SIGNAL-TRANSDUCTION DURING CANNIBALISTIC SEXUAL PHAGOCYTOSIS - CALCIUM IS NOT THE TRIGGER BUT GTP-BINDING PROTEIN FUNCTION IS ESSENTIAL

After fertilization, the zygote giant cell of Dictyostelium discoideum chemoattracts and subsequently engulfs hundreds of amoebae of the sam e species and strains from which it was derived. A pharmacological app roach indicates that, while it may have some role, calcium is not the trigger for this cannibalistic phagocytic process. Of several agents t hat perturb intracellular calcium levels [A23187, LaCl 8-diethylamino- octyl-3,4,5-trimethoxylbenzoate (TMB-8), and chlorotetracycline], only A23187 had an effect in reducing amoebal ingestion. In keeping with t his, agents which interfered with downstream effecters of calcium func tion did not alter sexual phagocytosis. Calmidazolium and trifluoperaz ine, which inhibit calmodulin function, were ineffective, as were a pr otein kinase C inhibitor (staurosporine) and activator (phorbol 12-myr istate 13-acetate). On the other hand, the nucleotide analogues GTP ga mma S and GDP beta S both inhibited sexual phagocytosis indicating a r ole for GTP-binding protein activity at some stage in the process. Sub -fractionation of cells from non-phagocytic and phagocytic stage cell cultures followed by immunolocalization after SDS-PAGE and western blo tting revealed a number of GTP-binding proteins in both the cell membr ane and intracellular membrane fractions that might function during th e events of sexual phagocytosis.