Mechanism of allosteric regulation of the rod cGMP phosphodiesterase activity by the helical domain of transducin α subunit

Abstract

The G protein a subunit (Gα) is composed of two distinct folding domains: a GTP-binding Ras-like domain and an a helical domain (HD). We have recently reported that the helical domain (HD(t)) of the vertebrate visual transducin α subunit (Gα(t)) synergizes activation of retinal cyclic GMP phosphodiesterase (PDE) by activated Gα(t) (Liu, W., and Northup, J. K., (1998) Proc. Natl. Acad. Sci. U.S.A. 95, 12878-12883). Here, we examine the molecular basis for this HD-based signaling regulation, and we provide a new model for the activation of the target effector. The HD proteins derived from visual transducin or taste gustducin α subunits, but no other Gα HD proteins, each attenuate the PDE catalytic core (Pαβ) and synergize Gα(t) stimulation of the holoPDE (Pαβγ2) with similar apparent affinities. The data from studies of both HD(t)-mediated attenuation and stimulation indicate that the HD(t) and the PDE inhibitory subunit (Pγ) interact with PDE at independent sites and that Pαβ contains the binding sites for HD. The saturation of both processes by HD(t) displays positive cooperativity with Hill coefficients of 1.5 for the attenuation of Pαβ activity and 2.1 for synergism of holoPDE activation. Our data suggest the that Gα(t)-HD(t) regulates PDE by allosterically decreasing the affinity of Pαβ for Pγ and thus simultaneously facilitating the interaction of the activated Gα(t)- Ras-like domain with Pγ. Thus, we propose a new model for the high efficiency of PDE activation as well as deactivation, and, overall, a novel mechanism for controlling fidelity, sensitivity, and efficacy of G protein signaling.