Cyclin synthesis, modification and destruction during meiotic maturation of the starfish oocyte

Abstract

The pattern of protein synthesis in oocytes of the starfish Marthasterias glacialis changes during 1-methyladenine-induced meiotic maturation. One of the newly synthesized proteins, a major 54-kDa polypeptide, was synthesized continuously after activation but was destroyed abruptly just before appearance of the polar bodies at each meiotic division. This protein thus resembles the cyclin proteins identified in cleaving sea urchin and clam embryos. RNA extracted from oocytes before and after maturation encoded virtually identical polypeptides when translated in the reticulocyte lysate. However there was poor correspondence between the in vitro translation products and the labeling pattern of intact cells. There was no exact in vitro counterpart to the in vivo-labeled cyclin. Instead, a major polypeptide of 52 kDA was seen which appears to be a precursor of the 54-kDa form of cyclin. The 52-kDa polypeptide was identified as cyclin by hybrid arrest of translation. Cyclin mRNA is not translated to a significant extent before oocyte activation and is present in oocytes as a nonadenylated form. It becomes polyadenylated when the oocytes mature. This behavior is also seen in the case of the mRNA for the small subunit of ribonucleotide reductase, another abundant maternal mRNA whose translation is activated at maturation.