Light-driven proton pump. The chromophore contains 78% all-trans- and 22% 13-cis-retinal in the dark and 90% all-trans- and 10% 13-cis-retinal upon illumination with >500 nm light. In light-adapted form, max=555 nm. In the mutated form, max=498 nm. The covalent binding of retinal to the apoprotein, bacterioopsin, generates bacteriorhodopsin. Different protein aggregation states can be observed, leading to variations in the absorbance maxima. Belongs to the archaeal/bacterial/fungal opsin family. Is called bop2 in PubMed:21135094.