ATP-binding cassette (ABC) transporters couple ATP hydrolysis to the translocation of diverse substrates across the cell membrane. Over 1,100 membrane transporters have been identified in the ABC family, yet their conserved mechanism of transport has only recently been explored. Thirteen genetic diseases are associated with mutations in human.ABC transporters and several exporters have been shown to be polyspecific for transporting drug molecules and have been implicated in multidrug resistance to antibiotics in bacteria and chemotherapeutics in humans. The multidrug exporter MsbA was recently crystallized in three functionally relevant conformations, providing a structural explanation for the mechanism of transport. Molecular modeling and simulation will be used to investigate the detailed mechanism of substrate entry, translocation and force generation in MsbA. The binding and transport of MsbA's native substrate and various drugs will be assessed. Physics-based approaches as well as geometric interpolation will be used to generate a series of conformations connecting the known structural states. Each of these intermediate conformational states along the functional pathway will then be subjected to simulation to examine the role of substrate, protein and bilayer at each stage in the functional transformation. Simulations will be performed with the multiscale coarse-grained method of Voth and coworkers. Multiscale coarse-graining involves the development of a reduced model with forces and fluctuations generated from atomistic simulations to yield physically accurate thermodynamics. The extension of multiscale simulation methods to such a large and dynamic system will be an important milestone in the study of functional dynamics in large assemblies. Coarse-grained simulations will be used here to further our understanding of the polyspecificity and multidrug resistance associated with ABC transporters. PUBLIC HEALTH RELEVANCE: Multidrug resistance to antibiotics in bacteria and chemotherapeutics in humans is a growing public health concern. Investigating the molecular mechanisms behind these phenomena is crucial to combating this issue and will also further our understanding of hereditary diseases such as cystic fibrosis, Tangier disease, Stargardt disease, age-related macular degeneration, adrenoleukodystrophy, Dubin-Johnson syndrome and progressive familial intrahepatic cholestasis.