Resolving a controversy on the vectoriality of proton transfers in Anabaena Sensory Rhodopsin
Anabaena' Sensory Rhodopsin (ASR) has an extensive hydrophilic cytoplasmic network and interacts with a soluble cytoplasmic transducer, functioning as a photochromic receptor for the chromatic adaptation. Two separate earlier studies have produced contradicting results on the signal relay of this unique protein, namely, the vectoriality of proton transfers. Spectroscopic measurements of truncated ASR in membranes and liposomes indicated that protons shuttle in the cytoplasmic side, whereas photoelectric signals of intact ' E. coli' cells suggested that protons move in the extracellular direction in full-length ASR. By studying full-length wild-type and mutant ASR using time-resolved spectroscopy in the visible and infrared ranges, we clarified that in full-length ASR, protons move in the cytoplasmic direction as well, and that Asp217 serves as a proton acceptor in full-length ASR. Mutation of the Schiff base counterion Asp75 to Glu lowers the pKa of the Schiff base, creating a strong artificial acceptor that rapidly draws most of the protons in the extracellular direction. We hypothesize that the controversy on the proton transfer vectoriality may have stemmed from the low membrane potential and/or lower internal pH retained in intact 'E. coli' cells expressing ASR.