Substrate-binding component of uncharacterized Tripartite ATP-independent Periplasmic transporter; the type 2 periplasmic-binding protein fold
This family represents the TRAP Transporters that are specific to various ligands, including sialic acid (N-acetyl neuraminic acid), glutamate, ectoine, xylulose, C4-dicarboxylates such as succinate, malate and fumarate, and keto acids such as pyruvate and alpha-ketobutyrate. TRAP transporters are a large family of solute transporters ubiquitously found in bacteria and archaea. This CD also included some eukaryotic homologs that have not been functionally characterized. TRAP transporters are comprised of a periplasmic substrate-binding protein (SBP; often called the P subunit) and two unequally sized integral membrane components: a large transmembrane subunit involved in the translocation process (the M subunit) and a smaller membrane of unknown function (the Q subunit). 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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