Molecular insights into the capsular polysaccharide transporter Wza-Wzc complex

Capsular polysaccharides (CPS), which form the protective outer capsule surrounding many Gram-negative bacterial pathogens, are critical virulence determinants. Their biosynthesis is primarily carried out via the conserved Wzx/Wzy-dependent pathway. In Escherichia coli, Group 1 CPS transport through the bacterial envelope is thought to be mediated by the Wza- Wzc complex. In this study, we present the first structural characterization of the complete Wza-Wzc complex from E. coli K12, determined using single-particle cryogenic electron microscopy. The structure revealed an elongated, continuous channel spanning the entire envelope, which is crucial for efficient CPS secretion, as supported by mutagenesis studies. Multiple structural snapshots of the ADP-bound Wza-Wzc complex captured intermediate conformations of the double membrane assembly, highlighting its remarkable intrinsic dynamics. In-depth analysis of the isolated Wza translocon and Wzc co-polymerase, revealed new mechanistic details of both complex formation and CPS transport. Importantly, we identified the jellyroll domain of Wzc as a previously unrecognized CPS-binding module, likely guiding CPS repeat units into a proposed Wzc-Wzy polymerization platform. Collectively, these findings provide new structural and functional insights into CPS synthesis and transport, advancing our understanding of bacterial capsule formation and virulence mechanisms.