Assessing the halophilic character of ADP-dependent sugar kinases from the archeon order Methanosarcinales

Halophilic organisms have evolved to live in environments of high salinity, therefore their
molecular machinery has adapted to carry out its functions in presence of molar concentrations of
salt. Most of the work aimed to understand the structural adaptations of these proteins has been
done using proteins from the archeon class Halobacteria. Proteins from these organisms are
characterized by a low abundance of basic residues and a high amount of acidic residues, which
accumulate on the protein surface, coupled with a reduction of bulky hydrophobic residues in its
core [1]. Nevertheless, halophilic organisms have been reported in a wide variety of taxa, including
other archaea orders, which their adaptation mechanisms have not been explored. To evaluate the
ubiquity of the protein structural adaptations found in Halobacteria, we built homology models
of ADP-dependent kinases from halophilic and non-halophilic organisms of the archaeal order
Methanosarcinales and compared them to models from Halobacterial and Eucariotic proteins.
Our results show that proteins from halophilic organisms of the Methanosarcinales order do not
show the classical bias in amino acid composition observed in Halobacteria, like the reduction of
the hydrophobic core and negative surface charge. However, experimental characterization of the
ADP-dependent phosphofructokinase of the halophilic organism Methanohalobium evestigatum
(from Methanosarcinales order) confirmed that the protein is indeed halotolerant, and this
character can be further exacerbated in presence of osmolytes commonly found on halophilic
archaea, like betaine [2]. These results suggest that the adaptations required to maintain the
structure and function of a protein in extreme salt concentrations can vary widely between
different organisms. These adaptations do not rely exclusively on the amino acidic composition,
being instead a product of the coevolutionary process between the protein and its intracellular
environment. Fondecyt 1150460

References

[1] Graziano, G., & Merlino, A. (2014). Molecular bases of protein halotolerance. Biochimica et Biophysica Acta
(BBA) - Proteins and Proteomics, 1844(4), 850–858
[2] Sowers, K. R., & Gunsalus, R. P. (1995). Halotolerance in Methanosarcina spp.: Role of N (sup (epsilon))-
Acetyl-(beta)-Lysine,(alpha)-Glutamate, Glycine Betaine, and K (sup+) as Compatible Solutes for Osmotic
Adaptation. Applied and environmental microbiology, 61(12), 4382-4388.