Part of the ABC transporter complex PtxABC involved in phosphite import. Responsible for energy coupling to the transport system (Probable). ATP + H2O + phosphite(out) = ADP + H(+) + phosphate + phosphite(in) The complex is composed of two ATP-binding proteins (PtxA), two transmembrane proteins (PtxC) and a solute-binding protein (PtxB). Belongs to the ABC transporter superfamily. Phosphonates importer (TC 3.A.1.9.1) family.