AbstractThe effects of the cytosol activator protein obtained from rat reticulocytes (RCAP) were investigated in a heterologous membrane system — partially purified cell membranes from dog renal cortex. RCAP enhanced the response of dog renal cortical adenylate cyclase to bovine parathyroid hormone (1–34) [bPTH (1–34)] from two‐ to three‐fold. RCAP also enhanced the response to 5 μMarginine vasopressin, 10 μMglucagon, and 10 μMisoproterenol. Analysis of double‐reciprocal plots of substrate concentration and enzyme activity indicated that bPTH (1–34) alone and together with RCAP increased the Vmaxof the adenylate cyclase enzyme and did not alter the apparent Kmof the enzyme for MgATP. Membranes from dog renal cortex contain 42K and 39K proteins that are ADP‐ribosylated by cholera toxin and pertussis toxin, respectively, and appear to be the stimulatory (Ns) and inhibitory (Ni) guanine nucleotide binding proteins described in many other hormone‐responsive membrane preparations. Similar to its effects in rat reticulocytes, RCAP inhibited ADP‐ribosylation of Nsand enhanced ADP‐ribosylation of Ni. The muscarinic agonist, carbachol, inhibited PTH‐responsive adenylate cyclase activity in dog renal cortical membranes and this inhibition was reversed by RCAP. These results indicate that RCAP enhances stimulation of adenylate cyclase by a variety of hormones in a heterologous membrane preparation and supports the hypothesis that RCAP's site of action is common to all adenylate cyclase systems. RCAP may facilitate coupling between Nsand the catalytic unit of adenylate cyclase by a pertussis toxin‐like effect to inactivate Ni. The dual effects of RCAP upon ADP‐ribosylation of Niand Nsα subunits suggest that a binding site for RCAP may exist at a site