Biphasic regulation of adenylate cyclase by cholera toxin in neuroblastoma × glioma hybrid cells is due to the activation and subsequent loss of the α subunit of the stimulatory GTP binding proteins (G s)

Abstract

Exposure of neuroblastoma × glioma hybrid (NG108-15) cells to low concentrations of cholera toxin produced a stimulation of both basal and forskolin-amplified adenylate cyclase activity in membranes prepared from these cells. Higher concentrations of cholera-toxin revesed this effect. Mn 2+ activation of adenylate cyclase indicated that this effect was not due to a modification of the intrinsic activity of this enzyme. Cholera toxin was demonstrated to produce a concentration and time-dependent loss of G sα from membranes of these cells. Loss of G sα from membranes of these cells was preceded by its ADP-ribosylation. The effects of cholera toxin were specific for G sα, as no alterations in levels of the pertussis toxin-sensitive G-proteins G i2, G i3 and G 0, were noted in parallel. Equally, no alteration in levels of G-protein β-subunit were produced by the cholera toxin treatment. These experiments demonstrate that cholera toxin-catalysed ADP-ribosylation does not simply maintain an activated population of G s at the plasma membrane and that alterations in levels of G s at the plasma membrane can modify adenylate cyclase activity.