Substrate binding domain of TAXI proteins of the tripartite ATP-independent periplasmic transporters; the type 2 periplasmic binding protein fold.
This group includes Thermus thermophilus GluBP (TtGluBP) of TAXI-TRAP family and closely related proteins. TRAP transporters are ubiquitous in prokaryotes, but absent from eukaryotes. They are comprised of an SBP (substrate-binding protein) of the DctP or TAXI families and two unequally sized integral membrane components. Although TtGluBP is predicted to be an L-glutamate and/or an L-glutamine-binding protein, the substrate spectrum of TAXI proteins remains to be defined. A sequence-homology search also shows that TtGluBP shares low sequence homology with putative immunogenic proteins of uncharacterized function. The substrate-binding domain of TAXI proteins belongs to the type 2 periplasmic-binding fold protein (PBP2) superfamily, whose members are involved in chemotaxis and uptake of nutrients and other small molecules from the extracellular space as a primary receptor. PBP2 typically comprises 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. After binding their specific ligand with high affinity, they can interact with a cognate membrane transport complex comprised of two integral membrane domains and tworeceptor cytoplasmically located ATPase domains. This interaction triggers the ligand translocation across the cytoplasmic membrane energized by ATP hydrolysis.
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