Is M1-L121E a good mimic on microbial rhodopsin? A viewpoint from excited-state dynamics

Microbial rhodopsin, an important photoreceptor protein, has been widely used in several fields, such as optogenetics, biotechnology, and biodevices etc. However, current microbial rhodopsins are all transmembrane proteins, which both complicates the investigation on the photoreaction mechanism and limits their further applications. Therefore, a suitable mimic for microbial rhodopsin can not only provide a better model for understanding the mechanism, but also can extend the applications. The human protein CRABPII turns out to be a good template for design mimics on rhodopsin, due to the convenience in synthesis and the stability after mutations. Recently, Geiger et al. designed a new CRABPII-based mimic M1-L121E on microbial rhodopsin with the correct 13-cis (13C) isomerization after irritation. However, it still remains a question how similar it is compared with the natural microbial rhodopsin, in particular in the aspect of the photoreaction dynamics. In this article, we investigated the excited-state dynamics of this mimic by measuring its transient absorption spectra. Our results reveal that there are two components in the solution of mimic M1-L121E at PH=8, known as protonated Schiff base (PSB) and unprotonated Schiff base (USB) states. In both states, the photoreaction process from 13-cis (13C) to all-trans (AT) is faster than that from the inverse direction. In addition, the photoreaction process in PSB state is faster than that in the USB state. In the end, we compared the isomerization time of the PSB state with the properties of the microbial rhodopsin, and confirmed that the mimic M1-L121E indeed captures the main feature of the rhodopsin and is a good model of microbial rhodopsin in the photoreaction dynamics. However, our results also reveal significant differences in the excited-state dynamics of the mimic relative to the natural microbial rhodopsin, including the slower PSB isomerization rates in both 13C-AT and AT-13C directions, as well as the unusual USB photoreaction dynamics at PH=8. Such unique properties have not been observed in the natural rhodopsin, which could further deepen the understanding in photoreaction mechanism of the photosensitive proteins.