Two protonation switches control rhodopsin activation in membranes

Mohana Mahalingam, Karina Martínez-Mayorga, Michael F. Brown, Reiner Vogel

Research output: Contribution to journalArticle

119 Scopus citations

Abstract

Activation of the G protein-coupled receptor (GPCR) rhodopsin is initiated by light-induced isomerization of the retinal ligand, which triggers 2 protonation switches in the conformational transition to the active receptor state Meta II. The first switch involves disruption of an interhelical salt bridge by internal proton transfer from the retinal protonated Schiff base (PSB) to its counterion, Glu-113, in the transmembrane domain. The second switch consists of uptake of a proton from the solvent by Glu-134 of the conserved E(D)RY motif at the cytoplasmic terminus of helix 3, leading to pH-dependent receptor activation. By using a combination of UV-visible and FTIR spectroscopy, we study the activation mechanism of rhodopsin in different membrane environments and show that these 2 protonation switches become partially uncoupled at physiological temperature. This partial uncoupling leads to ≈50% population of an entropy-stabilized Meta II state in which the interhelical PSB salt bridge is broken and activating helix movements have taken place but in which Glu-134 remains unprotonated. This partial activation is converted to full activation only by coupling to the pH-dependent protonation of Glu-134 from the solvent, which stabilizes the active receptor conformation by lowering its enthalpy. In a membrane environment, protonation of Glu-134 is therefore a thermodynamic rather than a structural prerequisite for activating helix movements. In light of the conservation of the E(D)RY motif in rhodopsin-like GPCRs, protonation of this carboxylate also may serve a similar function in signal transduction of other members of this receptor family.

Original languageEnglish (US)
Pages (from-to)17795-17800
Number of pages6
JournalProceedings of the National Academy of Sciences of the United States of America
Volume105
Issue number46
DOIs
StatePublished - Nov 18 2008

Keywords

  • FTIR spectroscopy
  • G protein-coupled receptor
  • Ionic lock
  • Membrane protein
  • UV-visible spectroscopy

ASJC Scopus subject areas

  • General

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