Studies concerning a GTP regulatory subunit of rat luteal adenylate cyclase.

The ADP-ribosylation of rat luteal membrane proteins has been investigated. In the presence of cholera toxin two membrane proteins, Mr 115,000 and 46,000, incorporated [32P]ADP-ribose from [32P]NAD+. The larger protein also incorporated [32P]ADP-ribose in the absence of cholera toxin. The smaller protein was identified as the guanine nucleotide regulatory protein of adenylate cyclase. To facilitate studies concerning this species, a simple and convenient method of measuring ADP-ribose incorporation into the protein was developed. Levels of the protein were found to be approximately equal to those of the hCG receptor and 7- to 10-fold higher than those of the beta-adrenergic receptor. Luteinization of rat ovaries by injection of hCG indicated that G/F concentrations increased approximately 2-fold over a 5- to 11-day period, and correlated significantly with increased beta-adrenergic receptors, and beta-adrenergic and NaF-stimulated adenylate cyclase, but not with hCG receptors or hCG-stimulated adenylate cyclase. The distribution of the G/F protein in purified luteal membrane preparations mimicked the distribution of adenylate cyclase activity. No evidence could be found for hormonally induced alterations in ADP-ribose incorporation into this protein under either ribosylation or adenylate cyclase conditions. The exact role of the protein in the activation of rat luteal adenylate cyclase has yet to be determined.