Figure 8.

Diversity of LLO flipping mechanisms beyond the MOP superfamily. Cartoon representations of LLO flippases MurJ, LtaA, Wzm-Wzt and PglK with their flipping mechanisms. Dashed lines indicate a hydrophobic groove or opening. MurJ and LtaA utilize alternating access. Wzm-Wzt forms a continuous channel inside the transmembrane region to transport substrate. PglK was proposed to use an unconventional mechanism in which only the outer-facing conformation is involved in flipping. The lipid tail does not enter into the protein but interacts only with the helix on the periplasmic side while the headgroup enters the positive charged cavity. Shown below are electrostatic surface representations highlighting the cavity where substrates bind, with cationic surfaces in blue and anionic surfaces in red.

Abbreviations: ABC, ATP-binding cassette; LLO, lipid-linked oligosaccharide; MFS, major facilitator superfamily; MOP, multidrug/oligosaccharidyl-lipid/polysaccharide; PDB ID, Protein Data Bank identifier.