G-protein βγ subunit-dependent phosphorylation of 62-kDa protein in the early signaling pathway of starfish oocyte maturation induced by 1- methyladenine

Abstract

In starfish oocytes, maturation is induced by a hormone, 1- methyladenine (1-MA), that binds to the receptors exposed to the outer surface of the plasma membrane. The signal of 1-MA stimulates the heterotrimeric G protein, resulting in dissociation of the βγ subunit of G protein (Gβγ) from a pertussis toxin-sensitive G(i)-type α subunit. To investigate the targets for Gβγ we analyzed 1-MA- or Gβγ-dependent phosphorylation using in vivo and in vitro systems. A 62-kDa protein was phosphorylated immediately after 1-MA treatment in intact oocytes. In the cell-free preparations, the 62-kDa protein was also phosphorylated on serine residue(s) immediately after addition of 1-MA or Gβγ. The Gβγ-dependent phosphorylation of the 62-kDa protein was inhibited by wortmannin or LY294002, which are mechanistically different inhibitors of phosphatidylinositol 3-kinase (PI3K). LY294002 also inhibited Gβγ- as well as 1-MA-induced maturation of oocytes. Taken together, these results indicate that the 62-kDa protein functions downstream of Gβγ and PI3K in the early signaling: pathway of 1-MA-induced starfish oocyte maturation. The phosphorylation of the 62-kDa protein may be required for the activation of maturation-promoting factor.