Crystal structure of the multidrug transporter P-glycoprotein from C. elegans

P-glycoprotein (P-gp) is an ATP-binding cassette (ABC) transporter that confers multidrug resistance in cancer cells(1,2). 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(3). Here we present biochemical characterization of P-gp from C. elegans and its crystal structure at 3.4 Å resolution. This work provides the following new information towards a mechanistic understanding of P-gp: 1. 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 drug accesses the transporter in the membrane(4). 2. 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 possibly mediate drug binding and/or function as hinges to gate the pathway. 3. 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 ABC importers(5), suggesting that ABC exporters and importers may share a similar mechanism to achieve alternating access for transport. 4. A carefully derived model of human P-gp, based on the C. elegans P-gp structure, not only is compatible with decades of biochemical analysis(6–12), but also provides insights to explain perplexing functional data regarding the F335A mutant(13,14).