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

Proc Natl Acad Sci U S A. 2009 Jun 30;106(26):10660-5. doi: 10.1073/pnas.0900072106. Epub 2009 Jun 17.


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 (Galphabetagamma). Kinetic analysis of transducin (G(t)alphabetagamma) activation shows that an intermediary R*xG(t)alphabetagamma.GDP complex is formed that precedes GDP release and formation of the nucleotide-free R*xG 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 (GalphaCT) obtained from crystal structures of the GPCR opsin. Molecular modeling allows reconstruction of the fully elongated C-terminal alpha-helix of G(t)alpha (alpha5) and shows how alpha5 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 GalphaCT peptide in the crystal structure and reproduces the hydrogen-bonding networks between the C-terminal reverse turn of GalphaCT 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 alpha5 relative to the S-interaction and shows different alpha5 contacts with the NPxxY(x)(5,6)F region and the second cytoplasmic loop of R*. From the 2 alpha5 interactions, we derive a "helix switch" mechanism for the transition of R*xG(t)alphabetagamma.GDP to the nucleotide-free R*xG protein complex that illustrates how alpha5 might act as a transmission rod to propagate the conformational change from the receptor-G protein interface to the nucleotide binding site.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Algorithms
  • Binding Sites
  • Guanosine Diphosphate / chemistry
  • Guanosine Diphosphate / metabolism
  • Guanosine Triphosphate / chemistry
  • Guanosine Triphosphate / metabolism
  • Kinetics
  • Models, Chemical*
  • Models, Molecular*
  • Protein Binding
  • Protein Conformation
  • Protein Structure, Secondary
  • Protein Structure, Tertiary
  • Rhodopsin / chemistry*
  • Rhodopsin / metabolism
  • Transducin / chemistry*
  • Transducin / metabolism


  • Guanosine Diphosphate
  • Guanosine Triphosphate
  • Rhodopsin
  • Transducin