Structural and kinetic modeling of an activating helix switch in the rhodopsin-transducin interface

Extracellular signals prompt G protein-coupled receptors (GPCRs) to adopt an active conformation (R*) and catalyze GDP/GTP exchange in the alpha-subunit of intracellular G proteins (G alpha beta gamma). Kinetic analysis of transducin (G(t)alpha beta gamma) activation shows that an intermediary R*.G(t)alpha beta gamma.GDP complex is formed that precedes GDP release and formation of the nucleotide-free R*.G protein complex. Based on this reaction sequence, we explore the dynamic interface between the proteins during formation of these complexes. We start from the R* conformation stabilized by a G(t)alpha C-terminal peptide (G alpha CT) obtained from crystal structures of the GPCR opsin. Molecular modeling allows reconstruction of the fully elongated C-terminal alpha-helix of G(t)alpha(alpha 5) and shows how alpha 5 can be docked to the open binding site of R*. Two modes of interaction are found. One of them-termed stable or S-interaction-matches the position of the G alpha CT peptide in the crystal structure and reproduces the hydrogen-bonding networks between the C-terminal reverse turn of G alpha CT and conserved E(D) RY and NPxxY(x)(5,6)F regions of the GPCR. The alternative fit-termed intermediary or I-interaction-is distinguished by a tilt (42 degrees) and rotation (90 degrees) of alpha 5 relative to the S-interaction and shows different alpha 5 contacts with the NPxxY(x)(5,6)F region and the second cytoplasmic loop of R*. From the 2 alpha 5 interactions, we derive a helix switch'' mechanism for the transition of R*.G(t)alpha beta gamma.GDP to the nucleotide-free R*.G protein complex that illustrates how alpha 5 might act as a transmission rod to propagate the conformational change from the receptor-G protein interface to the nucleotide binding site.