Structure and Assembly of Escherichia Coli Outer Membrane Protein A

Abstract

Outer membrane protein A (OmpA) of Escherichia coli is a paradigm for the biogenesis of outer membrane proteins; however, the structure and assembly of OmpA remain controversial. Studies to date support the hypothesis that native OmpA is a single-domain large pore, and a two-domain narrow pore conformer is a folding intermediate. The in vitro refolding of OmpA to the large pore conformation requires that the protein be isolated from outer membranes with an intact disulfide bond and then adequately incubated in lipids at temperatures ≥ 26 °C to overcome the energy of activation for refolding. The in vivo maturation of the protein involves covalent modification of serines of the N-terminal domain by oligo-(R)-3-hydroxybutyrates as the protein is escorted across the cytoplasm by SecB for post-translational secretion across the SEC translocase in the inner membrane. After cleavage of the signal sequence, protein chaperones, such a Skp, DegP and SurA, guide OmpA across the periplasm to the BAM complex in the outer membrane. During this passage, a disulfide bond is formed between C290 and C302 by DsbA, and the hydrophobicity of segments of the C-terminal domain which are destined for incorporation as β-barrels in the outer membrane bilayer is increased by covalent attachment of oligo-(R)-3-hydroxybutyrates. With the aid of the BAM complex, OmpA is then assembled into the outer membrane as a single-domain large pore.