Site of pertussis toxin-induced ADP-ribosylation on Gi is critical for receptor modulation of GDP interaction with Gi.

In this study, the influence of the inhibitory mu-opioid receptor on the potencies of 5'-guanosine alpha-thiotriphosphate (GTP gamma S) and GDP at the inhibitory GTP-binding protein (Gi) were investigated in an adenylyl cyclase system. It was hoped that a receptor-mediated change in the potency of either GTP gamma S or GDP in affecting adenylyl cyclase activity may elucidate how a receptor alters cyclase activity via its G-protein. In an adenylyl cyclase system employing 5'-adenylyl imidodiphosphate as substrate, GTP gamma S, a nonhydrolyzable analog of GTP, inhibited forskolin-stimulated adenylyl cyclase activity in the absence of morphine; morphine failed to significantly affect the apparent potency of GTP gamma S. GDP blocked the GTP gamma S-induced inhibition of adenylyl cyclase; morphine profoundly diminished the ability of GDP to block the inhibitory effect of GTP gamma S. The IC50 values of GTP gamma S were 0.02 +/- 0.01, 0.18 +/- 0.04, and 2.2 +/- 0.5 microM in the absence of other drugs, in the presence of a combination of 100 microM GDP and morphine, and in the presence of 100 microM GDP, respectively. GDP blocked the inhibitory effect of GTP gamma S (0.3 microM) in a concentration-dependent manner; the EC50 for GDP was 16 +/- 2.6 microM in the absence of morphine and 170 +/- 32 microM in the presence of morphine. Exposure of 7315c cells to pertussis toxin for 3 h resulted in a small decrease in the potency of GTP gamma S in inhibiting cyclase. However, the relative potency of GDP in blocking the GTP gamma S-mediated inhibition of cyclase was increased: the EC50 values of GDP were 11 +/- 4 and 0.81 +/- 0.2 microM in untreated and pertussis toxin-treated membranes, respectively. In untreated membranes, there was a brief lag in the GTP gamma S-induced inhibition of adenylyl cyclase; morphine diminished this lag. In membranes treated with pertussis toxin, there was an exaggerated lag in the onset of GTP gamma S inhibition of adenylyl cyclase activity; morphine could no longer affect this lag. Thus, uncoupling the mu-opioid receptor from Gi appeared to increase the affinity of Gi for GDP. These data suggest that the effect of an inhibitory receptor is to decrease the affinity of Gi for GDP by virtue of its interaction with the carboxy-terminal region of Gi alpha.(ABSTRACT TRUNCATED AT 400 WORDS)