Structure–function relationships of complement receptor type 1

Human complement receptor type 1 (CR1) is a large, multifunctional glycoprotein which is a member of the regulators of complement activation family. Like other members of this family, it is composed mainly of tandemly arranged modules, each about 60-70 amino acids long, known as complement control protein repeats (CCPs). Each domain folds independently and contains a hydrophobic core wrapped in beta sheets. These domains mediate interactions with C3/C4-derived fragments. CR1 is the most versatile inhibitor of both classical and alternative pathway C3 and C5 convertases due to its decay-accelerating activity and co-factor activity for C3b/C4b cleavage. Moreover, CR1 plays a major role in immune complex clearance due to its high affinity for C3b and C4b. CR1 is an excellent model to study structure-function relationships because its functions are mediated by two distinct but highly homologous sites, each composed of three CCPs. CR1 derivatives carrying just one active site were used to define critical sequences/amino acids. This was achieved by testing functional profiles of the proteins carrying a mutated active site produced by substituting peptides/amino acids with their counterparts from the other site. These mutated proteins, of which we analyzed over 100, permitted the fine mapping of the functional sites. CR1 on primate erythrocytes varies in size. In most cases it is smaller and has fewer active sites than does human CR1. This variation was used to determine that increased copy number (3,000 to 20,000 versus 300 for human CR1) compensates for a smaller size. Moreover, studies of primate CR1 led to the finding that subtle differences in the critical areas, as compared to human sites, produce active sites with a broader functional repertoire. These alterations ensure that short CR1 forms possess similar biologic activities to the large CR1 forms. There is much interest in producing therapeutic agents to inhibit unwanted complement activation. Based on these structure-function analyses, smaller and more potent complement inhibitors derived from CR1 can be produced.