P-glycoprotein (Pgp) is an ATP-binding cassette exporter that utilizes the energy from ATP binding and hydrolysis to transport hydrophobic cytotoxins and drugs out of cells, impacting the absorption, distribution, metabolism, and excretion of many xenobiotics, therapeutics, and metabolites. This action reduces the effectiveness of treatments for patients with cardiovascular conditions, cancers, and immunocompromised individuals. They often face polypharmacy, increasing their susceptibility to drug-drug interactions and cytotoxicity. A better understanding of how Pgp transports a diverse array of drugs and how this influences their pharmacokinetics is critical and can help clinicians and researchers develop strategies to mitigate adverse effects and improve therapeutic outcomes. Our recent cryo-EM structures of P-glycoprotein in intermediate transport conformations revealed an unwound segment in the upper region of transmembrane helix 1 (TM1) that caps a narrow substrate translocation tunnel. In contrast, this region remained a straight helix in both the pre-transport inward-facing and substrate-released outward-facing states. These observations suggest that local unwinding and rewinding of TM1 may be critical for substrate passage through the tunnel. Here, we provide biochemical evidence supporting this mechanism. Substitution of Gly73 in human Pgp and surrounding residues with helix-stabilizing alanine severely impairs drug transport. These findings indicate that Pgp may not “open wide” as proposed by a simple alternating access mechanism to let the drug naturally diffuse out. Instead, the loss of function from alanine substitutions strongly suggests that TM1 actively unwinds and rewinds to facilitate drug passage through the narrow tunnel. This reinforces the critical role of TM1 in drug translocation and encourages further research into the drug translocation mechanism, aiming to develop a new generation of therapeutics that can modulate Pgp function by avoiding recognition or inhibiting with high potency. Supported by NIH grant GM141216.
Tran et al. (Sun,) studied this question.