title: Molecular determinants differentiating photocurrent properties of two channelrhodopsins from chlamydomonas.
abstract: A light signal is converted into an electrical one in a single molecule named channelrhodopsin, one of the archaea-type rhodopsins in unicellular green algae. Although highly homologous, two molecules of this family, channelrhodopsin-1 (ChR1) and -2 (ChR2), are distinct in photocurrent properties such as the wavelength sensitivity, desensitization, and turning-on and -off kinetics. However, the structures regulating these properties have not been completely identified. Photocurrents were analyzed for several chimera molecules made by replacing N-terminal segments of ChR2 with the homologous counterparts of ChR1. We found that the wavelength sensitivity of the photocurrent was red-shifted with negligible desensitization and slowed turning-on and -off kinetics when replacement was made with the segment containing the fifth transmembrane helix of ChR1. Therefore, this segment is involved in the determination of photocurrent properties, the wavelength sensitivity, and the kinetics characterizing ChR1 and ChR2. Eight amino acid residues differentiating this segment were exchanged one-by-one, and the photocurrent properties of each targeted mutant ChR2 were further analyzed. Among them, position Tyr(226)(ChR1)/Asn(187)(ChR2) is one of the molecular determinants involved in the wavelength sensitivity, desensitization, and turning-on and -off kinetics. It is suggested that these amino acid residues directly or indirectly interact with the chromophore as well as with the protein structure determining the photocurrent kinetics. Some of the chimera channelrhodopsins are suggested to have several advantages over the wild-type ChR2 in the introduction of light-induced membrane depolarization for the purpose of artificial stimulation of neurons in vivo and visual prosthesis for photoreceptor degeneration.
authors.affiliation: Department of Developmental Biology and Neuroscience, Tohoku University Graduate School of Life Sciences, Sendai 980-8575, Japan.
authors.forename: Hongxia
authors.initials: H
authors.lastname: Wang
authors.forename: Yuka
authors.initials: Y
authors.lastname: Sugiyama
authors.forename: Takuya
authors.initials: T
authors.lastname: Hikima
authors.forename: Eriko
authors.initials: E
authors.lastname: Sugano
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Tomita
authors.forename: Tetsuo
authors.initials: T
authors.lastname: Takahashi
authors.forename: Toru
authors.initials: T
authors.lastname: Ishizuka
authors.forename: Hiromu
authors.initials: H
authors.lastname: Yawo
mesh_terms: D020418:Algal Proteins / Q000502:physiology*; D000595:Amino Acid Sequence; D000818:Animals; D016825:Chlamydomonas reinhardtii / Q000502:physiology* / Q000528:radiation effects*; D004594:Electrophysiology; D005453:Fluorescence; D015640:Ion Channel Gating / Q000502:physiology*; D008027:Light; D008969:Molecular Sequence Data; D025604:Rhodopsins, Microbial / Q000235:genetics / Q000378:metabolism*; D017386:Sequence Homology, Amino Acid
chemical_list: D020418:Algal Proteins; D025604:Rhodopsins, Microbial