FIGURE 1.

Factor B structure. A, scissile bond lying between the linker region and the VWA domain in the factor B primary sequence (9). CCP, complement control protein repeat. B, factor B three-dimensional structure (15). C, scissile bond sequestered by salt bridges that bind the Arg²³⁴ side chain to Glu²⁰⁷ and Glu⁴⁴⁶ (15). D, factor B VWA apex with its cation-free MIDAS (15). E, C3bBb VWA apex with MIDAS occupied with Mg²⁺ that is coordinated by five conserved VWA side chains plus the carboxyl terminus of C3b (11). The coordination residues are shown as stick figures. Coordinates of factor B and of C3bBb stabilized by the staphylococcal complement inhibitor SCIN were obtained from the Protein Data Bank, accession numbers 2OK5 and 2WIN, respectively. Molecular structures are illustrated using the PyMOL Molecular Graphics System (42).