Direct observation of the pH-dependent equilibrium between metarhodopsins I and II and the pH-independent interaction of metarhodopsin II with transducin C-terminal peptide

Keita Sato, Takefumi Morizumi, Takahiro Yamashita, Yoshinori Shichida

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Bovine rhodopsin contains 11-cis-retinal as a light-absorbing chromophore that binds to a lysine residue of the apoprotein opsin via a protonated Schiff base linkage. Light isomerizes 11-cis-retinal into the all-trans form, which eventually leads to the formation of an enzymatically active state,metarhodopsin II (MII). It is widely believed that MII forms a pH-dependent equilibriumwithmetarhodopsin I (MI), but direct evidence for this equilibrium has not been reported. Here, we confirmed this equilibrium by direct observation of the mutual conversions ofMI andMII upon changing the pH of theMI/MIImixture.We also observed a reversible binding of the synthetic peptide constituting the C-terminal 11 amino acids of the transducin α-subunit to MII, which resulted in change of the amounts of MI and MII in the equilibrium. Interestingly, addition of the peptide did not induce a simple pKa shift but rather induced an increase of the MII fraction at high pH. These results indicate that in addition to theMII that is formed from MI in a pH-dependentmanner there also exists another MII, which is in equilibriumwith MI in a pH-independentmanner and can bind to the peptide. Therefore, there is no need for proton uptake by the protein moiety of opsin for the binding to the peptide.

Original languageEnglish
Pages (from-to)736-741
Number of pages6
Issue number4
Publication statusPublished - Feb 2 2010


ASJC Scopus subject areas

  • Biochemistry

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