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. Is essential for lipopolysaccharide (LPS) sialylation and serum resistance. Belongs to the bacterial solute-binding protein 7 family.