STIMULATION OF XENOPUS OOCYTE MATURATION BY INHIBITION OF THE G-PROTEIN ALPHA-S SUBUNIT, A COMPONENT OF THE PLASMA-MEMBRANE AND YOLK PLATELET MEMBRANES

Oocytes of Xenopus laevis undergo maturation when injected with an aff inity-purified antibody against the COOH-terminal decapeptide of the a lpha subunit of the G-protein G(s), an antibody that inhibits G(s) act ivity. Germinal vesicle breakdown, chromosome condensation, and polar body formation occur, with a time course similar to that for oocytes t reated with progesterone. The alpha(s) antibody-injected oocytes also acquire the ability to be activated by sperm. Coinjection of the catal ytic subunit of cAMP-dependent protein kinase, or incubation with cycl oheximide, inhibits maturation in response to injection of the alpha(s ) antibody; these experiments show that the alpha(s) antibody acts at an early point in the pathway leading to oocyte maturation, before for mation of maturation promoting factor, and like progesterone, its acti on requires protein synthesis. Immunogold electron microscopy shows th at alpha(s) is present in the yolk platelet membranes as well as the p lasma membrane. These results support the hypothesis that progesterone acts by inhibiting alpha(s) and suggest that the target of progestero ne could include yolk platelet membranes as well as the plasma membran e.