Biochemical correlates of progesterone-induced plasma membrane depolarization during the first meiotic division in Rana oocytes

Abstract

Changes in protein synthesis, protein phosphorylation and lipid phosphorylation in the amphibian oocyte plasma membrane have been correlated with electrical changes following steroid induction of the completion of the first meiotic division. The oocyte first depolarizes from about -60 mV (inside negative) to about -25 mV 1 to 2 hr before breakdown of the large nucleus followed by a further depolarization beginning 3 to 6 hr after nuclear breakdown. The initial depolarization is associated with appearance of previously described cycloheximide-sensitive cytoplasmic factor(s) which induce both nuclear breakdown and plasma membrane depolarization. We found a similar ED50 (0.4 microM) for cycloheximide inhibition of nuclear breakdown, membrane depolarization, and [3H]-leucine incorporation. Emetine (1 nM to 1 mM) was inactive. The period of cycloheximide sensitivity (first 5 hr) is essentially the same for plasma membrane depolarization and nuclear breakdown. The onset of the second depolarization phase following nuclear breakdown is associated with a marked increase in the rate of [3H]-leucine and [32PO4] incorporation into membrane protein and lipid. Polyacrylamide gel electrophoresis of membrane protein and lipoprotein indicated that a major newly synthesized membrane component is proteolipid. An increase in [32PO4] incorporation into membrane phosphatidylserine and phosphatidylethanolamine (with a decrease in phosphatidylcholine [32PO4] begins during the second depolarization phase and coincides with the appearance of excitability in the oocyte plasma membrane. In toto, the bulk of the biochemical changes (proteins, phosphoproteins, proteolipids, phospholipids) appear to be associated with plasma membrane components and coincide with stepwise changes in membrane permeability to specific ions (e.g. Cl-).