Mediates the transbilayer movement of Und-PP-GlcNAc-ManNAcA-Fuc4NAc (lipid III) from the inner to the outer leaflet of the cytoplasmic membrane during the assembly of enterobacterial common antigen (ECA). Required for the assembly of the phosphoglyceride-linked form of ECA (ECA(PG)) and the water-soluble cyclic form of ECA (ECA(CYC)). Could also mediate the translocation of Und-PP-GlcNAc. Bacterial outer membrane biogenesis; enterobacterial common antigen biosynthesis. Probably part of a complex composed of WzxE, WzyE and WzzE. Null mutation results in the accumulation of lipid III and cell death. Belongs to the polysaccharide transport (PST) (TC 2.A.66.2) family.