Microbial rhodopsins are light-driven membrane proteins that convert photon energy into biological functions via photocycles initiated by retinal chromophore isomerization. Although their structural and functional properties under ambient conditions have been extensively characterized, the structural basis of adaptation and function of microbial rhodopsins inhabiting extreme environments remains largely unexplored. This study investigates the pressure-dependent spectroscopic properties of Parvularcula oceani xenorhodopsin (PoXeR), a deep-sea inward proton-pumping microbial rhodopsin from Parvularcula oceani, and compares its behavior with that of the terrestrial microbial Gloeobacter rhodopsin (GR). Spectroscopic analyses under hydrostatic pressure reveal that PoXeR exhibits inherent robustness under pressure, whereas GR undergoes irreversible structural perturbations. Circular dichroism spectroscopy demonstrates that PoXeR undergoes pressure-induced oligomerization, which contributes to its enhanced structural stability and functional robustness. These results suggest that oligomerization-mediated structural stabilization is a key adaptive strategy that enables PoXeR to maintain its function under extreme conditions and elucidates the molecular mechanisms of environmental adaptation in microbial rhodopsins.
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