Part of the tripartite ATP-independent periplasmic (TRAP) transport system SiaPT involved in the uptake of sialic acid (N-acetyl-beta-neuraminate). This protein specifically binds sialic acid with high affinity. N-Acetylneuraminate (sialic acid) can then be incorporated into the lipooligosaccharides (LOS) as a terminal non-reducing sugar, protecting the bacterium from complement-mediated killing by normal human serum. The complex comprises the extracytoplasmic solute receptor protein SiaP, and the fused transmembrane protein SiaT. Belongs to the bacterial solute-binding protein 7 family.