Peptidase S1A, complement C1r/C1S/mannan-binding (IPR024175)

Short name: Pept_S1A_C1r/C1S/mannan-bd

Family relationships


These proteins belong to MEROPS peptidase family S1 (chymotrypsin family, clan PA(S)), subfamily S1A.

This family contains the mammalian mannan-binding lectin-associated serine proteases 1 and 2 (MASP1 and MASP2) and complement components C1s and C1r. The C1 complex, containing C1q, C1s, and C1r, triggers the classical complement pathway. When C1q interacts with antibody, C1r becomes autocatalytically activated. Activated C1r in turn activates C1s, which then cleaves C2 and C4 in the classical pathway.

Mannose-binding lectin (MBL) complexes with MASP1, MASP2, and a smaller alternative splice product of the MASP2 gene. Binding of MBL to carbohydrates on the surface of microorganisms triggers activation of the associated MASPs. Then MASP1 activates C3 and C2, whereas MASP2 activates C4 and C2 [PMID: 10946292]. Based on the fact that the gene structures of MASP1, C1r, and C1s are similar except that C1r and C1s lack introns in the region encoding the trypsin domain, it has been proposed that the MASP proteins evolved earlier than C1r and C1s [PMID: 10475605].

These sequences typically contain a signal sequence, followed by a CUB domain, an EGF-like domain (which often is not detected), a second CUB domain, two sushi domains (sometimes only one is detected), and a trypsin domain.

GO terms

Biological Process

GO:0006956 complement activation

Molecular Function

GO:0004252 serine-type endopeptidase activity

Cellular Component

GO:0005576 extracellular region

Contributing signatures

Signatures from InterPro member databases are used to construct an entry.