Archaerhodopsin

Like the homologous bacteriorhodopsin (bR) protein, archaerhodopsins harvest energy from sunlight to pump H+ ions out of the cell, establishing a proton motive force that is used for ATP synthesis.

[1] The term archaerhodopsin is a portmanteau of archaea (the domain in which the proteins are found) and rhodopsin (a photoreceptor responsible for vision in the mammalian eye).

[3][8][6] The crystal structures of both proteins were solved by Kunio Ihara, Tsutomo Kouyama and co-workers at Nagoya University, together with collaborators at the Spring-8 synchrotron.

[9] The crystal structure of AR3 was solved by Anthony Watts at Oxford University and Isabel Moraes at the National Physical Laboratory, together with collaborators at Diamond Light Source.

Atomic force microscope images of the claret membranes of several archaerhodopsins, show that the proteins are trimeric and are arranged in a hexagonal lattice.

[24][25] Archaerhodopsins are active transporters, using the energy from sunlight to pump H+ ions out of the cell to generate a proton motive force that is used for ATP synthesis.

Removal of the retinal cofactor (e.g. by treatment with hydroxylamine) abolishes the transporter function and dramatically alters the absorption spectra of the proteins.

Retinal is covalently bonded via Schiff base to a lysine residue on helix G.[14][16][note 1] The conserved DLLxDGR sequence, close to the extracellular-facing N-terminus of both proteins, forms a tightly curved omega loop that has been implicated in bacterioruberin binding.

[16] Archaerhodopsins drive the hyperpolarization of the cell membrane by secreting protons in presence of light, thereby inhibiting action potential firing of neurons.

[30] When expressed within intracellular membranes, the proton pump activity increases the cytosolic pH, this functionality can be used for optogenetic acidification of lysosomes and synaptic vesicles when targeted to these organelles.