3.E.1.6.9
Bacteriorhodopsin (thermophilic rhodopsin; TR) of 260 aas and 7 TMSs.  53% identical to xanthorhodopsin (TC# 3.E.1.6.2).  It is a photoreceptor protein with extremely high thermal stability and a light-driven electrogenic proton pump. The x-ray crystal structure revealed the presence of a putative binding site for a carotenoid antenna and a larger number of hydrophobic residues and aromatic-aromatic interactions than in most microbial rhodopsins (Tsukamoto et al. 2016). The structural changes upon thermal stimulation involved a thermally induced structure in which an increase of hydrophobic interactions in the extracellular domain, the movement of extracellular domains, the formation of a hydrogen bond, and the tilting of transmembrane helices were observed. An extracellular LPGG motif between helices F and G may play an important role in thermal stability, acting as a "thermal sensor" (Tsukamoto et al. 2016).