Activation of a pertussis toxin-sensitive, inhibitory G-protein is necessary for steroid-mediated oocyte maturation in spotted seatrout |
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Authors: | Pace Margaret C Thomas Peter |
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Affiliation: | The University of Texas at Austin Marine Science Institute, 750 Channel View Drive, Port Aransas, TX 78373, USA |
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Abstract: | 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 Gi 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 Gi protein. MIS significantly increased [35S]GTPγS binding to ovarian membranes, confirming that MIS receptor binding activates a G-protein, and immunoprecipitation studies showed the increased [35S]GTPγS binding was associated with Gαi1-3 proteins. Radioligand binding studies in ovarian membranes using GTPγS 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 Gi 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. |
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Keywords: | Oocyte maturation G-protein Germinal vesicle breakdown Membrane progestin receptor Spotted seatrout |
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