The Retinal Specific Protein RGS-r Competes with the γ Subunit of cGMP Phosphodiesterase for the α Subunit of Transducin and Facilitates Signal Termination

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