Extreme conditions, extreme (protein) adaptations

Life under extreme conditions requires extreme adaptations. Successful organisms must have evolved mechanisms to perform biochemistry under conditions that mesophiles (like humans and E. coli) cannot withstand. In an attempt to understand how proteins can remain stable under such extremes, we surveyed proteins from three representative extremes: thermophilic, psychrophilic and halophilic. We found that thermophilic proteins tend to have prominent hydrophobic cores and increased electrostatic interactions to maintain activity at high temperatures, while psychrophilic proteins have compact hydrophobic cores and a more neutral protein surface to maintain flexibility and activity under cold temperatures. Halophilic proteins tend to be marked by strong negative surface charges due to increased acidic amino acid content and peptide insertions, which can compensate for their host’s extreme saline conditions. In an attempt to understand the proposed halophilic adaptations, we explored the effects of different salts on the structure of the E. coli and Halobacterium salinarium ssp. NRC-1 cysteinyl-tRNA synthetase using circular dichroism and fluorescence spectroscopy. Sodium and potassium have the greatest impact on halophilic protein structure and stability. The protein exhibits reversible folding and unfolding in the presence and absence of the salts, and resists thermal denaturation in the presence of salt far better than its E. coli counterpart. Our data suggest that halophiles exploit their saline environment to increase the structure and function of their proteins. These insights could be applied to increase the stability and function of biotechnologically useful enzymes under extreme environmental conditions.