Mechanism of action of glycopeptide hormones and cholera toxin: what is the role of ADP-ribosylation?

The cultured murine Leydig tumor cell line MLTC-1 and the normal rat thyroid strain FRTL have adenylate cyclase activities that are stimulated by human chorionic gonadotropin (hCG) and thyrotropin, respectively. Both cell types also respond to choleragen. Activation of adenylate cyclase in membranes by choleragen required NAD whereas stimulation of the enzyme by hormones did not. With [alpha-32P]NAD as a donor, ADP-ribosylation of membrane proteins was determined under the same conditions used to assay adenylate cyclase activity. Under these conditions, choleragen, but not the hormones, caused the ADP-ribosylation of subunits of the regulatory component (G/F) of adenylate cyclase in both FRTL and MLTC-1 membranes. In the absence of any effectors, several membrane proteins became labeled but the hormones did not cause the specific labeling of these or any other membrane proteins. Pretreatment of intact MLTC-1 cells with hCG did not block the ability of choleragen to ADP-ribosylate G/F in isolated membranes; labeling was actually enhanced in a manner related to the length of exposure to hCG. In contrast, pretreatment of the cells with choleragen inhibited ADP-ribosylation of G/F by the toxin in isolated membranes. Extracts of membranes from untreated, hCG-treated, and choleragen-treated MLTC-1 cells were used to reconstitute adenylate cyclase activity in membranes from the cyc- variant of S49 lymphoma cells which lacks a functional G/F. Toxin but not hormone treatment caused an increase in the basal activity of adenylate cyclase in the reconstituted system. Our results indicate that ADP-ribosylation of the regulatory component of adenylate cyclase is required for choleragen action but not for hormone action.