Single molecule studies of protein unfolding in highly saline solutions

Life on earth has been found thriving in a number of extreme environments, including those of high salinity, high and low temperatures and pH, and high pressure. Organisms which live in the presence of large quantities of salt are known as halophilic (meaning salt-loving), such as in the Dead Sea.

Proteins are fundamental components of all living organisms. They are large, complex molecules that carry out many processes within a cell. Halophilic proteins are of great interest due to their ability to remain soluble, flexible and functional under highly saline conditions. Intriguingly, these proteins are unstable in a low saline environment, suggesting a delicate balance between the intermolecular interactions of the protein, salt and solvent. How have halophilic proteins adapted to survive in highly saline environments?

To probe the effect of salt on the mechanical stability of a protein, a combination of molecular biology and single molecule force spectroscopy (SMFS) was used. Protein engineering was utilised to create chimeric polyprotein constructs including a obligate halophilic and a mesophilic protein. SMFS experiments have been carried out using these polyprotein constructs in 0.5 M and 2 M KCl. The studies suggest that an increase in the hydrophobic interactions of a mesophilic protein cause an increase in its mechanical stability. The results also indicate that an obligate halophilic protein does not have an increased mechanical stability in the increased salt concentration. Further studies in combination with molecular dynamics simulations have the potential to gain atomistic information on the mechanical unfolding behaviour of a halophilic protein.