THE RETINAL SPECIFIC PROTEIN RGS-R COMPETES WITH THE GAMMA-SUBUNIT OFCGMP PHOSPHODIESTERASE FOR THE ALPHA-SUBUNIT OF TRANSDUCIN AND FACILITATES SIGNAL TERMINATION

In vertebrate photoreceptor cells, transducin mediates signaling betwe en rhodopsin and cGMP phosphodiesterase by transiently binding its gam ma subunit (PDE gamma). For the termination of signaling GTP hydrolysi s by the transducin a subunit (TD alpha) GTPase is required. This reac tion can be accelerated by GTPase-activating proteins (GAPs), e.g. PDE gamma. Recently we identified a second retinal GAP that interacts wit h TD alpha, RGS-r. Here we compare the GAP function of RGS-r and PDE g amma. Both proteins stimulated single turnover GTPase of TD alpha; how ever, RGS-r was more effective than PDE gamma. When added together, PD E gamma competitively inhibited the RGS-r-stimulated GTPase. In additi on, the interaction of TD alpha in its GTP-bound form (TD alpha(GTP ga mma S)), the transition state (TD alpha(GDPAMF)) and the GDP-bound fo rm (TD alpha(GDP)) with RGS-r and PDE, respectively, was measured by s urface plasmon resonance, PDE gamma displayed highest affinity for TD alpha(GTP gamma S), weaker affinity for TD alpha(GDPAMF), and weakest affinity for TD alpha(GDP). RGS-r exhibited only a comparable high af finity for TD alpha(GDPAMF). We conclude that the observed competitio n between RGS-r and PDE gamma for TD alpha occurs when the hydrolysis of GTP is initiated. By competing with PDE gamma and removing it from TD alpha as well as increasing P-i release, RGS-r apparently facilitat es signal termination and TD alpha recycling.