Part of the tripartite ATP-independent periplasmic (TRAP) transport system SiaPT involved in the uptake of sialic acid (N-acetyl-beta-neuraminate). N-Acetylneuraminate can then be incorporated into the lipopolysaccharides 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. In the N-terminal section; belongs to the TRAP transporter small permease family. In the C-terminal section; belongs to the TRAP transporter large permease family.