Stimulation of phospholipase C by guanine‐nucleotide‐binding protein βγ subunits

We have previously shown that soluble fractions obtained from human HL‐60 granulocytes contain a phospholipase C which is markedly stimulated by the stable GTP analogue guanosine 5′‐[3‐O‐thio]triphosphate (Camps, M., Hou, C., Jakobs, K. H. and Gierschik, P. (1990) Biochem. J. 271, 743–748]. To investigate whether this stimulation was due to a soluble α subunit of a heterotrimeric guanine‐nucleotide‐binding protein or a soluble low‐molecular‐mass GTP‐binding protein, we have examined the effect of purified guanine‐nucleotide‐binding protein βγ dimers on the phospholipase‐C‐mediated formation of inositol phosphates by HL‐60 cytosol. We found that βγ subunits, purified from bovine retinal transducin (βγ_t), markedly stimulated the hydrolysis of phosphatidylinositol 4,5‐bisphosphate by this phospholipase C preparation. The stimulation of phospholipase C by βγ_t was not secondary to a phospholipase‐A₂‐mediated generation of arachidonic acid, was prevented by the GDP‐liganded transducin α subunit and was additive to activation of phospholipase C by guanosine 5′‐[3‐O‐thio]triphosphate. βγ_t also stimulated soluble phospholipase C from human and bovine peripheral neutrophils, as well as membrane‐bound, detergent‐solubilized phospholipase C from HL‐60 cells. Stimulation of soluble HL‐60 phospholipase C was not restricted to βγ_t, but was also observed with highly purified βγ subunits from bovine brain. Fractionation of HL‐60 cytosol by anion‐exchange chromatography revealed the existence of at least two distinct forms of phospholipase C in HL‐60 granulocytes. Only one of these forms was sensitive to stimulation by βγ_t, demonstrating that stimulation of phospholipase C by βγ subunits is isozyme specific. Taken together, our results suggest that guanine‐nucleotide‐binding protein βγ subunits may play an important and active role in mediating the stimulation of phospholipase C by heterotrimeric guanine‐nucleotide‐binding proteins.