PstB

PstB is the ATP-binding component of the PstSCAB phosphate-specific transport system, an ATP-binding cassette (ABC) transporter found in many bacteria. In organisms such as Escherichia coli, the transporter comprises four proteins: PstS, a periplasmic phosphate-binding protein; PstC and PstA, the membrane-embedded subunits forming the channel; and PstB, the cytoplasmic ATPase that provides the energy for transport. PstB typically forms a dimer and couples ATP hydrolysis to phosphate translocation by undergoing conformational changes that are transmitted to PstC/PstA through direct interactions.

Structurally, PstB belongs to the ABC transporter family and contains the conserved nucleotide-binding domain with the Walker A and Walker B motifs and the LSGGQ signature sequence. ATP binding and hydrolysis induce rearrangements within the ATPase that drive the conformational changes needed to move phosphate from the periplasm into the cytoplasm, in coordination with the periplasmic phosphate-binding protein PstS.

Regulation of the PstSCAB-PhoU system is governed by the Pho regulon. Under phosphate limitation, the PhoR-PhoB two-component system activates transcription of pstSCAB-phoU, while PhoU acts as a regulatory modulator that tunes transporter activity to prevent unnecessary energy expenditure and to integrate phosphate uptake with broader stress responses controlled by PhoB.

Physiological importance includes enabling high-affinity phosphate uptake under scarce conditions, supporting bacterial growth and fitness in phosphate-poor environments. The system’s regulation and the contribution of PstB can vary among species, but its role as the energy-providing subunit of a key phosphate transporter is widely conserved.