Roles of the Transducin a-Subunita4-Helix/ a4- b6Loopinthe Receptor and Effector Interactions

Abstract

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 a4-helix and a4-b6 loopofthetransducina-subunit(Gta)hasbeen implicated in interactions with the receptor and the effector. Ala-scanning mutagenesis of the a4-b6 region has been carried out to elucidate residues critical for the functions of transducin. The mutational analysis supports the role of the a4-b6 loopinthe R-Gt ainterface and suggests that the Gta residues Arg and Asp 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 Gta,our data indicatet hat none of the a4-b6 residues directly or significantly participate in the interaction with and activation of phosphodiesterase. However, Ile, Phe, and Leu form a network of interactions with the a3-helixofG ta, which is critical for the ability of Gt atoundergoanactivational conformational change. Thereby, Ile , Phe, and Leu play only an indirect role in the effector function of Gta.