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 Sacc haromyces cerevisiae strains (CYR1 ras1 ras2) and a recombinant RAS2 p rotein was used to evaluate the effect of lipids on adenylyl cyclase a ctivity. Incubation of wild-type membranes with lysophosphatidylinosit ol, lysophosphatidylserine, or lysophosphatidylcholine stimulated aden ylyl cyclase activity in the absence and presence of RAS between 2-10- fold depending upon the individual lipid. Unsaturated fatty acids pref erentially increased activity 2-3-fold in the presence of RAS. Other p hospholipids as well as several detergents had only marginal effects o n adenylyl cyclase activity. Lipids had no effect on either the bindin g or hydrolysis of GTP by RAS. LysoPI decreased both the K-m for ATP a nd the amount of RES required for enzyme activation. Four catalyticall y active deletion mutants of the CYR1 protein including one containing only the C-terminal 417 amino acids were similarly responsive to lyso PI when compared to the wild-type enzyme. These data suggest that lyso phospholipids and fatty acids, metabolites of the mitogenically respon sive enzyme phospholipase A(2), may represent a novel mechanism for mo dulating the activity of downstream effector molecules and their inter action with Ras proteins.