An intrinsic membrane protein with a m.w. of 65,000 that can bind human C8 has been identified after separation of human erythrocyte membrane proteins by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and electrotransfer to nitrocellulose sheets. The protein, tentatively designated as the C8-binding protein (C8bp) could be isolated from papain-treated erythrocyte (E) membranes by phenol-water extraction and isoelectric focusing. In a functional assay, with chicken (ch) E as target cells, C8bp inhibited the lysis of ch E C5b67 intermediates by human C8 and C9, whereas the lysis by rabbit C8 and C9 was not affected. Because the decay accelerating factor (DAF) from human erythrocyte membranes also inhibits the activity of C3/C5 convertases in an homologous system, we tested whether or not a DAF activity was present in C8bp. C8bp, however, did not accelerate the decay of the classic C3 convertases. Thus, it appears that C8bp and DAF are two different factors of E membranes with a similar molecular size inhibiting different sites of the activation cascade of complement while they can function synergistically to minimize the self-inflicted damage by complement.

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