Retinal counterion switch in the photoactivation of the G protein-coupled receptor rhodopsin

Proc Natl Acad Sci U S A. 2003 Aug 5;100(16):9262-7. doi: 10.1073/pnas.1531970100. Epub 2003 Jun 30.

Abstract

The biological function of Glu-181 in the photoactivation process of rhodopsin is explored through spectroscopic studies of site-specific mutants. Preresonance Raman vibrational spectra of the unphotolyzed E181Q mutant are nearly identical to spectra of the native pigment, supporting the view that Glu-181 is uncharged (protonated) in the dark state. The pH dependence of the absorption of the metarhodopsin I (Meta I)-like photoproduct of E181Q is investigated, revealing a dramatic shift of its Schiff base pKa compared with the native pigment. This result is most consistent with the assignment of Glu-181 as the primary counterion of the retinylidene protonated Schiff base in the Meta I state, implying that there is a counterion switch from Glu-113 in the dark state to Glu-181 in Meta I. We propose a model where the counterion switch occurs by transferring a proton from Glu-181 to Glu-113 through an H-bond network formed primarily with residues on extracellular loop II (EII). The resulting reorganization of EII is then coupled to movements of helix III through a conserved disulfide bond (Cys110-Cys187); this process may be a general element of G protein-coupled receptor activation.

Publication types

  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Animals
  • Cattle
  • GTP-Binding Proteins / chemistry*
  • GTP-Binding Proteins / physiology
  • Glutamic Acid / chemistry
  • Hydrogen-Ion Concentration
  • Ions*
  • Light
  • Models, Chemical
  • Models, Molecular
  • Mutagenesis, Site-Directed
  • Protein Binding
  • Retina / metabolism*
  • Rhodopsin / analogs & derivatives*
  • Rhodopsin / chemistry*
  • Rhodopsin / physiology
  • Spectrum Analysis, Raman
  • Temperature
  • Ultraviolet Rays

Substances

  • Ions
  • Glutamic Acid
  • metarhodopsins
  • Rhodopsin
  • GTP-Binding Proteins