Influence of the charge at D85 on the initial steps in the photocycle of bacteriorhodopsin

Calcium-Sensitive Microbial Rhodopsin VirChR1: A Femtosecond to Second Photocycle Study

Viral rhodopsins are light-gated cation channels representing a novel class of microbial rhodopsins. For viral rhodopsin 1 subfamily members VirChR1 and OLPVR1, channel activity is abolished above a certain calcium concentration. Here we present a calcium-dependent spectroscopic analysis of VirChR1 on the femtosecond to second time scale. Unlike channelrhodopsin-2, VirChR1 possesses two intermediate states P1 and P2 on the ultrafast time scale, similar to J and K in ion-pumping rhodopsins. Subsequently, we observe multifaceted photocycle kinetics with up to seven intermediate states. Calcium predominantly affects the last photocycle steps, including the appearance of additional intermediates P6Ca and P7 representing the blocked channel. Furthermore, the photocycle of the counterion variant D80N is drastically altered, yielding intermediates with different spectra and kinetics compared to those of the wt. These findings demonstrate the central role of the counterion within the defined reaction sequence of microbial rhodopsins that ultimately defines the protein function.

Two Consecutive Polar Amino Acids at the End of Helix E are Important for Fast Turnover of the Archaerhodopsin Photocycle

Archaerhodopsins (ARs) is one of the members of microbial rhodopsins. Threonine 164 (T164) and serine 165 (S165) residues of the AR from Halorubrum sp. ejinoor (HeAR) are fully conserved in ARs, although they are far from the proton transfer channel and the retinal Schiff base, and are likely involved in a hydrogen-bonding network at the end of the Helix E where most microbial rhodopsins assume a "bent structure". In the present work, T164 and/or S165 were replaced with an alanine (A), and the photocycles of the mutants were analyzed with flash photolysis. The amino acid replacements caused profound changes to the photocycle of HeAR including prolonged photocycle, accelerated decay of M intermediate and appearance of additional two intermediates which were evident in T164A- and T164A/S165A-HeAR photocyles. These results suggest that although T164 and S165 are located at the far end of the photoactive center, these two amino acid residues are important for maintaining the fast turnover of the HeAR photocycle. The underlying molecular mechanisms are discussed in relation to hydrogen-bonding networks involving these two amino acids. Present study may arouse our interests to explore the functional role of the well-conserved "bent structure" in different types of microbial rhodopsin.