Ser²⁶² determines the chloride-dependent colour tuning of a new halorhodopsin from Haloquadratum walsbyi

Light is an important environmental signal for all organisms on earth because it is essential for physiological signalling and the regulation of most biological systems. Halophiles found in salt-saturated ponds encode various archaeal rhodopsins and thereby harvest various wavelengths of light either for ion transportation or as sensory mediators. HR (halorhodopsin), one of the microbial rhodopsins, senses yellow light and transports chloride or other halides into the cytoplasm to maintain the osmotic balance during cell growth, and it exists almost ubiquitously in all known halobacteria. To date, only two HRs, isolated from HsHR (Halobacterium salinarum HR) and NpHR (Natronomonas pharaonis HR), have been characterized. In the present study, two new HRs, HmHR (Haloarcula marismortui HR) and HwHR (Haloquadratum walsbyi HR), were functionally overexpressed in Escherichia coli, and the maximum absorbance (λ_max) of the purified proteins, the light-driven chloride uptake and the chloride-binding affinity were measured. The results showed them to have similar properties to two HRs reported previously. However, the λ_max of HwHR is extremely consistent in a wide range of salt/chloride concentrations, which had not been observed previously. A structural-based sequence alignment identified a single serine residue at 262 in HwHR, which is typically a conserved alanine in all other known HRs. A Ser ²⁶² to alanine replacement in HwHR eliminated the chloride-independent colour tuning, whereas an Ala²⁶² to serine mutagenesis in HsHR transformed it to have chloride-independent colour tuning similar to that of HwHR. Thus Ser²⁶² is a key residue for the mechanism of chloride-dependent colour tuning in HwHR.