Natochin M, Granovsky A E, Muradov K G, Artemyev N O
Department of Physiology and Biophysics, University of Iowa College of Medicine, Iowa City, Iowa 52242, USA.
J Biol Chem. 1999 Mar 19;274(12):7865-9. doi: 10.1074/jbc.274.12.7865.
The visual GTP-binding protein, transducin, couples light-activated rhodopsin (R*) with the effector enzyme, cGMP phosphodiesterase in vertebrate photoreceptor cells. The region corresponding to the alpha4-helix and alpha4-beta6 loop of the transducin alpha-subunit (Gtalpha) has been implicated in interactions with the receptor and the effector. Ala-scanning mutagenesis of the alpha4-beta6 region has been carried out to elucidate residues critical for the functions of transducin. The mutational analysis supports the role of the alpha4-beta6 loop in the R*-Gtalpha interface and suggests that the Gtalpha residues Arg310 and Asp311 are involved in the interaction with R*. These residues are likely to contribute to the specificity of the R* recognition. Contrary to the evidence previously obtained with synthetic peptides of Gtalpha, our data indicate that none of the alpha4-beta6 residues directly or significantly participate in the interaction with and activation of phosphodiesterase. However, Ile299, Phe303, and Leu306 form a network of interactions with the alpha3-helix of Gtalpha, which is critical for the ability of Gtalpha to undergo an activational conformational change. Thereby, Ile299, Phe303, and Leu306 play only an indirect role in the effector function of Gtalpha.
视觉GTP结合蛋白转导素，在脊椎动物光感受器细胞中将光激活的视紫红质（R*）与效应酶cGMP磷酸二酯酶偶联起来。转导素α亚基（Gtα）中与α4螺旋和α4-β6环相对应的区域，被认为参与了与受体和效应器的相互作用。已对α4-β6区域进行丙氨酸扫描诱变，以阐明对转导素功能至关重要的残基。突变分析支持α4-β6环在R*-Gtα界面中的作用，并表明Gtα残基Arg310和Asp311参与了与R的相互作用。这些残基可能有助于R识别的特异性。与先前用Gtα合成肽获得的证据相反，我们的数据表明，α4-β6残基中没有一个直接或显著参与与磷酸二酯酶的相互作用和激活。然而，Ile299、Phe303和Leu306与Gtα的α3螺旋形成相互作用网络，这对Gtα进行激活构象变化的能力至关重要。因此，Ile299、Phe303和Leu306在Gtα的效应器功能中仅起间接作用。
