title: The periplasmic domain of Escherichia coli outer membrane protein A can undergo a localized temperature dependent structural transition.
abstract: Gram-negative bacteria such as Escherichia coli are surrounded by two membranes with a thin peptidoglycan (PG)-layer located in between them in the periplasmic space. The outer membrane protein A (OmpA) is a 325-residue protein and it is the major protein component of the outer membrane of E. coli. Previous structure determinations have focused on the N-terminal fragment (residues 1-171) of OmpA, which forms an eight stranded transmembrane β-barrel in the outer membrane. Consequently it was suggested that OmpA is composed of two independently folded domains in which the N-terminal β-barrel traverses the outer membrane and the C-terminal domain (residues 180-325) adopts a folded structure in the periplasmic space. However, some reports have proposed that full-length OmpA can instead refold in a temperature dependent manner into a single domain forming a larger transmembrane pore. Here, we have determined the NMR solution structure of the C-terminal periplasmic domain of E. coli OmpA (OmpA(180-325)). Our structure reveals that the C-terminal domain folds independently into a stable globular structure that is homologous to the previously reported PG-associated domain of Neisseria meningitides RmpM. Our results lend credence to the two domain structure model and a PG-binding function for OmpA, and we could indeed localize the PG-binding site on the protein through NMR chemical shift perturbation experiments. On the other hand, we found no evidence for binding of OmpA(180-325) with the TonB protein. In addition, we have also expressed and purified full-length OmpA (OmpA(1-325)) to study the structure of the full-length protein in micelles and nanodiscs by NMR spectroscopy. In both membrane mimetic environments, the recombinant OmpA maintains its two domain structure that is connected through a flexible linker. A series of temperature-dependent HSQC experiments and relaxation dispersion NMR experiments detected structural destabilization in the bulge region of the periplasmic domain of OmpA above physiological temperatures, which may induce dimerization and play a role in triggering the previously reported larger pore formation.
authors.affiliation: Biochemistry Research Group, Department of Biological Sciences, University of Calgary, Calgary, Alberta, Canada.
authors.forename: Hiroaki
authors.initials: H
authors.lastname: Ishida
authors.affiliation: Biochemistry Research Group, Department of Biological Sciences, University of Calgary, Calgary, Alberta, Canada.
authors.forename: Alicia
authors.initials: A
authors.lastname: Garcia-Herrero
authors.affiliation: Biochemistry Research Group, Department of Biological Sciences, University of Calgary, Calgary, Alberta, Canada. Electronic address: vogel@ucalgary.ca.
authors.forename: Hans J
authors.initials: HJ
authors.lastname: Vogel
keywords: Escherichia coli OmpA; Nanodisc; OmpA; Outer membrane protein A; Periplasmic peptidoglycan binding domain; TonB