G Protein Subtype Specificity of Rhodopsin Intermediates Metarhodopsin Ib and Metarhodopsin II

Rhodopsin is one of the members of the G protein-coupled receptor family that can catalyze a GDP-GTP exchange reaction on the retinal G protein transducin (Gt) upon photon absorption. There are at least two intermediate states, meta-Ib and meta-II, which exhibit direct interaction with Gt. Meta-Ib binds to GDP-bound Gt, while meta-II forms a complex with Gt having no nucleotide, suggesting that meta-Ib is a state that initially interacts with Gt. Here we investigated whether or not meta-Ib exhibits specific interaction with G protein similar to meta-II, by examining the binding efficiencies of meta-Ib and meta-II to Gi alpha and its mutants whose C-terminal 11 amino acids were replaced with those of Go alpha, Gq alpha and Gs alpha. The affinity of meta-Ib to the C-terminal 11 amino acids of Gt alpha was similar to those of Gi alpha and its mutant with Go alpha's C-terminal 11 amino acids, whereas meta-II exhibited affinity to the C-terminal 11 amino acids of Gi alpha mutant with Go alpha's C-terminal 11 amino acids about half of what was seen for Gt alpha and Gi alpha. Both intermediates exhibited no affinity to the Gi alpha mutants containing the C-terminal 11 amino acids of Gq alpha and Gs alpha. These results suggested that meta-Ib is the state that exhibits specific interaction with G protein as meta-II does, although meta-Ib exhibits a slightly lenient binding selectivity compared to that of meta-II.