Cholera toxin induces cAMP-independent degradation of Gs

Abstract

Cholera toxin stimulates adenylyl cyclase by catalyzing ADP-ribosylation of the alpha chain (alpha s) of Gs, a guanine nucleotide binding regulatory protein. In a rat pituitary cell line, GH3, the toxin-induced increase in GTP-dependent adenylyl cyclase activity is maximal at 1 h; adenylyl cyclase remains elevated for at least 32 h. Surprisingly, cholera toxin also induces a 74-95% decrease in the amount of immunoreactive alpha s in the same cells, as assessed on immunoblots probed with either of two antisera directed against separate alpha s peptide sequences. The decrease in immunoreactive alpha s, which begins after 1 h of toxin treatment and is complete by 8 h, is accompanied by a comparable decrease in the amount of biochemically active alpha s, as assessed by its ability to complement the biochemical defect of alpha s-deficient S49 cyc- membranes. Cholera toxin induces similar decreases in alpha s in wild type S49 lymphoma cells, in S49 kin- mutants, which lack cAMP-dependent protein kinase, and in S49 H21 a mutants, in which alpha s is unable to assume an active conformation upon binding GTP. The toxin-induced decrease in alpha s is somewhat temperature-dependent, but is not blocked by agents that increase lysosomal pH or by colchicine, which promotes breakdown of microtubules. alpha s in detergent-solubilized GH3 membranes is susceptible to proteolysis by an endogenous protease; this susceptibility is markedly increased in membranes from cells previously exposed to cholera toxin for 1 h. Taken together, these results suggest that cholera toxin-induced covalent modification of alpha s marks the protein for accelerated degradation. In addition, the persistence of elevated GTP-dependent adenylyl cyclase activity despite loss of a substantial fraction of alpha s suggests that the amount of alpha s membranes is greater than the amount necessary for maximal activation of cAMP synthesis by cholera toxin.