Structural insights into P-glycoprotein substrate polyspecificity: A dynamic conformational sampling model

P-glycoprotein (P-gp, also known as ABCB1) is a member of the ATP-binding cassette (ABC) transporter family that protects tissues from diverse xenobiotics and contributes to multidrug resistance in cancer. Despite extensive structural, biochemical, and biophysical studies, the molecular basis of its remarkable substrate polyspecificity remains incompletely understood. Recent structures of P-gp and its homologs, including zebrafish Abcb4, have revealed extensive conformational heterogeneity and provided unprecedented views of substrate and inhibitor recognition. In this review, we integrate structural and functional studies to examine how conformational variability of the drug-binding site contributes to P-gp polyspecificity. We highlight the continuum of inward-facing conformations and the coupling between local changes in drug-binding site architecture, including transmembrane-helix rotation and tilting, and the global opening and closing of the transporter's two pseudo-symmetric halves. We propose a dynamic conformational sampling model in which diverse substrates can recognize and possibly stabilize different conformational states of P-gp and, in some cases, adopt distinct binding poses. We further discuss how this dynamic view provides a framework for understanding the complex pharmacological behavior of P-gp and may inform the development of more effective inhibitors.