3.E.1.6.1
Green-light-absorbing H⁺ pumping proteorhopdopsin of 249 aas and 8 TMSs. It exhibits variable vectorality: H⁺ is pumped out at basic pH but not at acidic pH; see Friedrich et al., 2002). It presents a fast proton release and an alkaliphilic photocycle, consistent with its marine origin and the near-surface environment where this bacterium was collected. This proteorhodopsin has been used to measure membrane potentials and electrical spiking in E. coli (Kralj et al., 2011; Ward et al., 2011).  3-d structures of three proteorhodopsins show that they can exist as pentamers or hexamers, depending on the protein (Ran et al. 2013).  Protonation states of several carboxylic acids, the boundaries and distortions of transmembrane α-helices, and secondary structural elements in the loops have been identified (Shi et al. 2009). Proteorhodopsin molecules incorporated into mesostructured silica films exhibit native-like function, as well as enhanced thermal stability compared to surfactant or lipid environments (Jahnke et al. 2018).