P-glycoprotein (P-gp) is an
ATP-binding cassette transporter that confers multidrug resistance in
cancer cells. It also affects the absorption, distribution and clearance of
cancer-unrelated drugs and
xenobiotics. For these reasons, the structure and function of P-gp have been studied extensively for decades. Here we present biochemical characterization of P-gp from Caenorhabditis elegans and its crystal structure at a resolution of 3.4 ångströms. We find that the apparent affinities of P-gp for anticancer drugs actinomycin D and
paclitaxel are approximately 4,000 and 100 times higher, respectively, in the membrane bilayer than in
detergent. This affinity enhancement highlights the importance of membrane partitioning when a
drug accesses the transporter in the membrane. Furthermore, the transporter in the crystal structure opens its
drug pathway at the level of the membrane's inner leaflet. In the helices flanking the opening to the membrane, we observe extended loops that may mediate
drug binding, function as hinges to gate the pathway or both. We also find that the interface between the transmembrane and
nucleotide-binding domains, which couples
ATP hydrolysis to transport, contains a ball-and-socket joint and
salt bridges similar to the
ATP-binding cassette importers, suggesting that
ATP-binding cassette exporters and importers may use similar mechanisms to achieve alternating access for transport. Finally, a model of human P-gp derived from the structure of C. elegans P-gp not only is compatible with decades of biochemical analysis, but also helps to explain perplexing functional data regarding the Phe335Ala mutant. These results increase our understanding of the structure and function of this important molecule.