Thermostable light-driven inward proton pump rhodopsins

Abstract

Schizorhodopsins (SzR), a sub-family of microbial rhodopsin, are light-driven inward proton pumps using a retinal chromophore. Here, we studied new SzRs, MtSzR and MsSzR, from methanogenic archaea (genus Methanoculleus) reported as mesophilic and sampled at a hot spring, respectively. While MtSzR and MsSzR exhibited light-driven inward proton transport, they were more stable against high temperature compared with SzR (SzR1) from non-high-temperature environment. The Arrhenius plots of heat-denaturation rates and amino acid sequences suggested different mechanisms of each high thermostability. To our knowledge, these are the first thermostable rhodopsins besides outward proton pumps, suggesting functional diversities of rhodopsins in high-temperature environments.