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

Area of Science:

• Molecular Evolution
• Protein Phylogenetics
• Archaea Biology

Background:

• Extreme halophiles possess diverse retinal proteins involved in crucial cellular functions.
• Understanding the evolutionary relationships of these proteins is key to deciphering their functional diversification.

Purpose of the Study:

• To establish the molecular phylogenetic relationship of archaeal retinal proteins.
• To investigate the evolutionary history and functional divergence within the archaeal rhodopsin family.

Main Methods:

• Analysis of amino acid sequences from 25 archaeal retinal proteins across 13 halophile strains.
• Phylogenetic analysis, including likelihood mapping and branch length comparisons.
• Calculation of relative evolutionary rates for different rhodopsin types.

Main Results:

• Identification of a distinct archaeal rhodopsin family (ARF), unrelated to other known proteins like GPCRs.
• ARF divided into four functional clusters: H+ pump (bacteriorhodopsin), Cl- pump (halorhodopsin), and two sensors (sensory and phoborhodopsin).
• Gene duplication events predating halophilic archaea speciation were inferred, with relative evolution rates calculated as bacteriorhodopsin:halorhodopsin:sensory rhodopsin:phoborhodopsin = 5:4:3:10.

Conclusions:

• The ARF evolved through gene duplication and subsequent divergent evolution, reflecting speciation events in halophilic archaea.
• Differential evolutionary rates suggest functional and structural constraints, with sensory rhodopsin evolving faster from a phoborhodopsin-like ancestor.
• This study demonstrates how gene duplication and functional differentiation shape protein family evolution within archaea.