 Complement receptor 1

Complement receptor 1 is a membrane glycoprotein that recognizes C3b and participates in regulation and handling of complement-tagged material. It is distinct from complement receptor 3, the Mac-1 complex associated with recognition of iC3b. Naming a receptor by its complement ligand does not make all complement-receptor proteins or all cleaved complement fragments equivalent.^[ref]

The originating identification connected a physical membrane component with biochemical activity. D. T. Fearon investigated an erythrocyte membrane glycoprotein, termed gp205, that could reduce activity of the alternative-pathway C3 convertase by displacing Bb from the complex. It also promoted cleavage-inactivation of C3b in the presence of the C3b-inactivating enzyme. These activities motivated testing whether the same membrane component was the C3b receptor on several blood-cell classes.^[ref]

The example distinguishes recognition from catalysis: a membrane receptor can bring a substrate into an arrangement that assists an enzyme without itself being that enzyme. Likewise, observing less complement activation after adding a membrane preparation does not alone identify the responsible protein. Purification and independent binding/function evidence link an inhibitory activity to a particular molecular component. The receptor's presence on red blood cells also shows why a complement-binding cell need not be a phagocyte.