Enhancement of Phototransduction Protein Interactions

The G protein cascade of vision depends on two peripheral membrane proteins: the G protein, transducin (Gt), 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 protein-protein interactions, especially if electrostatic attraction is introduced with positively charged lipids. These interactions are less enhanced on highly curved surfaces, but are most enhanced by unsaturated or bulky acyl chains. On positively charged membranes, Gt assembles at a high enough density to form two-dimensional arrays with short-range crystalline order. Cationic membranes also support extremely efficient activation of PDE by the GTPgS (guanosine 5*-O-(thiotriphosphate)) form of Gat (Gat-GTPgS), minimizing functional heterogeneity of transducin and allowing activation with nanomolar Gat-GTPgS. Quantification of PDE activation and of the amount of Gat-GTPgS bound to PDE indicated that Gt activates PDE maximally when bound in a 1:1 molar ratio. No cooperativity was observed, even at nanomolar concentrations. Thus, under these conditions, the one binding site for Gat-GTPgS on PDE that stimulates catalysis must be of higher affinity than one or more additional sites which are silent with respect to activation of PDE.