrid analysis. Together, our results suggest that the nuclear ABIN-2 defines a novel transcriptional coactivator and acts presumably by recruiting a chromatin-remodeling complex to the site of the target gene.
Evidence
8:
Inferred from Physical InteractionIntAct
The polarization of eukaryotic cells is controlled by the concerted activities of asymmetrically localized proteins. The PAR proteins, first identified in Caenorhabditis elegans, are common regulators of cell polarity conserved from nematode and flies to man. However, little is known about the molecular mechanisms by which these proteins and protein complexes establish cell polarity in mammals. We have mapped multiprotein complexes formed around the putative human Par orthologs MARK4 (microtubule-associated protein/microtubule affinity-regulating kinase 4) (Par-1), Par-3, LKB1 (Par-4), 14-3-3zeta and eta (Par-5), Par-6a, -b, -c, and PKClambda (PKC3). We employed a proteomic approach comprising tandem affinity purification (TAP) of protein complexes from cultured cells and protein sequencing by tandem mass spectrometry. From these data we constructed a highly interconnected protein network consisting of three core complex "modules" formed around MARK4 (Par-1), Par-3.Par-6, and LKB1 (Par-4). The network confirms most previously reported interactions. In addition we identified more than 50 novel interactors, some of which, like the 14-3-3 phospho-protein scaffolds, occur in more than one distinct complex. We demonstrate that the complex formation between LKB1.Par-4, PAPK, and Mo25 results in the translocation of LKB1 from the nucleus to the cytoplasm and to tight junctions and show that the LKB1 complex may activate MARKs, which are known to introduce 14-3-3 binding sites into several substrates. Our findings suggest co-regulation and/or signaling events between the distinct Par complexes and provide a basis for further elucidation of the molecular mechanisms that govern cell polarity.
The LKB1 tumor suppressor is a protein kinase that controls the activity of adenosine monophosphate-activated protein kinase (AMPK). LKB1 activity is regulated by the pseudokinase STRADalpha and the scaffolding protein MO25alpha through an unknown, phosphorylation-independent, mechanism. We describe the structure of the core heterotrimeric LKB1-STRADalpha-MO25alpha complex, revealing an unusual allosteric mechanism of LKB1 activation. STRADalpha adopts a closed conformation typical of active protein kinases and binds LKB1 as a pseudosubstrate. STRADalpha and MO25alpha promote the active conformation of LKB1, which is stabilized by MO25alpha interacting with the LKB1 activation loop. This previously undescribed mechanism of kinase activation may be relevant to understanding the evolution of other pseudokinases. The structure also reveals how mutations found in Peutz-Jeghers syndrome and in various sporadic cancers impair LKB1 function.
Here, we investigate the mechanism and function of LKB1, a Ser/Thr kinase mutated in Peutz-Jegher syndrome (PJS). We demonstrate that LKB1 physically associates with p53 and regulates specific p53-dependent apoptosis pathways. LKB1 protein is present in both the cytoplasm and nucleus of living cells and translocates to mitochondria during apoptosis. In vivo, LKB1 is highly upregulated in pyknotic intestinal epithelial cells. In contrast, polyps arising in Peutz-Jegher patients are devoid of LKB1 staining and have reduced numbers of apoptotic cells. We propose that a deficiency in apoptosis is a key factor in the formation of multiple benign intestinal polyps in PJS patients, and possibly for the subsequent development of malignant tumors in these patients.
The LKB1 gene encodes a serine/threonine kinase mutated in Peutz-Jeghers cancer syndrome. Despite several proposed models for LKB1 function in development and in tumour suppression, the detailed molecular action of LKB1 remains undefined. Here, we report the identification and characterization of an LKB1-specific adaptor protein and substrate, STRAD (STe20 Related ADaptor). STRAD consists of a STE20- like kinase domain, but lacks several residues that are indispensable for intrinsic catalytic activity. Endogenous LKB1 and STRAD form a complex in which STRAD activates LKB1, resulting in phosphorylation of both partners. STRAD determines the subcellular localization of wild-type, but not mutant LKB1, translocating it from nucleus to cytoplasm. One LKB1 mutation previously identified in a Peutz-Jeghers family that does not compromise its kinase activity is shown here to interfere with LKB1 binding to STRAD, and hence with STRAD-dependent regulation. Removal of endogenous STRAD by siRNA abrogates the LKB1-induced G(1) arrest. Our results imply that STRAD plays a key role in regulating the tumour suppressor activities of LKB1.
The LKB1 tumor suppressor is a protein kinase that controls the activity of adenosine monophosphate-activated protein kinase (AMPK). LKB1 activity is regulated by the pseudokinase STRADalpha and the scaffolding protein MO25alpha through an unknown, phosphorylation-independent, mechanism. We describe the structure of the core heterotrimeric LKB1-STRADalpha-MO25alpha complex, revealing an unusual allosteric mechanism of LKB1 activation. STRADalpha adopts a closed conformation typical of active protein kinases and binds LKB1 as a pseudosubstrate. STRADalpha and MO25alpha promote the active conformation of LKB1, which is stabilized by MO25alpha interacting with the LKB1 activation loop. This previously undescribed mechanism of kinase activation may be relevant to understanding the evolution of other pseudokinases. The structure also reveals how mutations found in Peutz-Jeghers syndrome and in various sporadic cancers impair LKB1 function.
Apoptosis triggered by inadequate or inappropriate adherence to substrate e.g. after disruption of the interactions between normal epithelial cells and the extracellular matrix.
Resistance to anoikis, the subtype of apoptosis triggered by lack of adhesion, contributes to malignant transformation and the development of metastasis. Although several lines of evidence suggest that p53 plays a critical role in anoikis, the pathway(s) that connect cell detachment to p53 remain undefined. Here, through the use of a kinome-wide loss-of-function screen, we identify the serine-threonine kinase SIK1 (salt-inducible kinase 1) as a regulator of p53-dependent anoikis. Inactivation of SIK1 compromised p53 function in anoikis and allowed cells to grow in an anchorage-independent manner. In vivo, SIK1 loss facilitated metastatic spread and survival of disseminated cells as micrometastases in lungs. The presence of functional SIK1 was required for the activity of the kinase LKB1 in promoting p53-dependent anoikis and suppressing anchorage-independent growth, Matrigel invasion, and metastatic potential. In human cancers, decreased expression of the gene encoding SIK1 closely correlated with development of distal metastases in breast cancers from three independent cohorts. Together, these findings indicate that SIK1 links LKB1 to p53-dependent anoikis and suppresses metastasis.
The process in which cells digest parts of their own cytoplasm; allows for both recycling of macromolecular constituents under conditions of cellular stress and remodeling the intracellular structure for cell differentiation.
The series of molecular signals initiated by binding of a Wnt protein to a frizzled family receptor on the surface of the target cell, followed by propagation of the signal via beta-catenin, and ending with a change in transcription of target genes. In this pathway, the activated receptor signals via downstream effectors that result in the inhibition of beta-catenin phosphorylation, thereby preventing degradation of beta-catenin. Stabilized beta-catenin can then accumulate and travel to the nucleus to trigger changes in transcription of target genes.
Germline mutations of the serine/threonine kinase LKB1 (also known as STK11) lead to Peutz-Jeghers syndrome (PJS) that is associated with increased incidence of malignant cancers. However, the tumor suppressor function of LKB1 has not been fully elucidated. We applied yeast two-hybrid screening and identified that a novel WD-repeat protein WDR6 was able to interact with LKB1. Immunofluorescence staining revealed that WDR6 was localized in cytoplasm, similar to the localization of LKB1. Expression of LKB1 was able to inhibit colony formation of Hela cells. Interestingly, coexpression of WDR6 with LKB1 enhanced the inhibitory effect of LKB1 on Hela cell proliferation. Consistently, WDR6 was able to synergize with LKB1 in cell cycle G1 arrest in Hela cells. Coexpression of WDR6 and LKB1 was able to induce a cyclin-dependent kinase (CDK) inhibitor p27(Kip1). Furthermore, the stimulatory effect of LKB1 on p27(Kip1) promoter activity was significantly elevated by coexpression with WDR6. Collectively, these results provided initial evidence that WDR6 is implicated in the cell growth inhibitory pathway of LKB1 via regulation of p27(Kip1).
The LKB1 gene encodes a serine/threonine kinase mutated in Peutz-Jeghers cancer syndrome. Despite several proposed models for LKB1 function in development and in tumour suppression, the detailed molecular action of LKB1 remains undefined. Here, we report the identification and characterization of an LKB1-specific adaptor protein and substrate, STRAD (STe20 Related ADaptor). STRAD consists of a STE20- like kinase domain, but lacks several residues that are indispensable for intrinsic catalytic activity. Endogenous LKB1 and STRAD form a complex in which STRAD activates LKB1, resulting in phosphorylation of both partners. STRAD determines the subcellular localization of wild-type, but not mutant LKB1, translocating it from nucleus to cytoplasm. One LKB1 mutation previously identified in a Peutz-Jeghers family that does not compromise its kinase activity is shown here to interfere with LKB1 binding to STRAD, and hence with STRAD-dependent regulation. Removal of endogenous STRAD by siRNA abrogates the LKB1-induced G(1) arrest. Our results imply that STRAD plays a key role in regulating the tumour suppressor activities of LKB1.
A series of molecular signals in which an intracellular signal is conveyed to trigger the apoptotic death of a cell. The pathway is induced by the cell cycle regulator phosphoprotein p53, or an equivalent protein, and ends when the execution phase of apoptosis is triggered.
Here, we investigate the mechanism and function of LKB1, a Ser/Thr kinase mutated in Peutz-Jegher syndrome (PJS). We demonstrate that LKB1 physically associates with p53 and regulates specific p53-dependent apoptosis pathways. LKB1 protein is present in both the cytoplasm and nucleus of living cells and translocates to mitochondria during apoptosis. In vivo, LKB1 is highly upregulated in pyknotic intestinal epithelial cells. In contrast, polyps arising in Peutz-Jegher patients are devoid of LKB1 staining and have reduced numbers of apoptotic cells. We propose that a deficiency in apoptosis is a key factor in the formation of multiple benign intestinal polyps in PJS patients, and possibly for the subsequent development of malignant tumors in these patients.
Germline mutations of the serine/threonine kinase LKB1 (also known as STK11) lead to Peutz-Jeghers syndrome (PJS) that is associated with increased incidence of malignant cancers. However, the tumor suppressor function of LKB1 has not been fully elucidated. We applied yeast two-hybrid screening and identified that a novel WD-repeat protein WDR6 was able to interact with LKB1. Immunofluorescence staining revealed that WDR6 was localized in cytoplasm, similar to the localization of LKB1. Expression of LKB1 was able to inhibit colony formation of Hela cells. Interestingly, coexpression of WDR6 with LKB1 enhanced the inhibitory effect of LKB1 on Hela cell proliferation. Consistently, WDR6 was able to synergize with LKB1 in cell cycle G1 arrest in Hela cells. Coexpression of WDR6 and LKB1 was able to induce a cyclin-dependent kinase (CDK) inhibitor p27(Kip1). Furthermore, the stimulatory effect of LKB1 on p27(Kip1) promoter activity was significantly elevated by coexpression with WDR6. Collectively, these results provided initial evidence that WDR6 is implicated in the cell growth inhibitory pathway of LKB1 via regulation of p27(Kip1).
Negative regulation of epithelial cell proliferation involved in prostate gland developmentdefinition[GO:0060770]‹silver
Any process that decreases the rate, frequency or extent of epithelial cell proliferation that contributes to the progression of the prostate gland over time.
The LKB1 tumor suppressor is a protein kinase that controls the activity of adenosine monophosphate-activated protein kinase (AMPK). LKB1 activity is regulated by the pseudokinase STRADalpha and the scaffolding protein MO25alpha through an unknown, phosphorylation-independent, mechanism. We describe the structure of the core heterotrimeric LKB1-STRADalpha-MO25alpha complex, revealing an unusual allosteric mechanism of LKB1 activation. STRADalpha adopts a closed conformation typical of active protein kinases and binds LKB1 as a pseudosubstrate. STRADalpha and MO25alpha promote the active conformation of LKB1, which is stabilized by MO25alpha interacting with the LKB1 activation loop. This previously undescribed mechanism of kinase activation may be relevant to understanding the evolution of other pseudokinases. The structure also reveals how mutations found in Peutz-Jeghers syndrome and in various sporadic cancers impair LKB1 function.
Germline mutations in STK11 (also known as LKB1) are found in individuals with Peutz-Jeghers syndrome (PJS) manifesting with gastrointestinal polyps that contain a prominent stromal component. Epithelia in polyps of Stk11(+/-) mice can retain a functional copy of Stk11 (refs. 2,3), and loss of heterozygosity is not an obligate feature of human polyps, raising the possibility of non-epithelial origins in tumorigenesis. Here we show that either monoallelic or biallelic loss of murine Stk11 limited to Tagln-expressing mesenchymal cells results in premature postnatal death as a result of gastrointestinal polyps indistinguishable from those in PJS. Stk11-deficient mesenchymal cells produced less TGFbeta, and defective TGFbeta signaling to epithelial cells coincided with epithelial proliferation. We also noted TGFbeta signaling defects in polyps of individuals with PJS, suggesting that the identified stromal-derived mechanism of tumor suppression is also relevant in PJS.
Here, we investigate the mechanism and function of LKB1, a Ser/Thr kinase mutated in Peutz-Jegher syndrome (PJS). We demonstrate that LKB1 physically associates with p53 and regulates specific p53-dependent apoptosis pathways. LKB1 protein is present in both the cytoplasm and nucleus of living cells and translocates to mitochondria during apoptosis. In vivo, LKB1 is highly upregulated in pyknotic intestinal epithelial cells. In contrast, polyps arising in Peutz-Jegher patients are devoid of LKB1 staining and have reduced numbers of apoptotic cells. We propose that a deficiency in apoptosis is a key factor in the formation of multiple benign intestinal polyps in PJS patients, and possibly for the subsequent development of malignant tumors in these patients.
The process of creating protein oligomers, compounds composed of a small number, usually between three and ten, of component monomers that are not all identical. Oligomers may be formed by the polymerization of a number of monomers or the depolymerization of a large protein polymer.
The LKB1 gene encodes a serine/threonine kinase mutated in Peutz-Jeghers cancer syndrome. Despite several proposed models for LKB1 function in development and in tumour suppression, the detailed molecular action of LKB1 remains undefined. Here, we report the identification and characterization of an LKB1-specific adaptor protein and substrate, STRAD (STe20 Related ADaptor). STRAD consists of a STE20- like kinase domain, but lacks several residues that are indispensable for intrinsic catalytic activity. Endogenous LKB1 and STRAD form a complex in which STRAD activates LKB1, resulting in phosphorylation of both partners. STRAD determines the subcellular localization of wild-type, but not mutant LKB1, translocating it from nucleus to cytoplasm. One LKB1 mutation previously identified in a Peutz-Jeghers family that does not compromise its kinase activity is shown here to interfere with LKB1 binding to STRAD, and hence with STRAD-dependent regulation. Removal of endogenous STRAD by siRNA abrogates the LKB1-induced G(1) arrest. Our results imply that STRAD plays a key role in regulating the tumour suppressor activities of LKB1.
Any process that modulates the frequency, rate or extent of the protein kinase B signaling cascade, a series of reactions mediated by the intracellular serine/threonine kinase protein kinase B.
Any process that results in a change in state or activity of a cell (in terms of movement, secretion, enzyme production, gene expression, etc.) as a result of a stimulus indicating damage to its DNA from environmental insults or errors during metabolism.
Any process that results in a change in state or activity of a cell or an organism (in terms of movement, secretion, enzyme production, gene expression, etc.) as a result of a glucagon stimulus.
Any process that results in a change in state or activity of a cell or an organism (in terms of movement, secretion, enzyme production, gene expression, etc.) as a result of a ionizing radiation stimulus. Ionizing radiation is radiation with sufficient energy to remove electrons from atoms and may arise from spontaneous decay of unstable isotopes, resulting in alpha and beta particles and gamma rays. Ionizing radiation also includes X-rays.
Any process that results in a change in state or activity of a cell or an organism (in terms of movement, secretion, enzyme production, gene expression, etc.) as a result of a lipid stimulus.
A homeostatic process involved in the maintenance of an internal steady state within a defined tissue of an organism, including control of cellular proliferation and death and control of metabolic function.
The process whose specific outcome is the progression of the vasculature over time, from its formation to the mature structure. The vasculature is an interconnected tubular multi-tissue structure that contains fluid that is actively transported around the organism.
ISSOrtholog Curator
Enzymatic activity
This protein acts as an enzyme. It is known to catalyze the following reaction
EC 2.7.11.1: ATP + a protein ⇄ ADP + a phosphoprotein.
We recently demonstrated that the LKB1 tumour suppressor kinase, in complex with the pseudokinase STRAD and the scaffolding protein MO25, phosphorylates and activates AMP-activated protein kinase (AMPK). A total of 12 human kinases (NUAK1, NUAK2, BRSK1, BRSK2, QIK, QSK, SIK, MARK1, MARK2, MARK3, MARK4 and MELK) are related to AMPK. Here we demonstrate that LKB1 can phosphorylate the T-loop of all the members of this subfamily, apart from MELK, increasing their activity >50-fold. LKB1 catalytic activity and the presence of MO25 and STRAD are required for activation. Mutation of the T-loop Thr phosphorylated by LKB1 to Ala prevented activation, while mutation to glutamate produced active forms of many of the AMPK-related kinases. Activities of endogenous NUAK2, QIK, QSK, SIK, MARK1, MARK2/3 and MARK4 were markedly reduced in LKB1-deficient cells. Neither LKB1 activity nor that of AMPK-related kinases was stimulated by phenformin or AICAR, which activate AMPK. Our results show that LKB1 functions as a master upstream protein kinase, regulating AMPK-related kinases as well as AMPK. Between them, these kinases may mediate the physiological effects of LKB1, including its tumour suppressor function.
Activated by forming a complex with STRAD (STRADA or STRADB) and CAB39/MO25 (CAB39/MO25alpha or CAB39L/MO25beta): STRADA (or STRADB)-binding promotes a conformational change of STK11/LKB1 in an active conformation, which is stabilized by CAB39/MO25alpha (or CAB39L/MO25beta) interacting with the STK11/LKB1 activation loop.
The LKB1 tumor suppressor is a protein kinase that controls the activity of adenosine monophosphate-activated protein kinase (AMPK). LKB1 activity is regulated by the pseudokinase STRADalpha and the scaffolding protein MO25alpha through an unknown, phosphorylation-independent, mechanism. We describe the structure of the core heterotrimeric LKB1-STRADalpha-MO25alpha complex, revealing an unusual allosteric mechanism of LKB1 activation. STRADalpha adopts a closed conformation typical of active protein kinases and binds LKB1 as a pseudosubstrate. STRADalpha and MO25alpha promote the active conformation of LKB1, which is stabilized by MO25alpha interacting with the LKB1 activation loop. This previously undescribed mechanism of kinase activation may be relevant to understanding the evolution of other pseudokinases. The structure also reveals how mutations found in Peutz-Jeghers syndrome and in various sporadic cancers impair LKB1 function.
Protein involved in apoptotic programmed cell death. Apoptosis is characterized by cell morphological changes, including blebbing, cell shrinkage, nuclear fragmentation, chromatin condensation and chromosomal DNA fragmentation, and eventually death. Unlike necrosis, apoptosis produces cell fragments, called apoptotic bodies, that phagocytic cells are able to engulf and quickly remove before the contents of the cell can spill out onto surrounding cells and cause damage. In general, apoptosis confers advantages during an organism's life cycle.
Protein participating in autophagy, a process of intracellular bulk degradation in which cytoplasmic components including organelles are sequestered within double-membrane vesicles that deliver the contents to the lysosome/vacuole for degradation. There are three primary forms of autophagy: chaperone-mediated autophagy, microautophagy and macroautophagy. During macroautophagy, the sequestering vesicles, termed autophagosomes, fuse with the lysosome or vacuole resulting in the delivery of an inner vesicle (autophagic body) into the lumen of the degradative compartment.
Protein involved in the complex series of events by which the cell duplicates its contents and divides into two. The eukaryotic cell cycle can be divided in four phases termed G1 (first gap period), S (synthesis, phase during which the DNA is replicated), G2 (second gap period) and M (mitosis). The prokaryotic cell cycle typically involves a period of growth followed by DNA replication, partition of chromosomes, formation of septum and division into two similar or identical daughter cells.
Protein induced by DNA damage or protein involved in the response to DNA damage. Drug- or radiation-induced injuries in DNA introduce deviations from its normal double-helical conformation. These changes include structural distortions which interfere with replication and transcription, as well as point mutations which disrupt base pairs and exert damaging effects on future generations through changes in DNA sequence. Response to DNA damage results in either repair or tolerance.
Protein which catalyzes the phosphorylation of serine or threonine residues on target proteins by using ATP as phosphate donor. Such phosphorylation may cause changes in the function of the target protein. Protein kinases share a conserved catalytic core common to both serine/ threonine and tyrosine protein kinases.
A reference proteome is a set of protein sequences derived from a complete proteome which constitutes a defined standard for a particular user community. Reference proteomes are manually defined according to a number of criteria. They cover the proteomes of well- studied model organisms and other proteomes of interest for biomedical and biotechnological research. Reference proteomes have been selected to provide broad coverage of the tree of life, and constitute a representative cross-section of the taxonomic diversity to be found within UniProtKB.
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