Development of an Outward Proton Pumping Rhodopsin with a New Record in Thermostability by Means of Amino Acid Mutations.

We have developed a methodology for identifying further thermostabilizing mutations for an intrinsically thermostable membrane protein. The methodology comprises the following steps: (1) identifying thermostabilizing single mutations (TSSMs) for residues in the transmembrane region using our physics-based method; (2) identifying TSSMs for residues in the extracellular and intracellular regions, which are in aqueous environment, using an empirical force field FoldX; and (3) combining the TSSMs identified in steps (1) and (2) to construct multiple mutations. The methodology is illustrated for thermophilic rhodopsin whose apparent midpoint temperature of thermal denaturation is ∼91.8 °C. The TSSMs previously identified in step (1) were F90K, F90R, and Y91I with Δ ∼5.6, ∼5.5, and ∼2.9 °C, respectively, and those in step (2) were V79K, T114D, A115P, and A116E with Δ ∼2.7, ∼4.2, ∼2.6, and ∼2.3 °C, respectively (Δ denotes the increase in ). In this study, we construct triple and quadruple mutants, F90K+Y91I+T114D and F90K+Y91I+V79K+T114D. The values of Δ for these multiple mutants are ∼11.4 and ∼13.5 °C, respectively. of the quadruple mutant (∼105.3 °C) establishes a new record in a class of outward proton pumping rhodopsins. It is higher than of rhodopsin (∼100.8 °C) that was the most thermostable in the class before this study.