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). Bacterial outer membrane biogenesis; enterobacterial common antigen biosynthesis. Probably part of a complex composed of WzxE, WzyE and WzzE. Belongs to the polysaccharide transport (PST) (TC 2.A.66.2) family.