Complement factor I (CFAI) is a trypsin-like serine protease that plays an essential role in regulating the immune response by controlling all complement pathways. It cleaves three peptide bonds in the alpha-chain of C3b and two in the alpha-chain of C4b which inactivate these proteins. It consists of an N-terminal FI and membrane attack domain (FIMAC or Factor I-like modules FIM), a scavenger receptor cysteine-rich (SRCR) domain (Pfam:PF00530), LDLRA1 and LDLRA2 Pfam:PF00057) and a C-terminal catalytic serine protease domain (PF00089) [1,2]. FIMs are also present in complement proteins C6 and C7, and participate in protein-protein interactions that critical to the progress of a complement-mediated immune response. In CFAI, the FIMAC domain is important for the ability of FI to degrade C4b and C3b [1-2]. It consists of a a small N-terminal FOLN subdomain and a larger C-terminal KAZAL domain. This entry represents the N-terminal region of FIMAC, whose structure is a truncated form of the epidermal growth factor (EGF) domain [3].

1: Structural basis for complement factor I control and its disease-associated sequence polymorphisms. Roversi P, Johnson S, Caesar JJ, McLean F, Leath KJ, Tsiftsoglou SA, Morgan BP, Harris CL, Sim RB, Lea SM; Proc Natl Acad Sci U S A. 2011;108:12839-12844. PMID:21768352

2: Regulator-dependent mechanisms of C3b processing by factor I allow differentiation of immune responses. Xue X, Wu J, Ricklin D, Forneris F, Di Crescenzio P, Schmidt CQ, Granneman J, Sharp TH, Lambris JD, Gros P; Nat Struct Mol Biol. 2017;24:643-651. PMID:28671664

3: Solution structure of factor I-like modules from complement C7 reveals a pair of follistatin domains in compact pseudosymmetric arrangement. Phelan MM, Thai CT, Soares DC, Ogata RT, Barlow PN, Bramham J; J Biol Chem. 2009;284:19637-19649. PMID:19419965