Abstract
SummaryMany pathogenic bacteria encase themselves in a polysaccharide capsule that provides a barrier to the physical and immunological challenges of the host. The mechanism by which the capsule assembles around the bacterial cell is unknown. Wzi, an integral outer-membrane protein from Escherichia coli, has been implicated in the formation of group 1 capsules. The 2.6 Å resolution structure of Wzi reveals an 18-stranded β-barrel fold with a novel arrangement of long extracellular loops that blocks the extracellular entrance and a helical bundle that plugs the periplasmic end. Mutagenesis shows that specific extracellular loops are required for in vivo capsule assembly. The data show that Wzi binds the K30 carbohydrate polymer and, crucially, that mutants functionally deficient in vivo show no binding to K30 polymer in vitro. We conclude that Wzi is a novel outer-membrane lectin that assists in the formation of the bacterial capsule via direct interaction with capsular polysaccharides.
Highlights
Many species of bacteria, including both Gram-positive and Gram-negative organisms, are enveloped by a capsule
Capsules are comprised of high-molecular-weight capsular polysaccharide (CPS) chains that are exported across the bacterial cell envelope and retained at the cell surface
CPS chains in groups 2 and 3 are thought to be linked to the surface via a diacylglycerolphosphate moiety (Gotschlich et al, 1981), it was recently proposed that lipopolysaccharide (LPS) structures influence CPS association (Jimenez et al, 2012)
Summary
Many species of bacteria, including both Gram-positive and Gram-negative organisms, are enveloped by a capsule. E. coli capsules have been subdivided into four groups based on structural, biochemical, and genetic criteria, and these identify model systems applicable to capsules from across the Bacteria kingdom (reviewed in Whitfield, 2006). CPS chains in groups 2 and 3 are thought to be linked to the surface via a diacylglycerolphosphate moiety (Gotschlich et al, 1981), it was recently proposed that lipopolysaccharide (LPS) structures influence CPS association (Jimenez et al, 2012). The mechanism by which group 1 CPS chains are assembled into a coherent cell-associated capsule structure rather than being secreted as exopolysaccharides (EPSs) has yet to be elucidated. The controlled assembly of polar polymers into well-defined three-dimensional structures is of interest in chemistry and biology
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