Structural insights into Escherichia coli polymyxin B resistance protein D with X-ray crystallography and small-angle X-ray scattering

Abstract

Background: Polymyxin B resistance protein D (PmrD) plays a key role in the polymyxin B-resistance pathway, as it is the signaling protein that can act as a specific connecter between PmrA/PmrB and PhoP/PhoQ. We conducted structural analysis to characterize Escherichia coli (E. coli) PmrD, which exhibits different features compared with PmrD in other bacteria. Results: The X-ray crystal structure of E. coli PmrD was determined at a 2.00 angstrom resolution, revealing novel information such as the unambiguous secondary structures of the protein and the presence of a disulfide bond. Furthermore, various assays such as native gel electrophoresis, surface plasmon resonance (SPR), size-exclusion chromatography, dynamic light scattering (DLS), and small-angle X-ray scattering (SAXS) measurements, were performed to elucidate the structural and functional role of the internal disulfide bond in E. coli PmrD. Conclusions: The structural characteristics of E. coli PmrD were clearly identified via diverse techniques. The findings help explain the different protective mechanism of E. coli compared to other Gram-negative bacteria.