Specific interactions of chemoattractant factor receptors with G-proteins

Stimulation of leukocytes with chemoattractant ligands activates phospholipid turnover and calcium release, ultimately leading to chemotaxis, degranulation, and the inflammatory response. The leukocyte response to these ligands is transduced by the interaction of transmembrane receptors with GTP-binding regulatory proteins (G-proteins). To examine the mechanisms of signal transduction by these receptors, we transfected cDNA clones encoding the receptors for the active cleavage product of the fifth component of complement (C5a) and platelet-activating factor (PAF) into COS-7 cells, then measured the production of inositol phosphates (IP) in response to stimulation with these chemoattractant ligands. Cells transfected with the C5a receptor showed no increase in IP production when stimulated with ligand (5-120 nM). However, in cells co-transfected with these receptors and with the cDNA for G alpha 16, a G-protein alpha subunit that is specific to cells of hematopoietic lineage, addition of ligand caused up to a 5-fold increase in IP production. This interaction was specific, as co-transfection of receptors with the G-proteins G alpha q or G alpha 11 did not allow ligand-dependent increase in IP production. In contrast, ligand-dependent activation of IP production was seen in COS cells transfected solely with the PAF receptor. These results indicate that the C5a receptor utilizes signaling pathways distinct from the PAF receptor and suggest that a pertussis toxin-resistant G-protein, G alpha 16, may play a role in the leukocyte response to inflammatory ligands.