Modeling of the complex between transducin and photoactivated rhodopsin, a prototypical G-protein-coupled receptor.

Obtaining a reliable 3D model for the complex formed by photoactivated rhodopsin (R*) and its G-protein, transducin (Gtalphabetagamma), would significantly benefit the entire field of structural biology of G-protein-coupled receptors (GPCRs). In this study, we have performed extensive configurational sampling for the isolated C-terminal fragment of the alpha-subunit of transducin, Gtalpha 340-350, within cavities of photoactivated rhodopsin formed by different energetically feasible conformations of the intracellular loops. Our results suggested a new 3D model of the rhodopsin-transducin complex that fully satisfied all available experimental data on site-directed mutagenesis of rhodopsin and Gtalphabetagamma as well as data from disulfide-linking experiments. Importantly, the experimental data were not used as a priori constraints in model building. We performed a thorough comparison of existing computational models of the rhodopsin-transducin complex with each other and with current experimental data. It was found that different models suggest interactions with different molecules in the rhodopsin oligomer, that providing valuable guidance in design of specific novel experimental studies of the R*-Gtalphabetagamma complex. Finally, we demonstrated that the isolated Gtalpha 340-350 fragment does not necessarily bind rhodopsin in the same binding mode as the same segment in intact Gtalpha.