. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 152 G. T. BAXTER AND D. E. MORSE. GppNHp & CT Activate GDP-p-S Inhibits IPifcl im^mm> Activates Diacylglycerol-PKC Cascade I Amplifies Larval Response to Morphogenic Inducers Figure 5. Schematic diagram of the hypothetical signal transduction pathway suggested to mediate the facilitation by lysine. Binding of lysine to an extracellular domain of the receptor protein (drawn here as a trans- membrane protein) activates dissociation of the intracellular trimeric G protein. The released fi and 7 subunits of the G protein
. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 152 G. T. BAXTER AND D. E. MORSE. GppNHp & CT Activate GDP-p-S Inhibits IPifcl im^mm> Activates Diacylglycerol-PKC Cascade I Amplifies Larval Response to Morphogenic Inducers Figure 5. Schematic diagram of the hypothetical signal transduction pathway suggested to mediate the facilitation by lysine. Binding of lysine to an extracellular domain of the receptor protein (drawn here as a trans- membrane protein) activates dissociation of the intracellular trimeric G protein. The released fi and 7 subunits of the G protein then activate the downstream cascade (apparently including a diacylglycerol-dependent activation of PKC) that results in the facilitation of larval settlement and metamorphosis. GppNHp and cholera toxin (OT) directly activate the G protein and mimic the facilitating effect of lysine. GDP-fi-S inhibits the G protein and inhibits facilitation by lysine. but does not inhibit facilitation by diacylglycerol (i;/. Baxter and Morse, 1987). of the lysine receptors on the isolated cilia (Baxter, 1991) suggests that they may be true chemosensory receptors, as the binding of amino acids to transporters generally is sodium-dependent (Boge and Rigal, 1981; c/.Jaeckle and Manahan, 1989). Because the purified cilia are predom- inantly free of other cell structures, we can exclude the binding of lysine to synaptic receptors in this preparation. Baxter (! 991) first observed that cholera toxin-catalyzed labelling of the putative G protein «-subunit in the isolated cilia is stimulated significantly by the diamino acid re- ceptor ligand lysine. This result, analogous to the direct increase in susc ; :nlity to labelling caused by GppNHp (Fig. 4), suggests it the lysine receptor is functionally coupled to a cholo xin-sensitive G protein in the pu- rified cilia. Transmembrane receptors coupled to G pro- teins display a marked decrease in their affinity (increased Kj) for ligand in the presence of G
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