Molecular mechanism of phosphate import by the bacterial PstSCAB transporter

Inorganic phosphate (Pi) is essential for all living organisms. PstSCAB, a bacterial high-affinity ABC transporter, imports Pi under limiting conditions via five subunits: PstA and PstC forming the transmembrane domain (TMD), periplasmic PstS that switches between free and TMD-docked forms for Pi capture and delivery, and two cytosolic PstB subunits for ATP binding and hydrolysis. Its malfunction affects the virulence of pathogenic bacteria, making it pharmaceutically attractive. However, complete structural pictures of PstSCAB in different states remain lacking. Here, we determine cryo-EM structures of PstSCAB in resting, pretranslocation, and catalytic intermediate states, which reveal that conformational changes in PstS and ATP binding/unbinding in PstB collectively induce rigid-body movements of TMD, generating inward- or outward-facing conformations. In TMD, Pi specificity is determined by positively charged Arg220 (PstA) and Arg237 (PstC). This study advances understanding of bacterial Pi import and supports drug development targeting PstSCAB.