Proteorhodopsins in giant viruses

Viral genomes can encode for many proteins do not directly promote virus production but can modify the functionality of the cells they infect to aid the virus indirectly. A good example of this is cyanophages which code for multiple proteins used in cyanobacteria photosynthesis, indirectly promoting phage reproduction. This study reports, for the first time, how giant viruses code for proteorhodopsins. Five viral proteins were of particular interest in this study which show similarity to previously studied bacterial proteorhodopsins.

Yutin and Koonin (2012) compared rhodopsin proteins across viral, archaeal, bacterial, and eukaryotic sequences, by aligning them and comparing for similar sequences. This showed the viral proteins to have high conservation, with 7 transmembrane helicases being especially conserved.

The viral proteins were also shown to have the presence of invariant lysine and aspartic acid in the viral sequences which suggests retinal binding and proton donation would be coded for by the viruses. However, there was no conservation of the carboxylate proton donor, characteristic of the light dependent reaction, suggesting that the viral proteorhodopsins are thought to be used in sensory rather than light dependant proton pumping. More work would be needed to confirm this function of the proteins and the reactions they are used in. There would also need to be more work into the source of the proteins as it is possible that there was masking in the environmental samples, it could be that the proteins may have been a product of unclassified rhodopsin genes from protist viral hosts.

The phylogenetic tree analysis showed early acquisition of the supposed proteorhodopsin genes from either proteorhodopsin coding bacteria or eukaryotes at the base of the tree. It is thought this would be acquired through horizontal gene transfer.

The role of proteorhodopsins in unicellular organisms is assumed to be that they are used as light driven reaction pumps, especially in oligotrophic environmental conditions. It has also been hypothesised that the rhodopsins are used in signalling and phototaxis of the organism.

The study has focused overall on the environmental presence of rhodopsin proteins. For this work to progress the proteins would need to be isolate from individual organisms to determine the sources of these rhodopsins and work out their function in the unicellular organism.

Yutin, N., & Koonin, E. V. (2012). Proteorhodopsin genes in giant viruses. Biology direct, 7, 1, 34.