Evolution of the archaeal rhodopsins: evolution rate changes by gene duplication and functional differentiation.

The amino acid sequences of 25 archaeal retinal proteins from 13 different strains of extreme halophiles were analyzed to establish their molecular phylogenetic relationship. On the basis of amino acid sequence similarity, these proteins apparently formed a distinct family designated as the archaeal rhodopsin family (ARF), which was not related to other known proteins, including G protein-coupled receptors. The archaeal rhodopsin family was further divided into four clusters with different functions; H+ pump (bacteriorhodopsin), Cl- pump (halorhodopsin), and two kinds of sensor (sensory rhodopsin and phoborhodopsin). These four rhodopsin clusters seemed to have occurred by gene duplication(s) before the generic speciation of halophilic archaea, based on phylogenetic analysis. Therefore, the degrees of differences in amino acid sequences within each cluster simply reflected the divergent evolution of halophilic archaea. By comparing the branch lengths after speciation points of the reconstituted tree, we calculated the relative evolution rates of the four archaeal rhodopsins bacteriorhodopsin:halorhodopsin:sensory rhodopsin: phoborhodopsin to be 5:4:3:10. From these values, the degrees of functional and structural restriction of each protein can be inferred. The branching topology of four clusters grouped bacteriorhodopsin and halorhodopsin versus sensory rhodopsin and phoborhodopsin by likelihood mapping. Using bacteriorhodopsin (and halorhodopsin) as an outgroup, the gene duplication point of sensory rhodopsin/phoborhodopsin was determined. By calculating the branch lengths between the gene duplication point and each halophilic archaea speciation point, we could speculate upon the relative evolution rate of pre-sensory rhodopsin and pre-phoborhodopsin. The evolution rate of pre-sensory rhodopsin was fivefold faster than that of pre-phoborhodopsin, which suggests that the original function of the ancestral sensor was similar to that of phoborhodopsin, and that sensory rhodopsin evolved from pre-sensory rhodopsin by the accumulation of mutations. The changes in evolution rate by gene duplication and functional differentiation were demonstrated in the archaeal rhodopsin family using the gene duplication date and halobacterial speciation date as common time stamps.