Translocation of group 1 capsular polysaccharide to the surface of Escherichia coli requires a multimeric complex in the outer membrane

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Abstract
Surface expression of the group 1 K30 capsular polysaccharide of Escherichia coli strain E69 (O9a:K30) requires WzaK30, a member of the outer membrane auxiliary (OMA) protein family. A mutation in wzaK30 severely restricts the formation of the K30 capsular structure on the cell surface, but does not interfere with the biosynthesis or polymerization of the K30 repeat unit. Here we show that WzaK30 is a surface‐exposed outer membrane lipoprotein. WzaK30 multimers form ring‐like structures in the outer membrane that are reminiscent of the secretins of type II and III protein translocation systems. We propose that WzaK30 forms an outer membrane pore through which the K30‐capsular antigen is translocated. This is the first evidence of a potential mechanism for translocation of high molecular weight polysaccharide across the outer membrane. The broad distribution of the OMA protein family suggests a similar process for polysaccharide export in diverse Gram‐negative bacteria.