Rhodopsin-G-protein interactions monitored by resonance energy transfer

Resonance energy transfer measurements were implemented to monitor the specific interactions between G-protein and rhodopsin in phospholipid vesicles reconstituted with the purified proteins. Fluorescently labeled G-protein was extracted from bleached rod outer segments (ROS) reacted with several sulfhydryl reagents: N-(1-pyrenyl)maleimide (P), monobromobimane (B), 7-(diethylamino)-3-(4-maleimidylphenyl)-4-methylcoumarin (C), and N-(4-anilino-1-naphthyl)maleimide (A). Limited labeling of ROS, resulting in the modification of less than a single -SH residue per G-protein molecule and less than 0.2 residue per rhodopsin, did not impair the specific in situ interactions between rhodopsin and G-protein. This was demonstrated by preservation of their light-activated tight association and Gpp(NH)p binding and their fast dissociation with excess GTP. The distribution of fluorescent label among the three subunits of G-protein revealed a highly reactive -SH group in the gamma subunit accessible to labeling when G-protein was bound specifically to bleached rhodopsin. Recombination of purified fluorescent derivatives of G-protein with purified rhodopsin reconstituted in lipid vesicles restored the light-activated Gpp(NH)p binding to a level comparable to that measured with unlabeled G-protein. Similar observations were obtained with ROS depleted of peripheral proteins. Likewise, modification of up to two -SH groups per rhodopsin molecule with the fluorescent reagents did not affect the functional recombination of G-protein with rhodopsin in reconstituted lipid vesicles or in depleted ROS. Interactions between rhodopsin and G-protein were monitored by resonance energy transfer measurements, with the following fluorescent conjugates as donor/acceptor couples: P-rhodopsin/C-G-protein, P-rhodopsin/B-G-protein, and P-G-protein/C-rhodopsin.(ABSTRACT TRUNCATED AT 250 WORDS)