Structural determinants for regulation of phosphodiesterase by a G protein at 2.0 Å

A multitude of heptahelical receptors use heterotrimeric G proteins to transduce signals to specific effector target molecules. The G protein transducin, Gt, couples photon-activated rhodopsin with the effector cyclic GMP phosophodiesterase (PDE) in the vertebrate phototransduction cascade. The interactions of the Gt α-subunit (αt) with the inhibitory PDE γ-subunit (PDEγ) are central to effector activation, and also enhance visual recovery in cooperation with the GTPase-activating protein regulator of G-protein signalling (RGS)-9 (refs 1,2,3). Here we describe the crystal structure at 2.0 Å of rod transducin Å·GDP·AlF-4 in complex with the effector molecule PDEγ and the GTPase-activating protein RGS9. In addition, we present the independently solved crystal structures of the RGS9 RGS domain both alone and in complex with αt/i1·GDP·AlF-4. These structures reveal insights into effector activation, synergistic GTPase acceleration, RGS9 specificity and RGS activity. Effector binding to a nucleotide-dependent site on αt sequesters PDEγ residues implicated in PDE inhibition, and potentiates recruitment of RGS9 for hydrolytic transition state stabilization and concomitant signal termination.