Enhancement of phototransduction protein interactions by lipid surfaces

The G protein cascade of vision depends on two peripheral membrane proteins : the Gr protein, transducin (G(t)), and cGMP phosphodiesterase (PDE), Each has covalently attached lipids, and interacts with transduction components on the membrane surface. We have found that their surface interactions are critically dependent on the nature of the lipid. Membranes enhance their p rotein-protein interactions, especially if electrostatic attraction is intr oduced with positively charged lipids. These interactions are less enhanced on highly curved surfaces, but are most enhanced by unsaturated or bulky a cyl chains. On positively charged membranes, G(t) assembles at a high enoug h density to form two-dimensional arrays with short-range crystalline order . Cationic membranes also support extremely efficient activation of PDE by the GTP gamma S (guanosine 5'-O-(thio-triphosphate)) form of G alpha(t) (G alpha(t)-GTP gamma S), minimizing functional heterogeneity of transducin an d allowing activation with nanomolar G alpha(t)-GTP gamma S. Quantification of PDE activation and of the amount of G alpha(t)-GTP gamma S bound to PDE indicated that G(t) activates PDE maximally when bound in a 1:1 molar rati o. No cooperativity was observed, even at nanomolar concentrations. Thus, u nder these conditions, the one binding site for G alpha(t)-GTP gamma S on P DE that stimulates catalysis must be of higher affinity than one or more ad ditional sites which are silent with respect to activation of PDE.