Excitation-wavelength- and protonation-dependent multiple photoreaction pathways in TAT rhodopsin from femtoseconds to seconds

TAT rhodopsin is a member of the microbial rhodopsin family, a large group of seven-transmembrane photoreceptive proteins that bind an all-trans-retinal chromophore. While the Schiff-base linkage between the retinal and the protein moiety is protonated in most microbial rhodopsins, it is partially deprotonated in TAT rhodopsin, leading to the co-existence of a visible-absorbing protonated state and a UV-absorbing deprotonated state under physiological conditions. Here, we investigated the photoreaction dynamics of TAT rhodopsin in both the visible-absorbing protonated and UV-absorbing deprotonated states. Photoexcitation of the visible-absorbing protonated state led to relaxation from the first-excited state (S1), accompanied by all-trans-to-13-cis retinal isomerization. However, its quantum efficiency is significantly lower than that of typical microbial rhodopsins. In contrast, excitation of the UV-absorbing deprotonated state generated an excited state, with an absorption spectrum markedly different from that of the S1 state of the protonated form. This state was assigned to a higher excited state and relaxed to the optically forbidden S1 state within 100 fs. The S1 state subsequently produced long-lived ground-state photointermediates, recovering the initial state over a timescale ranging from seconds to tens of seconds. Upon 355 and 400 nm excitation, the relaxation dynamics of the deprotonated form differed, indicating the presence of multiple species in the ground state, as further supported by fluorescence spectroscopy. These findings suggest that the photoreaction mechanism of TAT rhodopsin is distinct from, and more complicated than, that of other microbial rhodopsins and that its unique photoreaction dynamics would be essential for its function as a UV sensor.