1.B.73 The Capsule Biogenesis/Assembly (CBA) Family
Many 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 poorly understood. Wzi, an integral outer-membrane protein from E. coli, has been implicated in the formation of group 1 capsules (Rahn et al. 2003). The 2.6 Å resolution structure of Wzi revealed 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 (Bushell et al. 2013). Mutagenesis experiments showed that specific extracellular loops are required for in vivo capsule assembly. Wzi is a lectin that binds the K30 carbohydrate polymer. Mutants functionally deficient in vivo show no binding to K30 polymer in vitro.Apparently, Wzi is a novel outer-membrane lectin that assists in the formation of the bacterial capsule via direct interaction with capsular polysaccharides.