Archaerhodopsin-3 (AR3) is a light-driven proton pump from Halorubrum sodomense, but little is known about its photocycle. Recent interest has focused on AR3 because of its ability to serve both as a high-performance, genetically-targetable optical silencer of neuronal activity and as a membrane voltage sensor. We examined light-activated structural changes of the protein, retinal chromophore, and internal water molecules during the photocycle of AR3. Low-temperature and rapid-scan time-resolved FTIR-difference spectroscopy revealed that conformational changes during formation of the K, M, and N photocycle intermediates are similar, although not identical, to bacteriorhodopsin (BR). Positive/negative bands in the region above 3,600 cm( - 1), which have previously been assigned to structural changes of weakly hydrogen bonded internal water molecules, were substantially different between AR3 and BR. This included the absence of positive bands recently associated with a chain of proton transporting water molecules in the cytoplasmic channel and a weakly hydrogen bonded water (W401), which is part of a hydrogen-bonded pentagonal cluster located near the retinal Schiff base. However, many of the broad IR continuum absorption changes below 3,000 cm( - 1) assigned to networks of water molecules involved in proton transport through cytoplasmic and extracellular portions in BR were very similar in AR3. This work and subsequent studies comparing BR and AR3 structural changes will help identify conserved elements in BR-like proton pumps as well as bioengineer AR3 to optimize neural silencing and voltage sensing.
Keywords: Archaerhodopsin-3; Bacteriorhodopsin; Biomembranes; Energy transduction; FTIR difference spectroscopy; Membrane protein; Protein conformational changes; Proton pump; Water networks.