3.E.1.2.1
Halorhodopsin Cl^- uptake pump; homologous to bacteriorhodopsin (3.E.1.1.1) which can be converted from a proton pump with outwardly directed polarity into a chloride pump with inwardly directed polarity via a single amino acid substitution at position 85.  Cl^- transport does not depend on water hydrogen bonded to the chromophore as in the case of bacteriorhodopsin (Muroda et al. 2012).  However, inter-helical hydrogen bonds, mediated by a key arginine residue, largely govern the dynamics of the protein and water groups coordinating the chloride ion (Jardón-Valadez et al. 2014).  Helices E and F probably move considerable during chloride binding and ion transport (Schreiner et al. 2016).