title: Mechanism of activation of sensory rhodopsin I: evidence for a steric trigger.
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.
authors.affiliation: Department of Anatomy and Structural Biology, Albert Einstein College of Medicine, Bronx, NY 10461.
authors.forename: B
authors.initials: B
authors.lastname: Yan
authors.forename: K
authors.initials: K
authors.lastname: Nakanishi
authors.forename: J L
authors.initials: JL
authors.lastname: Spudich
mesh_terms: D001059:Apoproteins / Q000378:metabolism; D001436:Bacteriorhodopsins / Q000502:physiology*; D001665:Binding Sites; D006217:Halobacterium / Q000502:physiology*; D025602:Halorhodopsins; D010777:Photochemistry; D011487:Protein Conformation; D012172:Retinaldehyde / Q000031:analogs & derivatives / Q000502:physiology*; D025603:Sensory Rhodopsins; D015398:Signal Transduction; D013057:Spectrum Analysis; D013237:Stereoisomerism; D013329:Structure-Activity Relationship
chemical_list: D001059:Apoproteins; D025602:Halorhodopsins; C061497:SRI protein, Halobacterium; D025603:Sensory Rhodopsins; D001436:Bacteriorhodopsins; D012172:Retinaldehyde