title: Resonance Raman Study of an Anion Channelrhodopsin: Effects of Mutations near the Retinylidene Schiff Base.
abstract: Optogenetics relies on the expression of specific microbial rhodopsins in the neuronal plasma membrane. Most notably, this includes channelrhodopsins, which when heterologously expressed in neurons function as light-gated cation channels. Recently, a new class of microbial rhodopsins, termed anion channel rhodopsins (ACRs), has been discovered. These proteins function as efficient light-activated channels strictly selective for anions. They exclude the flow of protons and other cations and cause hyperpolarization of the membrane potential in neurons by allowing the inward flow of chloride ions. In this study, confocal near-infrared resonance Raman spectroscopy (RRS) along with hydrogen/deuterium exchange, retinal analogue substitution, and site-directed mutagenesis were used to study the retinal structure as well as its interactions with the protein in the unphotolyzed state of an ACR from Guillardia theta (GtACR1). These measurements reveal that (i) the retinal chromophore exists as an all-trans configuration with a protonated Schiff base (PSB) very similar to that of bacteriorhodopsin (BR), (ii) the chromophore RRS spectrum is insensitive to changes in pH from 3 to 11, whereas above this pH the Schiff base (SB) is deprotonated, (iii) when Ser97, the homologue to Asp85 in BR, is replaced with a Glu, it remains in a neutral form (i.e., as a carboxylic acid) but is deprotonated at higher pH to form a blue-shifted species, (iv) Asp234, the homologue of the protonated retinylidene SB counterion Asp212 in BR, does not serve as the primary counteranion for the protonated SB, and (v) substitution of Glu68 with an Gln increases the pH at which SB deprotonation is observed. These results suggest that Glu68 and Asp234 located near the SB exist in a neutral state in unphotolyzed GtACR1 and indicate that other unidentified negative charges stabilize the protonated state of the GtACR1 SB.
authors.affiliation: Molecular Biophysics Laboratory, Photonics Center, and Department of Physics, Boston University , Boston, Massachusetts 02215, United States.
authors.forename: Adrian
authors.initials: A
authors.lastname: Yi
authors.affiliation: Molecular Biophysics Laboratory, Photonics Center, and Department of Physics, Boston University , Boston, Massachusetts 02215, United States.
authors.forename: Natalia
authors.initials: N
authors.lastname: Mamaeva
authors.affiliation: Center for Membrane Biology, Department of Biochemistry and Molecular Biology, The University of Texas Health Science Center at Houston, McGovern Medical School , Houston, Texas 77030, United States.
authors.forename: Hai
authors.initials: H
authors.lastname: Li
authors.affiliation: Center for Membrane Biology, Department of Biochemistry and Molecular Biology, The University of Texas Health Science Center at Houston, McGovern Medical School , Houston, Texas 77030, United States.
authors.forename: John L
authors.initials: JL
authors.lastname: Spudich
authors.affiliation: Molecular Biophysics Laboratory, Photonics Center, and Department of Physics, Boston University , Boston, Massachusetts 02215, United States.
authors.forename: Kenneth J
authors.initials: KJ
authors.lastname: Rothschild
mesh_terms: D019943:Amino Acid Substitution; D044785:Cryptophyta / Q000737:chemistry* / Q000235:genetics; D011487:Protein Conformation; D012172:Retinaldehyde / Q000737:chemistry / Q000235:genetics; D012176:Retinoids / Q000737:chemistry / Q000235:genetics; D012243:Rhodopsin / Q000737:chemistry* / Q000235:genetics; D012545:Schiff Bases / Q000737:chemistry; D013059:Spectrum Analysis, Raman
chemical_list: D012176:Retinoids; D012545:Schiff Bases; C121835:retinylidene chromophore; D012243:Rhodopsin; D012172:Retinaldehyde