Identification and Isolation of GTP-Binding Regulatory Protein from Starfish (Asterias amurensis) Oocyte Plasma Membranes

A method for isolating a GTP-binding regulatory protein from starfish oocytes is described. The protein consists of three subunits with molecular weights of 40, 37, and about 8 kDa. It is shown that the 40-kDa subunit has a high GTPase activity and is susceptible to ADP-ribosylation by pertussis toxin. The latter property of this subunit proved to decrease upon its incubation with nonhydrolyzable GTP analogues. These data provide evidence that the plasma membrane of starfish oocytes contains a 40-kDa GTP-binding protein with properties characteristic of the alpha subunit of the inhibitory G _i protein. The role of this protein in the transmembrane signal transmission from the 1-methyladenine receptor to intracellular effectors is discussed.