Proteorhodopsin.

Proteorhodopsins are the most abundant retinal based photoreceptors and their phototrophic function might be relevant in marine ecosystems. Here, we describe their remarkable molecular properties with a special focus on the green absorbing variant. Its distinct features include a high pKa value of the primary proton acceptor stabilized through an interaction with a conserved histidine, a long-range interaction between the cytoplasmic EF loop and the chromophore entailing a particular mode of color tuning and a variable proton pumping vectoriality with complex voltage-dependence. The proteorhodopsin family represents a profound example for structure-function relationships. Especially the development of a biophysical understanding of green proteorhodopsin is an excellent example for the unique opportunities offered by a combined approach of advanced spectroscopic and electrophysiological methods. This article is part of a Special Issue entitled: Retinal Proteins-You can teach an old dog new tricks.