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J. Biol. Chem., Vol. 279, Issue 46, 48112-48119, November 12, 2004
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¶
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
In the phototransduction pathway of rhodopsin, the metarhodopsin (Meta) III retinal storage form arises from the active G-protein binding Meta II by a slow spontaneous reaction through the Meta I precursor or by light absorption and photoisomerization, respectively. Meta III is a side product of the Meta II decay path and holds its retinal in the original binding site, with the Schiff base bond to the apoprotein reprotonated as in the dark ground state. It thus keeps the retinal away from the regeneration pathway in which the photolyzed all-trans-retinal is released. This study was motivated by our recent observation that Meta III remains stable for hours in membranes devoid of regulatory proteins, whereas it decays much more rapidly in situ. We have now explored the possibility of regulated formation and decay of Meta III, using intrinsic opsin tryptophan fluorescence and UV-visible and Fourier transform infrared spectroscopy. We find that a rapid return of Meta III into the regeneration pathway is triggered by the G-protein transducin (Gt). Depletion of the retinal storage is initiated by a novel direct bimolecular interaction of Gt with Meta III, which was previously considered inactive. Gt thereby induces the transition of Meta III into Meta II, so that the retinylidene bond to the apoprotein can be hydrolyzed, and the retinal can participate again in the normal retinoid cycle. Beyond the potential significance for retinoid metabolism, this may provide the first example of a G-protein-catalyzed conversion of a receptor.
Received for publication, June 18, 2004 , and in revised form, August 20, 2004.
* This work was supported by Grant SFB 366 from the Deutsche Forschungsgemeinschaft (to M. H. and K. P. H.) and a grant from the Fonds der Chemischen Industrie (to K. P. H.).
¶ To whom correspondence should be addressed. Tel.: 49-30-450-524111; Fax: 49-30-450-524952; E-mail: martin.heck{at}charite.de.
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