MOP superfamily proteins mediate the transport of lipids important for biosynthetic pathways as well as the expulsion of small molecules. (a) Three subfamilies (MVF, OLF, PST) comprise flippases that transport lipid substrates, while another three subfamilies (MATE, AgnG, and Ank) transport small molecules. (b) MurJ is the prototypical MOP flippase belonging to the MVF family. Abbreviations: AgnG, Agrocin 84; Ank, ankylosis; LLO, lipid-linked oligosaccharide; LPS, lipopolysaccharide; MATE, multidrug and toxic compound extrusion; MOP, multidrug/oligosaccharidyl-lipid/polysaccharide; MVF, mouse virulence factor; OLF, oligosaccharidyl-lipid flippase; PST, polysaccharide transporter.