   3.4.21.45 | to 2.7 A resolution. The shape of factor I is bilobal, with the heavy and light chains making up the two halves of a brick. The arrangement of the domains in the larger heavy-chain lobe forms a ring structure, with the N-terminal FIMAC domain contacting the C-terminal LDLRA domains. This contact is linked covalently by a disulfide bridge between Cys15 and Cys237. Mapping of disease-associated gene polymorphisms and mutations known to alter cofactor-assisted C3b/C4b cleavage by factor I onto the factor I structure implies allosteric regulation of light-chain activity by contact of the heavy chain. Modeling of the ternary complex of factor I, factor H, and C3b |
