How Does a Microbial Rodhopsin RxR Realize Its Exceptionally High Thermostability with the Proton-Pumping Function Being Retained?

We often encounter a case where two proteins, whose amino-acid sequences are similar, are quite different with regard to the thermostability. As a striking example, we consider the two seven-transmembrane proteins, recently discovered Rubrobacter xylanophilus rhodopsin (RxR) and long-known bacteriorhodopsin from Halobacterium salinarum (HsBR). They commonly function as a light-driven proton pump across the membrane. Though their sequence similarity and identity are ~71 and ~45%, respectively, RxR is much more thermostable than HsBR. In this study, we solve the 3D structure of RxR using X-ray crystallography and find that the backbone structures of RxR and HsBR are surprisingly similar to each other: The root mean square deviation for the two structures calculated using the backbone Cα atoms of the seven helices is only 0.86 A, which makes the large stability difference more puzzling. We calculate the thermostability measure and its energetic and entropic components for RxR and HsBR using our recently deve...