Human membrane-bound C3 receptors. II. Physiochemical properties of the C3b and C3d receptors isolated from tonsil cells by immunoprecipitation.

The aim of the present study was to define the physicochemical structure of C3b and C3d receptors of lymphoid cells. C3b and C3d receptors were isolated from KBr lysates of the 20,000 g fraction of human tonsil homogenates by immunoprecipitation with an anti-C3 receptor serum (AC3RS). Sodium dodecyl sulphate (SDS) gel filtration and polyacrylamide gel electrophoresis (PAGE) of unreduced immunoprecipitates revealed a highly predominant component with an apparent molecular weight (mol. wt.) greater than 1 x 10(6) and a small component with a mol. wt. of 80,000. After reduction, the SDS-PAGE profile was made up of a constant major 38,000 mol. wt. component and a inconstant smaller 18,000 mol. wt. component. The 38,000 (and also the 18,000) component could be isolated only from C3 receptor-active lysates, and not from C3 receptor-negative lysates. Taken together, the results of this study suggest that the active C3 receptor molecule of tonsil cells is a lipoprotein complex with a mol. wt. greater than 1 x 10(6); its protein moiety consists predominantly of disulphide-bridged polypeptide chains with a mol. wt. of 38,000; C3b and C3d receptors are composed of equal-sized polypeptide chains, but the specific binding sites for C3b and C3d are located on different molecules.