Molecular dynamics simulations of transducin: interdomain and front to back communication in activation and nucleotide exchange.

The dynamic events that underlie the nucleotide exchange process for the Galpha subunit of transducin (Galpha(t)) were studied with nanosecond time-scale molecular dynamics simulations. The modeled systems include the active and inactive forms of the wild-type Galpha(t) and three of its mutants (GDP-bound form only): F332A, A322S, and Q326A that are known to exhibit various degrees of enhancement of their basal and receptor-catalyzed rates of nucleotide exchange (150-fold, 70-fold and WT-like, respectively). The results of these computational experiments reveal a number of nucleotide-dependent structural and dynamic changes (involving the alpha(B)-alpha(C) loop, the inter-domain orientation of the helical and GTPase domains and the alpha(5) helix) that were not observed in the various crystal structures of Galpha(t). Notably, the results show the existence of a front to back communication device (involving the beta(2)-beta(3) hairpin, the alpha(1) helix and the alpha(5) helix), strategically located near all elements susceptible to be involved in receptor-mediated activation/nucleotide exchange. The wild-type simulations suggest that the dynamic interplay between the elements of this device would be critical for the activation of the Galpha(t) subunit. This inference is confirmed by the results of the computational experiments on the mutants that show that even in their GDP-bound forms, the A322S and F332A mutants acquire an "active-like" structure and dynamics phenotype. The same is not true for the Q326A mutant whose structural and dynamic properties remain similar to those of the GDP-bound WT. Taken together the results suggest a nucleotide exchange mechanism, analogous to that found in the Arf family GTPases, in which a partially activated state, achievable from a receptor-mediated action of the front to back communication device either by displacement of the C-terminal alpha(5) helix, of the N-terminal alpha(N) helix, or of the Gbetagamma subunit, could precede the dissociation of GDP from the native Galpha subunit.