Gain, speed and sensitivity of GTP binding vs PDE activation in visual excitation

Abstract

About 2000 PDE molecules are gradually activated by one bleached rhodopsin molecule, R * , on a toad disk membrane to yield a final enzymic velocity of about 2.5 × 10 6 cGMP hydrolyzed sec −1 bleached rhodopsin −1. This amplified effect of a single photon requires GTP, whose function we originally proposed ( Yee and Liebman, 1978; Liebman and Pugh, 1979) to serve as a “memory” label attached to each PDE as it is contacted via lateral diffusion by R * . Thus, the binding of GTP was explicitly seen as an identically-amplified casual link in the amplified PDE activation. We have subjected our GTP-PDE coupling hypothesis to both stoichiometric and kinetic tests using radioactive GTP labelling techniques. We find agreement in principle with our original hypothesis with modifications to allow for (1) GTP binding to a separate G-protein (Γ) which activates PDE; (2) evidence that there are fewer PDE's activated than GTP's bound in response to a light flash; (3) evidence of reversible binding of Γ to PDE with incomplete activation of the latter; (4) multisecond delay of GTP binding compatible with lateral diffusionally-mediated activation of thousands of Γ's by single R * 's; (5) gain regulation by ATP that reduces both PDE activation and GTP binding.