Structural and Functional Variation in Outer Membrane Polysaccharide Export (OPX) Proteins from the Two Major Capsule Assembly Pathways Present in Escherichia coli

J Bacteriol. 2019 Jun 21;201(14):e00213-19. doi: 10.1128/JB.00213-19. Print 2019 Jul 15.

Abstract

Capsular polysaccharides (CPSs) are virulence factors for many important pathogens. In Escherichia coli, CPSs are synthesized via two distinct pathways, but both require proteins from the outer membrane polysaccharide export (OPX) family to complete CPS export from the periplasm to the cell surface. In this study, we compare the properties of the OPX proteins from the prototypical group 1 (Wzy-dependent) and group 2 (ABC transporter-dependent) pathways in E. coli K30 (Wza) and E. coli K2 (KpsD), respectively. In addition, we compare an OPX from Salmonella enterica serovar Typhi (VexA), which shares structural properties with Wza, while operating in an ABC transporter-dependent pathway. These proteins differ in distribution in the cell envelope and formation of stable multimers, but these properties do not align with acylation or the interfacing biosynthetic pathway. In E. coli K2, murein lipoprotein (Lpp) plays a role in peptidoglycan association of KpsD, and loss of this interaction correlates with impaired group 2 capsule production. VexA also depends on Lpp for peptidoglycan association, but CPS production is unaffected in an lpp mutant. In contrast, Wza and group 1 capsule production is unaffected by the absence of Lpp. These results point to complex structure-function relationships between different OPX proteins.IMPORTANCE Capsules are protective layers of polysaccharides that surround the cell surface of many bacteria, including that of Escherichia coli isolates and Salmonella enterica serovar Typhi. Capsular polysaccharides (CPSs) are often essential for virulence because they facilitate evasion of host immune responses. The attenuation of unencapsulated mutants in animal models and the involvement of protein families with conserved features make the CPS export pathway a novel candidate for therapeutic strategies. However, appropriate "antivirulence" strategies require a fundamental understanding of the underpinning cellular processes. Investigating export proteins that are conserved across different biosynthesis strategies will give important insight into how CPS is transported to the cell surface.

Keywords: Gram-negative bacteria; OPX proteins; capsule; exopolysaccharide; export; outer membrane proteins.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Bacterial Capsules / chemistry*
  • Bacterial Outer Membrane Proteins / chemistry*
  • Bacterial Outer Membrane Proteins / genetics
  • Biosynthetic Pathways
  • Escherichia coli / chemistry*
  • Escherichia coli / genetics
  • Escherichia coli Proteins / chemistry*
  • Escherichia coli Proteins / genetics
  • Lipoproteins / chemistry
  • Lipoproteins / genetics
  • Peptidoglycan / chemistry
  • Periplasmic Proteins / chemistry
  • Periplasmic Proteins / genetics
  • Polysaccharides, Bacterial / chemistry*
  • Polysaccharides, Bacterial / genetics
  • Protein Transport
  • Salmonella typhi / chemistry
  • Salmonella typhi / genetics

Substances

  • Bacterial Outer Membrane Proteins
  • Escherichia coli Proteins
  • KpsD protein, E coli
  • Lipoproteins
  • Lpp protein, E coli
  • Peptidoglycan
  • Periplasmic Proteins
  • Polysaccharides, Bacterial
  • Wza protein, E coli

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