Receptor-binding sites on C3 and C3b

Human erythrocytes carry C3b receptors: Daudi lymphoid cells carry exclusively C3d receptors. With these two types of cells, it could be shown that isolated, uncleaved human C3 and soluble C3b possess two stable binding sites: SBS1-specific C3b receptors, and SBS2-specific for C3d receptors. Upon binding of freshly cleaved C3b (nascent C3b) via its labile binding site (generated on C3b through cleavage of C3) to the C3 acceptors on cell surfaces, the SBS2 becomes concealed. Kinetic experiments show that immediately after the action of fetal calf serum or partially purified C3b-inactivator on EAC1423b the SBS2 is accessible again. The reappearance of SBS2 does coincide with cleavage of surface-bound C3b into C3c and C3d but not with the release of C3c from the cell; C3c remains attached and is released only with delay. Concomitant with the cleavage event the number of SBS1 is reduced, stressing the importance of an unaltered steric configuration of the C3b structure for the expression of SBS1. An alternative explanation might be that the SBS1 is located at the site connecting C3c and C3d, so that it becomes altered upon dissection of the two fragments.