MAP KINASE INACTIVATION IS REQUIRED ONLY FOR G2-M PHASE-TRANSITION INEARLY EMBRYOGENESIS CELL-CYCLES OF THE STARFISHES MARTHASTERIAS-GLACIALIS AND ASTROPECTEN-ARANCIACUS

Downregulation of MAP kinase is a universal consequence of fertilizati on in the animal kingdom. Here we show that oocytes of the starfishes Astropecten aranciacus and Marthasterias glacialis complete meiotic ma turation and form a pronucleus when treated with 1-methyladenine and t hen complete DNA replication and arrest at 62 if not fertilized. Relea se of G2 by fertilization or a variety of parthenogenetic treatments i s associated with inactivation of MAP kinase. Prevention of MAP kinase inactivation by microinjection of Ste11-Delta N, a constitutively act ive budding yeast MAP kinase kinase kinase, arrests fertilized eggs at G2 in either the first or the second mitotic cell cycle, in a dose-de pendent manner. G1 arrest is never observed. Conversely, inactivation of MAP kinase by microinjection of the MAP kinase-specific phosphatase Pyst-1 releases mature starfish oocytes from G2 arrest. The role of M AP kinase in arresting cell cycle at various stages in oocytes of diff erent animal species is discussed. (C) 1998 Academic Press.