The retinal specific protein RGS-r competes with the gamma subunit of cGMP phosphodiesterase for the alpha subunit of transducin and facilitates signal termination.

In vertebrate photoreceptor cells, transducin mediates signaling between rhodopsin and cGMP phosphodiesterase by transiently binding its gamma subunit (PDEgamma). For the termination of signaling GTP hydrolysis by the transducin alpha subunit (TDalpha) GTPase is required. This reaction can be accelerated by GTPase-activating proteins (GAPs), e.g. PDEgamma. Recently we identified a second retinal GAP that interacts with TDalpha, RGS-r. Here we compare the GAP function of RGS-r and PDEgamma. Both proteins stimulated single turnover GTPase of TDalpha; however, RGS-r was more effective than PDEgamma. When added together, PDEgamma competitively inhibited the RGS-r-stimulated GTPase. In addition, the interaction of TDalpha in its GTP-bound form (TDalphaGTPgammaS), the transition state (TDalphaGDPAMF) and the GDP-bound form (TDalphaGDP) with RGS-r and PDE, respectively, was measured by surface plasmon resonance. PDEgamma displayed highest affinity for TDalphaGTPgammaS, weaker affinity for TDalphaGDPAMF, and weakest affinity for TDalphaGDP. RGS-r exhibited only a comparable high affinity for TDalphaGDP*AMF. We conclude that the observed competition between RGS-r and PDEgamma for TDalpha occurs when the hydrolysis of GTP is initiated. By competing with PDEgamma and removing it from TDalpha as well as increasing Pi release, RGS-r apparently facilitates signal termination and TDalpha recycling.