Roles of the transducin alpha-subunit alpha 4-helix/alpha 4-beta 6 loop inthe receptor and effector interactions

The visual GTP-binding protein, transducin, couples light-activated rhodops in (R*) with the effector enzyme, cGMP phosphodiesterase in vertebrate phot oreceptor cells. The region corresponding to the alpha 4-helix and alpha 4- beta 6 loop of the transducin alpha-subunit (G(t)alpha) has been implicated in interactions with the receptor and the effector. Ala-scanning mutagenes is of the alpha 4-beta 6 region has been carried out to elucidate residues critical for the functions of transducin. The mutational analysis supports the role of the alpha 4-beta 6 loop in the R*-G(t)alpha interface and sugge sts that the G(t)alpha residues Arg(310) and Asp(311) are involved in the i nteraction with R*. These residues are likely to contribute to the specific ity of the R* recognition. Contrary to the evidence previously obtained wit h synthetic peptides of G(t)alpha, our data indicate that none of the (alph a 4-beta 6 residues directly or significantly participate in the interactio n with and activation of phosphodiesterase. However, Ile(299), Phe(303), an d Leu(306) form a network of interactions with the alpha 3-helix of G(t)alp ha, which is critical for the ability of G(t)alpha to undergo an activation al conformational change. Thereby, Ile(299), Phe303 and Leu(306) play only an indirect role in the effector function of G(t)alpha.