Metarhodopsin-II stabilization by crosslinked Gtα C-terminal peptides and implications for the mechanism of GPCR-G protein coupling

Metarhodopsin-II is the light-excited form of rhodopsin that triggers G protein function. Metarhodopsin-II is stabilized when the N-terminus of the carboxyl (340–350) tail peptide of the α-subunit of transducin (Gtα) is crosslinked to rhodopsin cysteine 140 or the 340–350 peptide C-terminus of Gtα is crosslinked to rhodopsin cysteine 316. When the N-terminus of the peptide is coupled to C316 the MI ↔ MII equilibrium is not affected. The evidence suggests that the N-terminus of the 340–350 region of Gtα is located near C140 when transducin stabilizes metarhodopsin-II and alternative explanations are suggested for the effectiveness of the 340–350 Gtα tail peptide when bound to C316.