Backbone dynamics and thermodynamics of Borrelia outer surface protein A

Nuclear spin relaxation experiments performed at 298K, 308K and 318K are used to characterize the intramolecular dynamics and thermodynamics of outer surface protein A (OspA), a key protein in the life-cycle of Borrelia burgdorferi, the causative agent of Lyme disease. It has recently been demonstrated that OspA specifically binds to the gut of the intermediate tick host (Ixodes scapularis), and that this interaction is mediated, at least in part, by residues in the C-terminal domain of OspA that are largely inaccessible to solvent in all X-ray structures of this protein. Our analysis of 15N relaxation parameters in OspA shows that the putative-binding region contains and is surrounded by flexible residues, which could facilitate accessibility to solvent and ligands. In addition, residues with similar activation energies are clustered in a manner that suggests locally collective motions. We have used molecular modeling to show that these collective motions are consistent with a hinge-bending mechanism that exposes residues implicated in binding. Characteristic temperatures describing the energy landscape of the OspA backbone are derived from the temperature dependence of the N-H bond vector order parameters, and a comparison is made between the N and C-terminal globular domains and the unusual single-layer beta-sheet connecting them. The average characteristic temperatures in the three regions indicate that, with an increase in temperature, a larger increase in accessible conformational states occurs for N-H bond vectors in the single-layer central beta-sheet than for bond vectors in the globular N and C-terminal domains. These conformational states are accessible without disruption of hydrogen bonds, providing a conformational entropic gain, upon increase in temperature, without a significant enthalpic penalty. This increase in heat capacity may help to explain the unexpected thermal stability of the unusual single-layer beta-sheet.