Pertussis toxin-sensitive activation of phospholipase C by the C5a and fMet-Leu-Phe receptors

Signal transduction pathways that mediate C5a and fMet-Leu-Phe (fMLP)-induced pertussis toxin (PTx)-sensitive activation of phospholipase C (PLC) have been investigated using a cotransfections assay system in COS-7 cells. The abilities of the receptors for C5a and fMLP to activate PLC beta 2 and PLC beta 3 through the G beta gamma subunits of endogenous G(1) proteins in COS-7 cells were tested because both PLC beta 2 and PLC beta 3 were shown to be activated by the beta gamma subunits of G proteins in in vitro reconstitution assays. Neither or the receptors can activate endogenous PLC beta 3 or recombinant PLC beta 3 in transfected COS-7 cells. However, both receptors can clearly activate PLC beta 2 in a PTx-sensitive manner, suggesting that the receptors may interact with endogenous PTx-sensitive G proteins and activate PLC beta 2 probably through the B beta gamma subunits. These findings were further corroborated by the results that PLC beta 3 could only be slightly activated by G(b)eta(1) gamma(1) or G beta(1) gamma(5) in the co-transfection assay, whereas the G beta gamma subunits strongly activated PLC beta 2 under the same conditions. PLC beta 3 can be activated by G(alpha q), G(alpha11), and G(alpha16) in the cotransfection assay. In addition, the G gamma(2) and G gamma(3) mutants wit substitution of the C-terminal Cys residue by a Ser residue, which can inhibit wild type G beta gamma-mediated activation of PLC beta 2, were able to inhibit C5a or fMLP-mediated activation of PLC beta 2. These G gamma mutants, however, showed little effect on m(1)-muscarinic receptor-mediated PLC activation, which is mediated by the G(q) class of G proteins. These results all confirm that the G beta gamma subunits are involved in PLC beta 2 activation by the two chemoattractant receptors and suggest that in COS-7 cells activation of PLC beta 3 by G beta gamma may not be the primary pathway for the receptors.