ABSTRACT
Human C8 is one of five complement components-C5b, C6, C7, C8, C9-that assemble on the surface of pathogenic organisms and other cells to form C5b-9, a cytolytically active, macromolecular complex commonly referred to as the membrane attack complex (MAC). Efforts to identify structure–function relationships within the MAC components have focused extensively on C8. It contains three subunits that participate in binding interactions associated with the formation and function of the MAC. Of the five MAC components, C8 has the most unusual and complex subunit organization. C5b is approximately 180 kDa and composed of two disulfide-linked subunits. C6, C7, and C9 are single-chain proteins of approximately 105 kDa, 92 kDa, and 72 kDa, respectively. MAC formation is regulated in part by the binding of CD59 to C8. Human CD59 is a 20 kDa, membrane-bound complement regulatory protein that protects human blood and vascular cells from injury arising through activation of complement.
