Activation of a pertussis toxin-sensitive, inhibitory G-protein is necessary for steroid-mediated oocyte maturation in spotted seatrout.
Oocyte maturation (OM) is initiated in lower vertebrates and echinoderms when maturation-inducing substances ( MIS) bind oocyte membrane receptors. This study tested the hypothesis that activation of a G(i) protein is necessary for MIS-mediated OM in spotted seatrout. Addition of MIS significantly decreased adenylyl cyclase activity in a steroid specific, pertussis toxin (PTX)-sensitive manner in oocyte membranes and microinjection of PTX into oocytes inhibited MIS-induced OM, suggesting the steroid activates a G(i) protein. MIS significantly increased [(35)S]GTPgammaS binding to ovarian membranes, confirming that MIS receptor binding activates a G-protein, and immunoprecipitation studies showed the increased [(35)S]GTPgammaS binding was associated with Galpha(i1-3) proteins. Radioligand binding studies in ovarian membranes using GTPgammaS and PTX demonstrated that the MIS binds a receptor coupled to a PTX-sensitive G-protein. This study provides the first direct evidence in a vertebrate model that MIS-induced activation of a G(i) protein is necessary for OM. These results support a mechanism of MIS action involving binding to a novel, G-protein coupled receptor and activation of an inhibitory G-protein, the most comprehensive and plausible model of MIS initiation of OM proposed to date.[1]References
- Activation of a pertussis toxin-sensitive, inhibitory G-protein is necessary for steroid-mediated oocyte maturation in spotted seatrout. Pace, M.C., Thomas, P. Dev. Biol. (2005) [Pubmed]
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