Allosteric substrate release by a sialic acid TRAP transporter substrate binding protein.

The tripartite ATP-independent periplasmic (TRAP) transporters enable Vibrio cholerae and Haemophilus influenzae to acquire sialic acid, aiding their colonization of human hosts. This process depends on SiaP, a substrate-binding protein (SBP) that captures and delivers sialic acid to the transporter. We identified 11 nanobodies that bind specifically to the SiaP proteins from H.