Abstract
Author SummaryLife on earth exhibits an enormous adaptive capacity and living organisms can be found even in extreme environments. The halophilic archea are a group of microorganisms that grow best in highly salted lakes (with KCl concentrations between 2 and 6 molar). To avoid osmotic shock, halophilic archea have the same ionic strength inside their cells as outside. All their macromolecules, including the proteins, have therefore adapted to remain folded and functional under such ionic strength conditions. As a result, the amino acid composition of proteins adapted to a hypersaline environment is very characteristic: they have an abundance of negatively charged residues combined with a low frequency of lysines. In this study, we have investigated the relationship between this biased amino-acid composition and protein stability. Three model proteins – one from a strict halophile, its homolog from a mesophile and a totally unrelated protein from a mesophile - have been largely redesigned by site-directed mutagenesis, and the resulting mutants have been characterized structurally and thermodynamically. Our results show that amino acids with short side-chains (like aspartic and glutamic acid) are preferred to the longer lysine because they succeed in reducing the interaction surface between the protein and the solvent, which is beneficial in an environment where water is in limited availability because it also has to hydrate the salt ions.
Highlights
Halophilic archea are extremophiles that thrive in highly saline environments such as natural salt lakes [1]
The halophilic Hv 1ALigN domain is stabilized by salt while the stability of the mesophilic Ec 1ALigN and protein L from Streptococcus magnus (ProtL) domains are completely independent of ionic strength
The Hv 1ALigN nonlinear stabilization at [KCl],0.5 M can be attributed to electrostatic effects whereas the linear dependence with salt found at [KCl].0.5 M shall obey to a different mechanism
Summary
Halophilic archea are extremophiles that thrive in highly saline environments such as natural salt lakes [1]. To maintain positive turgor pressure, salt concentration in the cytoplasm can reach 4 M [2] Proteins from these organisms have evolved to maximize stability and activity at high salt concentrations (haloadaptation) [3,4]. Comparative analyses between the proteomes of halophilic and non-halophilic bacteria have recognized a characteristic signature in the amino acid composition of proteins with hypersaline adaptation [5,6]. These features include a large increase in glutamic acids and, more frequently, aspartic acids; a drastic drop in the number of lysines (often replaced by arginines) [7]; and a decrease in the overall hydrophobic content [5,8]. An alternative hypothesis suggests that hydrated ions can interact with surface acidic residues (a.r.) to stabilize the folded conformation [14,15]
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