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

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 plasm a membrane. The signal of 1-MA stimulates the heterotrimeric. G protein, re sulting in dissociation of the beta gamma subunit of G protein (G beta gamm a) from a pertussis toxin-sensitive G(i)-type alpha subunit. To investigate the targets for G beta gamma, we analyzed 1-MA- or G beta gamma-dependent phosphorylation using in vivo and in vitro systems. A 62-kDa protein was ph osphorylated immediately after 1-MA treatment in intact oocytes. In the cel l-free preparations, the 62-kDa protein was also phosphorylated on serine r esidue(s) immediately after addition of 1-MA or G beta gamma. The G beta ga mma-dependent phosphorylation of the 62-kDa protein was inhibited by wortma nnin or LY294002, which are mechanistically different inhibitors of phospha tidylinositol 3-kinase (PI3K). LY294002 also inhibited G beta gamma- as wel l as 1-MA-induced maturation of oocytes. Taken together, these results indi cate that the 62-kDa protein functions downstream of G beta gamma 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 activatio n of maturation-promoting factor. (C) 1999 Academic Press.