This family of proteins is involved in binding extracellular solutes for transport across the bacterial cytoplasmic membrane. This family includes Swiss:P37735, a C4-dicarboxylate-binding protein [1] and the sialic acid-binding protein SiaP. The structure of the SiaP receptor has revealed an overall topology similar to ATP binding cassette ESR (extracytoplasmic solute receptors) proteins [2]. Upon binding of sialic acid, SiaP undergoes domain closure about a hinge region and kinking of an alpha-helix hinge component [2].

1: Purification, characterization and nucleotide sequence of the periplasmic C4-dicarboxylate-binding protein (DctP) from Rhodobacter capsulatus. Shaw JG, Hamblin MJ, Kelly DJ; Mol Microbiol 1991;5:3055-3062. PMID:1809844

2: Conservation of structure and mechanism in primary and secondary transporters exemplified by SiaP, a sialic acid binding virulence factor from Haemophilus influenzae. Muller A, Severi E, Mulligan C, Watts AG, Kelly DJ, Wilson KS, Wilkinson AJ, Thomas GH; J Biol Chem. 2006;281:22212-22222. PMID:16702222

3: Sialic acid transport in Haemophilus influenzae is essential for lipopolysaccharide sialylation and serum resistance and is dependent on a novel tripartite ATP-independent periplasmic transporter. Severi E, Randle G, Kivlin P, Whitfield K, Young R, Moxon R, Kelly D, Hood D, Thomas GH; Mol Microbiol. 2005;58:1173-1185. PMID:16262798

4: Caught in a TRAP: substrate-binding proteins in secondary transport. Fischer M, Zhang QY, Hubbard RE, Thomas GH; Trends Microbiol. 2010;18:471-478. PMID:20656493