OPSINS

Halorhodopsin from the archaeon Halobacterium salinarum (shR) belongs to the seven transmembrane helical retinal protein that pumps chloride ions inside the cell upon yellow-light absorption. As a result the cells can sustain a concentration gradient of anions against strong membrane potential (90−150 mV). Although shR has been investigated by mutational studies and its structure was resolved by high resolution X-ray crystallography (1.9 Angstrom), the structure−function relationships associated with spectral tuning and the ion translocation path remain to be understood at molecular level. Understanding the chloride pumping mechanism triggered by photo-absorption is essential for a variety of technological developments ranging from bio-inspired materials for solar energy conversion to studies of optogenetics (i.e., the use of light to control the activity of genetically modified neurons that express halorhodopsin and can be silenced with yellow light). Our work on the spectral tuning in shR as influenced by the chloride depletion, other anions substitutions and mutation of key amino acid residues along the ion translocation channel reveals several key functional aspects of the anion pump photoreceptor.