1. Out, out darn toxin: the role of MDR in intestinal homeostasis
Janine Bilsborough, Joanne L. Viney 
Gastroenterology 
Volume 127, Issue 1, Pages (July 2004)
DOI: /j.gastro

2. Figure 1
(A) Linear secondary structure of human P-gp showing putative structural features. ATP binding sites are circled. ATP-binding cassette reporters contain 2 mirror image halves separated by a flexible linker region that is essential for activity. (B) One model for the topology of P-gp suggests a single aqueous pore is formed by 12 membrane-spanning α-helices through the interaction of each of the 6 α-helices within the 2 hydrophobic transmembrane (TM) units. The ATP-binding sites are represented by gray circles (reviewed5). Different orientations of the TM helices have been proposed including (C) mirror, (D) orientational, and (E) cyclone (reviewed5). (F) An alternative dual channel model for P-gp proposes the 2 TM domains form membrane-spanning β-barrels. The cytoplasmic loops attach the α-helices to the TM β barrels to form the substrate binding sites. Substrate, bound to the α-helices, is transported through the β-barrel to the extracelluar domain (reviewed5).
Gastroenterology  , DOI: ( /j.gastro )