Receptor-independent activation of guanine nucleotide-binding regulatory proteins by terminal complement complexes

Activation of heterotrimeric guanine nucleotide-binding proteins (G proteins) by terminal complement complexes (TCC) was investigated on human lymphoblastoid B-cell line JY25 and its mutant JY5 deficient in glycosylphosphatidylinositol-anchored proteins. TCC assembly achieved by antibody-dependent activation of C7-deficient serum reconstituted with C7 increased specific guanosine-5'-(gamma-thio)triphosphate (GTP gamma S) binding, 4- and 8-fold, in JY25 and JY5 membranes, respectively, between 2 and 10 min, over the level without C7. TCC also increased GTPase activity 5- and 4-fold in JY25 and JY5, respectively, between 5 and 10 min. Increased GTPase activity was noted first with C5b-7 assembly, which increased further with C5b-8 and C5b-9. The presence of G proteins in anti-TCC immunoprecipitates of cell lysates was investigated by demonstration of G alpha subunit that can be ADP-ribosylated by pertussis toxin (PTX). Immunoprecipitated TCC complexes contained a PTX-sensitive 41-kDa Gi alpha/Go alpha subunit, as shown by SDS-PAGE and Western blotting. These complexes were functionally active as determined by GTP gamma S binding. We have further shown that enhanced TCC elimination from the plasma membrane induced by TCC-generated signals was inhibited by PTX. In conclusion the biological activities induced by TCC in nucleated cells may be mediated in part by activation of PTX-sensitive G proteins.