Irreversible trimer to monomer transition of thermophilic rhodopsin upon thermal stimulation

Assembly is one of the keys to understand biological molecules, and it takes place in spatial and temporal domains upon stimulation. Microbial rhodopsin (also called retinal protein) is a membrane-embedded protein that has a retinal chromophore within seven-transmembrane α-helices and shows homo-, di-, tri-, penta-, and hexameric assemblies. Those assemblies are closely related to critical physiological properties such as stabilizing the protein structure and regulating their photoreaction dynamics. Here we investigated the assembly and disassembly of thermophilic rhodopsin (TR), which is a novel proton-pumping rhodopsin derived from a thermophile living at 75 °C. TR was characterized using size-exclusion chromatography and circular dichroism spectroscopy, and formed a trimer at 25 °C, but irreversibly dissociated into monomers upon thermal stimulation. The transition temperature was estimated to be 68 °C. The irreversible nature made it possible to investigate the photochemical properties of both the trimer and the monomer independently. Compared with the trimer, the absorption maximum of the monomer is blue-shifted by 6 nm without any changes in the retinal composition, pKa value for the counterion or the sequence of the proton movement. The photocycling rate of the monomeric TR was similar to that of the trimeric TR. A similar trimer-monomer transition upon thermal stimulation was observed for another eubacterial rhodopsin GR but not for the archaeal rhodopsins AR3 and HwBR, suggesting that the transition is conserved in bacterial rhodopsins. Thus, the thermal stimulation of TR induces the irreversible disassembly of the trimer.