Part of the tripartite ATP-independent periplasmic (TRAP) transport system SiaPQM that catalyzes unidirectional Na(+)-dependent sialic acid uptake. Binds the common sialic acid N-acetylneuraminic acid (Neu5Ac) with a high affinity. The complex comprises the extracytoplasmic solute receptor protein SiaP, and the two transmembrane proteins SiaQ and SiaM. Belongs to the bacterial solute-binding protein 7 family.