Highly conserved glutamic acid in the extracellular IV-V loop in rhodopsins acts as the counterion in retinochrome, a member of the rhodopsin family.
Retinochrome is a member of the rhodopsin family having a chromophore retinal and functioning as a retinal photoisomerase in squid photoreceptor cells. Unlike vertebrate rhodopsins, but like many invertebrate rhodopsins, retinochrome does not have a glutamic acid at position 113 to serve as a counterion for the protonated retinylidene Schiff base. Here we investigated possible counterions in retinochrome by site-specific mutagenesis. Our results showed that the counterion is the glutamic acid at position 181, at which almost all the pigments in the rhodopsin family, including vertebrate and invertebrate rhodopsins, have a glutamic or aspartic acid. The remarkable exceptions are the long-wavelength visual pigments that have a histidine that, together with a nearby lysine, serves as a chloride-binding site. Replacement of Glu-181 of bovine rhodopsin with Gln caused a 10-nm red-shift of absorption maximum. Because the position at 181 is in the extracellular loop connecting the transmembrane helices VI and V, these results demonstrate the importance of this loop to function for spectral tuning in the rhodopsin family.
Terakita
A
A
Department of Biophysics, Graduate School of Science, Kyoto University and Core Research for Evolutional Science and Technology (CREST), Japan Science and Technology Corporation, Kyoto 606-8502, Japan. shichida@photo2.biophys.kyoto-u.ac.jp
Yamashita
T
T
Shichida
Y
Y
D000595:Amino Acid Sequence; D000818:Animals; D001224:Aspartic Acid / Q000235:genetics / Q000378:metabolism; D001436:Bacteriorhodopsins / Q000235:genetics / Q000378:metabolism; D001665:Binding Sites; D002417:Cattle; D002460:Cell Line; D017124:Conserved Sequence; D005110:Extracellular Space; D018698:Glutamic Acid / Q000235:genetics / Q000378:metabolism*; D006801:Humans; D008958:Models, Molecular; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D017433:Protein Structure, Secondary; D012168:Retinal Pigments / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D012243:Rhodopsin / Q000737:chemistry / Q000145:classification / Q000235:genetics / Q000378:metabolism; D013053:Spectrophotometry / Q000379:methods
D012168:Retinal Pigments; D001224:Aspartic Acid; D018698:Glutamic Acid; D001436:Bacteriorhodopsins; C023421:retinochrome; D012243:Rhodopsin