ABSTRACT
In the 1970s and early 1980s, four rhodopsins were discovered in the cytoplasmic membrane of the archaeon
Halobacterium salinarum
: the light-driven ion pumps bacteriorhodopsin (BR
) and halorhodopsin (HR
), and the phototaxis receptors sensory rhodopsin I (SRI
), and sensory rhodopsin II (SRII
). Many laboratories extensively characterized this family of proteins with a battery of diverse techniques, because they provide excellent model systems for the two fundamental functions of membranes: active transport and sensory signaling. Twenty-nine variants of BR, HR, SRI, and SRII were documented in related extremely halophilic archaea, such as
Natronomonas pharaonis
and
Haloarcula vallismortis
. Members of the “archaeal rhodopsin family” were generally assumed to be only in the haloarchaea and appeared to be restricted to the extreme halophilic environments of solar evaporation ponds and other regions of near-saturated salt concentration.
