Transducin interactions with rhodopsin. Evidence for positive cooperative behavior.

Transducin and rhodopsin belong to a family of guanine nucleotide-binding (G) protein-coupled receptor systems that provide signal transduction mechanisms resulting in numerous metabolic responses in a variety of cell types. A simple, direct binding assay has been developed to investigate the molecular interactions between transducin and rhodopsin. The binding curves generated by these studies are sigmoidal, indicating an allosteric response. The Scatchard plots of this data display an asymptotic, bell-shaped character representative of the positive cooperative behavior. A Hill coefficient, nH = 1.92, was determined and found to be in close agreement with previous kinetic studies of allosterism described for rhodopsin's catalytic mechanism (Wessling-Resnick, M., and Johnson G.L. (1987) J. Biol. Chem. 262, 3697-3705). The value for Kd app was determined to be 0.05 microM. Bmax values obtained from the binding studies suggest that oligomeric complexes of rhodopsin may be involved in interactions with transducin to form multiple high affinity binding sites for the G protein. The positive cooperative behavior demonstrated in this investigation can provide insight into the molecular basis for regulation of other G protein-coupled receptor systems.