Adenylate cyclase inhibition and GTPase stimulation by somatostatin in S49 lymphoma cyc variants are prevented by islet‐activating protein

Abstract
Cyc‐Variants of S49 lymphoma cells are defective in the stimulatory guanine nucleotide site of the adenylate cyclase but contain an inhibitory site. Treatment of cyc cells with islet‐activating protein (IAP), which causes ADP‐ribosylation of an M r 40 000 polypeptide in cyc membranes, abolishes adenylate cyclase inhibition by GTP and the peptide hormone, somatostatin, but not that induced by GTPγS. Furthermore, somatostatin‐induced stimulation of GTP hydrolysis is lost. Thus, the data indicate that IAP interferes with the adenylate cyclase system by an action at the inhibitory guanine nucleotide site.