Effects of Hydrostatic Pressure on the Thermodynamics of CspB-Bs Interactions with the ssDNA Template

Understanding the thermodynamic mechanisms of adaptation of biomacromolecules to high hydrostatic pressure can help shed light on how piezophilic organisms can survive at pressures reaching over 1000 atmospheres. Interaction of proteins with nucleic acids is one of the central processes that allow information flow encoded in the sequence of DNA. Here, we report the results of a study on the interaction of cold shock protein B from Bacillus subtilis (CspB-Bs) with heptadeoxythymine template (pDT7) as a function of temperature and hydrostatic pressure. Experimental data collected at different CspB-Bs:pDT7 ratios were analyzed using a thermodynamic linkage model that accounts for both protein unfolding and CspB-Bs:pDT7 binding. The global fit to the model provided estimates of the stability of CspB-Bs, ΔG_Prot^o, the volume change upon CspB-Bs unfolding, ΔV_Prot, the association constant for CspB-Bs:pDT7 complex, K_a^o, and the volume changes upon pDT7 single-stranded DNA (ssDNA) template binding, ΔV_Bind. The protein, CspB-Bs, unfolds with an increase in hydrostatic pressure (ΔV_Prot < 0). Surprisingly, our study showed that ΔV_Bind < 0, which means that the binding of CspB-Bs to ssDNA is stabilized by an increase in hydrostatic pressure. Thus, CspB-Bs binding to pDT7 represents a case of linked equilibrium in which folding and binding react differently upon an increase in hydrostatic pressure: protein folding/unfolding equilibrium favors the unfolded state, while protein-ligand binding equilibrium favors the bound state. These opposing effects set a "maximum attainable" pressure tolerance to the protein-ssDNA complex under given conditions.