Retinal counterion switch in the photoactivation of the G protein-coupled receptor rhodopsin
- Elsa C. Y. Yan*,†,
- Manija A. Kazmi†,
- Ziad Ganim*,
- Jian-Min Hou†,‡,
- Douhai Pan*,
- Belinda S. W. Chang†,§,
- Thomas P. Sakmar†, and
- Richard A. Mathies*,¶
- *Department of Chemistry, University of California, Berkeley, CA 94720; and †Howard Hughes Medical Institute, The Rockefeller University, 1230 York Avenue, New York, NY 10021
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Edited by Jeremy Nathans, Johns Hopkins University School of Medicine, Baltimore, MD, and approved May 29, 2003 (received for review April 9, 2003)
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.
Footnotes
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↵ ¶ To whom correspondence should be addressed. E-mail: rich{at}zinc.cchem.berkeley.edu.
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↵ ‡ Present address: Department of Chemistry, Gonzaga University, 502 East Boone Avenue, Spokane, WA 99258.
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↵ § Present address: Department of Zoology, University of Toronto, 25 Harbord Street, Toronto, ON, Canada M5S 3G5.
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A preliminary account of this work was first presented at the 10th International Conference on Retinal Proteins, August, 20–24, 2002, University of Washington, Seattle, WA.
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This paper was submitted directly (Track II) to the PNAS office.
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Abbreviations: SB, Schiff base; GPCR, G protein-coupled receptor; PSB, protonated SB; EII, extracellular loop II; DM, n-β-dodecyl maltoside.
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See commentary on page 9105.
- Copyright © 2003, The National Academy of Sciences
