Structure and Ligand-Binding Properties of the O Antigen ABC Transporter Carbohydrate-Binding Domain

A hallmark of Gram-negative bacteria is an asymmetric outer membrane containing lipopolysaccharides (LPS) in the extracellular leaflet. LPS molecules consist of lipid-A that is connected to the inner and outer core oligosaccharides. This LPS core structure is extended in the periplasm by the O antigen, a variable and serotype-defining polysaccharide. In the ABC transporter-dependent LPS biosynthesis pathway, the WzmWzt transporter secretes the complete O antigen across the inner membrane for ligation to the LPS core. In some O antigen transporters, the nucleotide binding domain (NBD) of Wzt is fused C terminally to a carbohydrate-binding domain (CBD) that interacts with the O antigen chain. Here, we present the crystal structure of the Aquifex aeolicus CBD that reveals a conserved flat and variable twisted jelly-roll surface. The CBD dimer is stabilized by mutual β-strand exchange. Microbial glycan array binding studies with the isolated CBD provide insights into its interaction with complex carbohydrates.