Salt-induced first-order structural transition in a DNA-interacting protein

Thermodynamics and structural transitions on protein surfaces remain relatively understudied and poorly understood. Wrapping of DNA on proteins provides a paradigm for studying protein surfaces. We used magnetic tweezers to investigate a prototypical DNA-interacting protein, i.e., the single-stranded DNA binding protein (SSB). SSB binds DNA with distinct binding modes the mechanism of which is still elusive. The measured thermodynamic parameters relevant to the SSB-DNA complex are salt-dependent and discontinuous at the bind-mode transitions. Our data indicate that free SSB undergoes salt-induced first-order structural transitions. The conclusion was supported by the infrared spectroscopy of SSB in salt solutions. Ultrafast infrared spectroscopy further suggests that the transitions are correlated with surface salt bridges. Our work not only unravels a long-standing mystery of the different binding site sizes of SSB, but also would inspire interests in thermodynamics of protein surfaces.