C1S - Complement C1s subcomponent - human protein (Function)
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C1S »  Complement C1s subcomponent   [ EC ]
Protein also known as:  Complement component 1 subcomponent s. Cleaved into:  Complement C1s subcomponent heavy chain; Complement C1s subcomponent light chain.
Gene name:  C1S
Family name: Peptidase S1
Entry whose protein(s) existence is based on evidence at protein level
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C1s B chain is a serine protease that combines with C1q and C1r to form C1, the first component of the classical pathway of the complement system. C1r activates C1s so that it can, in turn, activate C2 and C4.  
  • CuratedUniProtKB
GO molecular function 
Calcium ion bindingdefinition[GO:0005509] silver  
  • IEAInterPro 2 GO
Identical protein bindingdefinition[GO:0042802]  
  • IPIIntAct
Protein bindingdefinition[GO:0005515]  
  • IPIIntAct
Serine-type endopeptidase activitydefinition[GO:0004252]  
  • IDAUniProtKB
GO biological process 
  • IEAUniProtKB KW
Enzymatic activity 
This protein acts as an enzyme. It is known to catalyze the following reaction
EC Cleavage of Arg-|-Ala bond in complement component C4 to form C4a and C4b, and Lys(or Arg)-|-Lys bond in complement component C2 to form C2a and C2b: the 'classical' pathway C3 convertase.  
  • CuratedUniProtKB
It is regulated in the following manner
Inhibited by SERPING1.  
  • CuratedUniProtKB
More information is available from:
According to KEGG, this protein belongs to the following pathways:
Complement and coagulation cascades  hsa04610+716  
Pertussis  hsa05133+716  
Staphylococcus aureus infection  hsa05150+716  
Systemic lupus erythematosus  hsa05322+716  
According to Reactome, this protein belongs to the following pathways:
Classical antibody-mediated complement activation  REACT_7956  
Initial triggering of complement  REACT_8024  

Biophysicochemical properties

Kinetic parameters
Less efficient than MASP2 in C4 cleavage.
KM 1.9 uM for complement component C4 (at 37 degrees Celsius)
KM 12.3 uM for complement component C2 (at 37 degrees Celsius)


Biological process 
Complement pathway  definition   [KW-0180]
Immunity  definition   [KW-0391]
Innate immunity  definition   [KW-0399]
Molecular function 
Hydrolase  definition   [KW-0378]
Protease  definition   [KW-0645]
Serine protease  definition   [KW-0720]
Technical term 
Reference proteome  definition   [KW-1185]

Further external links

GeneWiki: C1S
GenomeRNAi: 716
PRO: PR:P09871