3.A.1.103.2
Heteropolysaccharide O-antigen exporter, Wzm/Wzt (Feng et al., 2004). The C-terminal cytoplasmic domain of Wzt (an IgG-like β-sandwich) determines the specificity of the transporter for either O8 or O9a O-PS (Cuthbertson et al., 2007). The transporter structure reveals a continuous transmembrane channel in a nucleotide-free state (Caffalette et al. 2019). Upon ATP binding, large structural changes within the nucleotide-binding and transmembrane regions push conserved hydrophobic residues at the substrate entry site towards the periplasm and provide a model for polysaccharide translocation. With ATP bound, the transporter forms a large transmembrane channel with openings toward the membrane and periplasm. The channel's periplasmic exit is sealed by detergent molecules that block solvent permeation. Molecular dynamics simulation data suggest that, in a biological membrane, lipid molecules occupy this periplasmic exit and prevent water flux in the transporter's resting state (Caffalette et al. 2019).