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Archaerhodopsin (alternatively known as bacterio-opsin) is a family of light-driven outward proton pump retinal proteins found in Archaea such as Halobacterium and Halorubrum. They drive the hyperpolarization of the cell membrane by secreting protons in presence of light. This process is associated to an increase in pH linked to the activity of these proteins. These characteristics allow for Archaerhodopsins to be commonly used tools for optogenetic studies as they behave as transmission inhibition factors in presence of light. This family includes:
 * Archaerhodopsin-1 (AR-1)
 * Archaerhodopsin-2 (AR-2)
 * Archaerhodopsin-3 (AR-3)
 * Archaerhodopsin-BD1 (AR-BD1)
 * Archaerhodopsin-4 (AR-4)

Etymology
Archaerhodopsin is named after Archaea, the domain in which the protein family was first found, and Rhodopsin (commonly known as visual purple), the protein family Archaerhodopsins belong to.


 * Archaea, from Ancient Greek ἀρχαῖα (arkhaîa, "ancient"), which is the plural and neuter form of ἀρχαῖος (arkhaîos, "ancient").

Gene
Archaerhodopsins are light-driven outward proton pump retinal proteins, similar to bacteriorhodopsins,. Archaerododhopsin 1 and 2 (aR1 and aR2) represent the first archaerhodopsins founded in Halobacterium family isolated from Australian strains by Mukahata et al., 1988, and Uegaki et al., 1991. Archaerododhopsin 1 and 2 (AR1 and AR2), are present in Halobacterial species, Halobacterium sp. Aus-1 and Aus-2 respectively.

In addition to (aR1) and (aR2), Archaerhodopsin-3 (aR3), is another microbial rhodopsin proton pump founded in Halorubrum sodomense species. That has recently been introduced as a fluorescent voltage sensor. The fourth Archaerhodopsin, Archaerodopsin-4 (aR4), represents a new member of the microbial rhodopsin family founded in Halobacterium species xz 515 collected from a salt take in Tibet,.

Archaerhodopsin proteins mentioned above carried the some ion pumps function but with different amino acid sequences which reflects difference in Halobacteria strains and related ecological habitats. For example, according to Sugiyama et al., 1989 and Ubgaki et al., 1991, the amino acid sequences which were deduced from the structural genes coding the polypeptides of both (aR1) an (aR2) indicated that all essential residues are conserved, .While Archaerhodopsin-3 protein encoded by  aop3 gene is closely related to Archaerodopsin-1 (AaR1) and Archaerodopsin-2 (aR2) with over 90 % homology .However, (aR4), was found membrane of Halobacterium species xz 515 collected from a salt take in Tibet , shows 87% sequence similarity to aR1, 97% to aR2 and 84% to aR3. Archaerhodopsin gene of xz 515 encoding the protein from helix C to helix G has been identified by H. Wang et al., 2000. The genes encoding archaerhodopsin proteins and its related Halobacterium strains, origins and subcelular location were illustrated in the present table:

Function

 * Rhodopsin, from Ancient Greek ῥόδον (rhódon, "rose"), because of its pinkish color, and ὄψις (ópsis, "sight").