Single-Molecule Force Spectroscopy Identifies a Small Cold Shock Protein as Being Mechanically Robust

Single-molecule force spectroscopy has emerged as a powerful approach to examine the stability and dynamics of single proteins. We have completed force extension experiments on the small cold shock protein B from Thermotoga maritima, using a specially constructed chimeric polyprotein. The protein's simple topology, which is distinct from the mechanically wellcharacterized /9-grasp and immunoglobulin (Ig)-like folds, in addition to the wide range of structural homologues resulting from its ancient origin, provides an attractive model protein for single-molecule force spectroscopy studies. We have determined that the protein has mechanical stability, unfolding at greater than 70 pN at a pulling velocity of 100 nm s(-1) . We reveal features of the unfolding energy landscape by measuring the dependence of the mechanical stability on pulling velocity, in combination with Monte Carlo simulations. We show that the cold shock protein has mechanically robust, yet malleable, features that may be important in providing the protein with stability and flexibility to function over a range of environmental conditions. These results provide insights into the relationship between the secondary structure and topology of a protein and its mechanical strength. This lays the foundation for the investigation of the effects of changes in environmental conditions on the mechanical and dynamic properties of cold shock proteins.