G-protein alpha and beta–gamma subunits interact with conformationally distinct signaling states of rhodopsin

Light activated rhodopsin interacts with domains on all three subunits of transducin. Two of these domains, the C-terminal regions of the alpha and gamma subunits mimic the ability of transducin to stabilize the active conformation of rhodopsin, metarhodopsin II, but display different roles in transducin activation process. Whether the interactions are with the same or different complimentary sites on Meta II is unknown. We have used chemo-selective thioalkylation of rhodopsin and UV/visible spectroscopy to show that interactions with transducin C-terminal domains can be selectively disrupted. These data provide evidence that formal structural determinants on Meta II for these domains of transducin are different. In a set of complimentary experiments we examined the reactivity of Meta II species produced in the presence of the Gtα and Gtγ subunit peptides to hydroxylamine. Analysis of the rates of Meta II decay confirms that the conformational states of Meta II when bound to Gtα and Gtβγ represent distinct signaling states of rhodopsin.