Gonad-stimulating substance (GSS) in starfish is the only known invertebrate peptide hormone responsible for final gamete maturation, rendering it functionally analogous to gonadotropins in vertebrates. In breeding season (stage V), GSS stimulates oocyte maturation to induce 1-methyladenine (1-MeAde) by ovarian follicle cells. The hormonal action of GSS is mediated through the activation of its receptor, G-proteins and adenylyl cyclase. It has been reported that GSS fails to induce 1-MeAde and cyclic AMP (cAMP) production in follicle cells of ovaries during oogenesis (stage IV). This study examined the regulatory mechanism how ovarian follicle cells acquire the potential to respond to GSS by producing 1-MeAde and cAMP. Because the failure of GSS action was due to G-proteins of follicle cells, the molecular structures of Gα. s, Gα. i, Gα. q and Gβ were identified in follicle cells of starfish Asterina pectinifera. The cDNA sequences of Gα. s, Gα. i, Gα. q and Gβ consisted of ORFs encoding 379, 354, 353 and 353 amino acids. The expression levels of Gα. s were extremely low in follicle cells at stage IV, whereas the mRNA levels increased markedly in stage V. On contrary, the mRNA levels of Gα. i were almost constant regardless of stage IV and V. These findings strongly suggest that de novo synthesis of Gα. s-proteins is contributed to the action of GSS on follicle cells to produce 1-MeAde and cAMP.
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