NMR STRUCTURE OF A RECEPTOR-BOUND G-PROTEIN PEPTIDE

HETEROTRIMERIC GTP-binding proteins (G proteins) regulate cellular act ivity by coupling to hormone or sensory receptors. Stimulated receptor s catalyse the release of GDP from G protein alpha-subunits1-4 and GTP bound to the empty alpha-subunits provides signals that control effec tors such as adenylyl cyclases, phosphodiesterases, phospholipases and ion channels4. Three cytoplasmic loops of the activated receptor are thought to interact with three sites on the heterotrimeric G protein t o provide high-affinity interaction and catalyse G-protein activation5 -8. The carboxyl terminus of the alpha-subunit is particularly importa nt for interaction with the receptor9-14. Here we study the structure of part of the active interface between the photon receptor rhodopsin and the G protein transducin, or G(t), using nuclear magnetic resonanc e. An 11-amino-acid peptide from the C terminus of the alpha-subunit o f G(t) (alpha(t) (340-350)) binds to rhodopsin and mimics the G protei n in stabilizing its active form, metarhodopsin II. The peptide alpha( t) (340-350) binds to both excited and unexcited rhodopsin and conform ational differences between the two bound forms suggest a mechanism fo r activation of G proteins by agonist-stimulated receptors. Insight in to receptor-catalysed GDP release will have broad application because the GTP/GDP exchange and the intrinsic GTPase activity of GTP-binding proteins constitute a widespread regulatory mechanism15.