Substrate-binding domain of a sialic acid binding Tripartite ATP-independent Periplasmic transport system (SiaP) and related proteins; the type 2 periplasmic-binding protein fold.
This subfamily includes the periplasmic-binding component of TRAP transport systems such as SiaP (a sialic acid binding virulence factor), TeaA (an ectoine binding protein), and an uncharacterized TM0322 from hyperthermophilic bacterium Thermotoga maritima. TRAP transporters are a large family of solute transporters ubiquitously found in bacteria and archaea. They are comprised of a periplasmic substrate-binding protein (SBP) and two unequally sized integral membrane components: a large transmembrane subunit involved in the translocation process and a smaller membrane of unknown function. The driving force of TRAP transporters is provided by electrochemical ion gradients (either protons or sodium ions) across the cytoplasmic membrane, rather than ATP hydrolysis. This substrate-binding domain belongs to the type 2 periplasmic binding fold protein superfamily (PBP2). The PBP2 proteins are typically comprised of two globular subdomains connected by a flexible hinge and bind their ligand in the cleft between these domains in a manner resembling a Venus flytrap.
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