Phosphate uptake in PhoX: Molecular mechanisms

PhoX is a high-affinity phosphate binding protein, present in Xanthomonas citri, a phytopathogen responsible for the citrus canker disease. Performing molecular dynamics simulations and different types of computational analyses, we study the molecular mechanisms at play in relation to phosphate binding, revealing the global functioning of the protein: PhoX naturally oscillates along its global normal modes, which allow it to explore both bound and unbound conformations, eventually attracting a nearby negative phosphate ion to the highly positive electrostatic potential on its surface, particularly close to the binding pocket. There, several hydrogen bonds are formed with the two main domains of the structure. Phosphate creates, in this way, a strong bridge that connects the domains, keeping itself between them, in a tight closed conformation, explaining its high binding affinity. •Both, phosphate-bound and phosphate-unbound PhoX can explore the entire configurational space of the protein.•This is contrary to the usually assumed correspondence between a closed/open conformation and bound/unbound state.•PhoX posseses an mostly positive electrostatic potential, which gives way to a strong attraction of the phosphate ion.•A patch containing S62, G63, G64, K199, with high positive potential, appears to be the one initiating the anion’s binding.•Binding pocket’s S34 and S62 from DI, and R159, S163 and G164 from DII, simultaneously form H-bonds with the anion.•These bridges connect and stabilize the interaction between domains, keeping PhoX in a highly stable and closed conformation.