Oocyte Adenylyl Cyclase Contains Ni, Yet the Guanine Nucleotide-dependent Inhibition by Progesterone is Not Sensitive to Pertussis Toxin

Membranes were obtained from Xenopus laevis oocytes after removal of follicular cells by collagenase treatment. [32P]ADP-ribosylation with pertussis toxin showed them to contain a single Mr = 40000 substrate for this toxin that co-migrates on sodium dodecylsufate-polyacrylamide gel electrophoresis with pure human erythrocyte Ni, the inhibitory regulatory component of adenylyl cyclase. [32P]ADP-ribosylation of oocyte membranes with cholera toxin also showed presence of a single substrate but of Mr = 42000. These results indicate, that the adenylyl cyclase system of oocytes, like that of somatic cells and unlike that of spermatozoids, contains the catalytic unit C and both of the known regulatory N components. The possible susceptibility to pertussis toxin of the guanine nucleotide-dependent inhibition of oocyte adenylyl cyclase by progesterone was investigated. This action of progesterone is mediated by a membrane bound receptor as opposed to a receptor of cytosolic or nuclear localization. However, the inhibitory effect of progesterone was unaffected by pertussis toxin, even though the oocyte membrane Ni was fully ADP-ribosylated with pertussis toxin, as revealed by lack of further [32P]ADP-ribosylation on subsequent re-incubation with pertussis toxin. These results indicate that the action of progesterone, in spite of being nucleotide-dependent, is either not mediated by Ni, suggesting the existence of an additional nucleotide regulatory component, or if mediated by Ni, involves a mode of regulation of this coupling protein that is different from that by which all other inhibitory hormones act on adenylyl cyclase.