The molecular basis of GTP-binding protein interaction with receptors

The molecular basis of the interaction between the visual receptor, rhodopsin, and the rod outer segment GTP-binding protein, transducin or Gt, was studied using a synthetic-peptide-competition approach to elucidate the site(s) on the Gt alpha-subunit (alpha t) involved in high-affinity binding to light-activated rhodopsin (R*). Synthetic peptides based on the amino acid sequence of portions of the molecule that interact with rhodopsin can themselves bind the rhodopsin and thus behave as competitive inhibitors of rhodopsin-G-protein interaction. This blockade was assessed by measuring the ability of peptides to inhibit Gt stabilization of the metarhodopsin II conformation of rhodopsin. Based upon this analysis, two regions near the C-terminal of alpha 1 are important for interaction with R*.