PstA

PstA is a membrane protein that forms part of the high-affinity phosphate-specific transport (Pst) system in many bacteria. It is typically encoded by the pstA gene as part of an operon with pstS, pstC, and pstB (pstSCAB). The PstSCAB complex is an ATP-binding cassette (ABC) transporter that imports inorganic phosphate (Pi) from the environment into the cytoplasm. In this system, PstS is a periplasmic or extracellular phosphate-binding protein, PstB is the cytoplasmic ATPase providing the energy for transport, and PstC and PstA form the transmembrane channel through which Pi moves, driven by ATP hydrolysis.

PstA and PstC usually form the transmembrane components of the transporter, coordinating substrate passage across the membrane. The PstSCAB transporter enables Pi uptake against a concentration gradient when Pi is limited and is the principal high-affinity system under phosphate-starved conditions. The activity of the transporter is linked to cellular phosphate homeostasis and regulation of phosphate-responsive genes.

Regulation of pstA expression is governed by the Pho regulon, controlled by the two-component system PhoR/PhoP. Under low phosphate conditions, PhoR phosphorylates PhoP, leading to activation of pstSCAB transcription and other phosphate-starvation genes. When phosphate is abundant, PhoP is not activated, resulting in repression of the regulon and transporter expression.

PstA is widely conserved across bacteria, though gene organization and regulatory nuances can vary. In pathogenic species, components of the PstSCAB system can influence aspects of virulence and fitness in phosphate-poor environments.