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J. Biol. Chem., Vol. 279, Issue 46, 48102-48111, November 12, 2004

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Transition of Rhodopsin into the Active Metarhodopsin II State Opens a New Light-induced Pathway Linked to Schiff Base Isomerization^*

Eglof Ritter, Kerstin Zimmermann, Martin Heck, Klaus Peter Hofmann, and Franz J. Bartl¶

From the Institut für Medizinische Physik und Biophysik, Charité-Universitätsmedizin Berlin, Campus Charité Mitte, Schumannstrasse 20-21, D-10098 Berlin, Germany and Zentrum für Biophysik und Bioinformatik, Humboldt-Universität zu Berlin, Invalidenstrasse 42, D-10015 Berlin, Germany

Rhodopsin bears 11-cis-retinal covalently bound by a protonated Schiff base linkage. 11-cis/all-trans isomerization, induced by absorption of green light, leads to active metarhodopsin II, in which the Schiff base is intact but deprotonated. The subsequent metabolic retinoid cycle starts with Schiff base hydrolysis and release of photolyzed all-trans-retinal from the active site and ends with the uptake of fresh 11-cis-retinal. To probe chromophore-protein interaction in the active state, we have studied the effects of blue light absorption on metarhodopsin II using infrared and time-resolved UV-visible spectroscopy. A light-induced shortcut of the retinoid cycle, as it occurs in other retinal proteins, is not observed. The predominantly formed illumination product contains all-trans-retinal, although the spectra reflect Schiff base reprotonation and protein deactivation. By its kinetics of formation and decay, its low temperature photointermediates, and its interaction with transducin, this illumination product is identified as metarhodopsin III. This species is known to bind all-trans-retinal via a reprotonated Schiff base and forms normally in parallel to retinal release. We find that its generation by light absorption is only achieved when starting from active metarhodopsin II and is not found with any of its precursors, including metarhodopsin I. Based on the finding of others that metarhodopsin III binds retinal in all-trans-C₁₅-syn configuration, we can now conclude that light-induced formation of metarhodopsin III operates by Schiff base isomerization ("second switch"). Our reaction model assumes steric hindrance of the retinal polyene chain in the active conformation, thus preventing central double bond isomerization.

Received for publication, June 18, 2004 , and in revised form, August 20, 2004.

^* This research was supported by the Deutsche Forschungsgemeinschaft/Sonderforschungsbereich 498 and by the Fonds der chemischen Industrie. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

^¶ To whom correspondence should be addressed. Tel.: 49-30-450-524111; Fax: 49-30-450-524952; E-mail: franz.bartl{at}charite.de.

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