Halorhodopsin

Halorhodopsin (NpHR) is a light-driven ion pump, specific for chloride ions, and found in phylogenetically ancient archaea, known as halobacteria. It is a seven-transmembrane protein of the retinylidene protein family, homologous to the light-driven proton pump bacteriorhodopsin, and similar in tertiary structure (but not primary sequence structure) to vertebrate rhodopsins, the pigments that sense light in the retina. Halorhodopsin also shares sequence similarity to channelrhodopsin, a light-driven ion channel. Halorhodopsin contains the essential light-isomerizable vitamin A derivative all-trans-retinal. Due to the intense attention on solving the structure and function of this molecule, halorhodopsin is one of the few membrane proteins whose crystal structure is known.

Halorhodopsin uses the energy of green/yellow light to move chloride ions into the cell, overcoming the membrane potential. Beside chlorides it transports other halides and nitrates into the cell. Potassium chloride uptake by cells helps to maintain osmotic balance during cell growth. By performing the same task, light-driven anion pumps can considerably reduce the use of metabolic energy. Halorhodopsin has been the subject of much study and its structure is accurately known. Its properties are similar to those of bacteriorhodopsin, and these two light-driven ion pumps transport cations and anions in opposite directions.

Halorhodopsin isoforms can be found in multiple species of halobacteria, including H. salinarum, and N. pharaonis. Much ongoing research is exploring these differences, and using them to parse apart the photocycle and pump properties. After bacteriorhodopsin, halorhodopsin may be the best type I (microbial) opsin studied. Peak absorbance of the halorhodopsin retinal complex is about 570 nm.

Just as the blue-light activated ion channel channelrhodopsin-2 opens up the ability to activate excitable cells (such as neurons, muscle cells, pancreatic cells, and immune cells) with brief pulses of blue light, halorhodopsin opens up the ability to silence excitable cells with brief pulses of yellow light. Thus halorhodopsin and channelrhodopsin together enable multiple-color optical activation, silencing, and desynchronization of neural activity, creating a powerful neuroengineering toolbox.