A. "Open-book" surface representation of C3b and regulator molecules, highlighting their domain organizations using different colors as shown in the neighboring schemes. Domains in complement regulators are labeled using domain numbering from CCPi to CCPiv based on their contact sites on C3b. The corresponding CCP domains in each regulator described in this work are shown in the bottom-left scheme. Domains CCP1-2 of MCP are shown in shaded grey because they are not involved in C3b binding. The table reports the extent of the regulator contact surface on C3b and the associated buried surface area. 