Simple SummaryP-glycoprotein (P-gp, ABCB1) is an ATP-binding cassette (ABC) transporter that contributes to the development of multidrug resistance (MDR) in cancer cells. P-gp pumps various amphipathic agents including anticancer drugs out of cells. The elucidation of the P-gp mechanism of drug transport is critical to develop strategies to overcome MDR. We used interdisciplinary approaches including cell biological, biochemical, mutational analysis, and molecular dynamics simulations to understand the conformational changes that occur in the homologous transmembrane helices (TMHs) 4 and 10 from the inward-open to the ATP-bound inward-closed states of P-gp during the transport cycle. We found that after substituting seven residues in either TMH4 or TMH10 with alanine, there was no significant effect on the transport function. However, the TMH4,10-14A mutant with 14 substitutions lost the ability to transport most of the substrates tested, revealing that conformational changes in both TMHs 4 and 10 are critical for P-gp’s transport function.AbstractP-glycoprotein (P-gp, ABCB1) transports structurally dissimilar hydrophobic and amphipathic compounds, including anticancer drugs, thus contributing to multidrug-resistant cancer. Cryo-EM structures of human P-gp revealed that TMHs 4 and 10 contribute to the formation of the drug-binding cavity and undergo conformational changes during drug transport. To assess the role of the conformational changes in TMH4 and TMH10 during drug transport, we generated two mutants (TMH4-7A and TMH10-7A), each containing seven alanine substitutions. Analysis of the drug efflux function of these mutants using 15 fluorescent substrates revealed that most of the substrates were transported, indicating that even seven mutations in an individual helix have no significant effect on transport function. We then designed the TMH4,10-14A mutant combining seven mutations in both TMHs 4 and 10. Interestingly, when the TMH4,10-14A mutant was tested with 15 substrates, there was no efflux observed for fourteen. The basal ATPase activity of the TMH4,10-14A mutant, similar to that of the WT protein, was inhibited by zosuquidar but was not stimulated by verapamil or rhodamine 6G. Molecular dynamics simulations indicated that the mutations cause TMHs 4 and 10 to pack tighter to their proximal helices, reducing their independent mobility. In aggregate, our findings demonstrate the critical role of the residues of homologous TMHs 4 and 10 for substrate transport, consistent with conformational changes observed in the structure of P-gp.