Structural studies of the hipersaline adaptation of proteins belonging to halophilic archaea

Abstract

Adaptation of organisms to extreme halophilic environments (> 1 ¿ 2 M) occurs through theaccumulation of large intracellular concentrations of KCl. Their major adaptive feature is theextensive modification of the constituting proteome. A biased set of amino acids is selected inorder to improve the stability and solubility of halophilic proteins: large hydrophobic residuesare penalized, specially lysines, whereas small, polar and often negatively charged residuesare favoured, such as aspartic acid, threonine and glutamic acid (Figure I4). Themodifications occur mainly at the surface, so the overall structure is conserved. Themolecular determinants for such a selection remain elusive despite of considerable efforts.Previous models based on weak unspecific K+¿carboxylate interactions have provedthemselves insufficient to explain some features of haloadaptation, such as the complex saltmodulationof enzymatic activity or the dependence of protein stability with Hofmeister anions.Figure I4. Halophilic amino acid composition. Residue abundance in halophilic proteins compared tomesophilic proteins expressed as the percentage of relative variation in the average amino acidcomposition.