Lipid A flippase

FIGURE 11-41 Structures of two ABC transporters of E, coli, (a) The lipid A flippase MsbA (PDB ID 1JSQ) and (b) the vitamin B12 importer BtuCD (PDB ID 1L7V). Both structures are homodimers. The two nucleotidebinding domains (NBDs, in red) extend into the cytoplasm. In (b), residues involved in ATP binding and hydrolysis are shown as ball-and-stick structures. Each monomer of MsbA has six transmembrane helical segments (blue), and each monomer of BtuCD has ten. 

The structure of P-glycoprotein has not been determined. However, X-ray crystallographic structures have been determined for bacterial members of the ABC transporter family the MsbA lipid A "flippases" from E. coli (72) and Vibrio cholera (73) and the cobal-amin uptake transporter BtuCD protein of E. coli (74). These structures are consistent with the overall picture of P-glycoprotein function described here. 

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