Stimulation of yeast adenylyl cyclase activity by lysophospholipids and fatty acids. Implications for the regulation of Ras/effector function by lipids.

A reconstituted system containing membranes prepared from various Saccharomyces cerevisiae strains (CYR1 ras1 ras2) and a recombinant RAS2 protein was used to evaluate the effect of lipids on adenylyl cyclase activity. Incubation of wild-type membranes with lysophosphatidylinositol, lysophosphatidylserine, or lysophosphatidylcholine stimulated adenylyl cyclase activity in the absence and presence of RAS between 2-10-fold depending upon the individual lipid. Unsaturated fatty acids preferentially increased activity 2-3-fold in the presence of RAS. Other phospholipids as well as several detergents had only marginal effects on adenylyl cyclase activity. Lipids had no effect on either the binding or hydrolysis of GTP by RAS. LysoPI decreased both the Km for ATP and the amount of RAS required for enzyme activation. Four catalytically active deletion mutants of the CYR1 protein including one containing only the C-terminal 417 amino acids were similarly responsive to lysoPI when compared to the wild-type enzyme. These data suggest that lysophospholipids and fatty acids, metabolites of the mitogenically responsive enzyme phospholipase A2, may represent a novel mechanism for modulating the activity of downstream effector molecules and their interaction with Ras proteins.