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  • Title: Mechanism of activation of sensory rhodopsin I: evidence for a steric trigger.
    Author: Yan B, Nakanishi K, Spudich JL.
    Journal: Proc Natl Acad Sci U S A; 1991 Nov 01; 88(21):9412-6. PubMed ID: 1946353.
    Abstract:
    Sensory rhodopsin I (SR-I) and bacteriorhodopsin (BR) from Halobacterium halobium show broad structural and spectroscopic similarities and yet perform distinct functions: photosensory reception and proton pumping, respectively. Probing the photoactive sites of SR-I and BR with 24 retinal analogs reveals differences in the protein environments near the retinal 13-methyl group and near the beta-ionone ring. 13-cis-Retinal does not form a retinylidene pigment with the SR-I apoprotein, although this isomer binds to the BR apoprotein even more rapidly than all-trans-retinal, the functional isomer of both pigments. The activation of both SR-I and BR requires all-trans/13-cis isomerization of retinal;however, a steric interaction between the retinal 13-methyl group and the protein is required for SR-I activation but not for that of BR. These results reveal a key difference between SR-I and BR that is likely to be the initial diverging point in their photoactivation pathways. We propose the 13-methyl group-protein interaction functions as a trigger for SR-I activation--i.e., converts photon absorption by the chromophore into protein conformational changes. A similar steric trigger is essential for activation of mammalian rhodopsin, indicating a common mechanism for receptor activation in archaebacterial and vertebrate retinylidene photosensors.
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