Po-Chao Wen, Brandy Verhalen, Stephan Wilkens, Hassane S. Mchaourab, and Emad
Tajkhorshid.
On the origin of large flexibility of P-glycoprotein in the
inward-facing state.
Journal of Biological Chemistry, 288:19211-19220, 2013.
(PMC: PMC3696692)
WEN2013-ET
P-glycoprotein (Pgp) is one of the most biomedically relevant transporters in the ATP
binding cassette (ABC) superfamily due to its involvement in developing multidrug
resistance in cancer cells. Employing molecular dynamics simulations and double electron-
electron resonance spectroscopy, we have investigated the structural dynamics of
membrane-bound Pgp in the inward-facing state and found that Pgp adopts an
unexpectedly wide range of conformations, highlighted by the degree of separation
between the two nucleotide-binding domains (NBDs). The distance between the two NBDs
in the equilibrium simulations covers a range of at least 20 Å, including, both, more
open and more closed NBD configurations than the crystal structure. The double electron-
electron resonance measurements on spin-labeled Pgp mutants also show wide
distributions covering both longer and shorter distances than those observed in the crystal
structure. Based on structural and sequence analyses, we propose that the transmembrane
domains of Pgp might be more flexible than other structurally known ABC exporters. The
structural flexibility of Pgp demonstrated here is not only in close agreement with, but also
helps rationalize, the reported high NBD fluctuations in several ABC exporters and possibly
represents a fundamental difference in the transport mechanism between ABC exporters
and ABC importers. In addition, during the simulations we have captured partial entrance
of a lipid molecule from the bilayer into the lumen of Pgp, reaching the putative drug
binding site. The location of the protruding lipid suggests a putative pathway for direct
drug recruitment from the membrane.
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