5kc0
Crystal structure of TmRibU, hexagonal crystal form
Structural highlights
FunctionRIBU_THEMA Substrate-binding (S) component of an energy-coupling factor (ECF) ABC-transporter complex. Mediates riboflavin uptake, may also transport FMN and roseoflavin. Probably a riboflavin-binding protein that interacts with the energy-coupling factor (ECF) ABC-transporter complex. Unlike classic ABC transporters this ECF transporter provides the energy necessary to transport a number of different substrates. The substrates themselves are bound by transmembrane, not extracytoplasmic soluble proteins (Probable). Expression of the complex plus RibU in E.coli allows riboflavin uptake; uptake does not occur in the absence of RibU or the EcfA1A2T complex.[1] Publication Abstract from PubMedECF transporters are a family of active membrane transporters for essential micronutrients, such as vitamins and trace metals. Found exclusively in archaea and bacteria, these transporters are composed of four subunits: an integral membrane substrate-binding subunit (EcfS), a transmembrane coupling subunit (EcfT), and two ATP-binding cassette ATPases (EcfA and EcfA'). We have characterized the structural basis of substrate binding by the EcfS subunit for riboflavin from Thermotoga maritima, TmRibU. TmRibU binds riboflavin with high affinity, and the protein-substrate complex is exceptionally stable in solution. The crystal structure of riboflavin-bound TmRibU reveals an electronegative binding pocket at the extracellular surface in which the substrate is completely buried. Analysis of the intermolecular contacts indicates that nearly every available substrate hydrogen bond is satisfied. A conserved aromatic residue at the extracellular end of TM5, Tyr130, caps the binding site to generate a substrate-bound, occluded state, and non-conservative mutation of Tyr130 reduces the stability of this conformation. Using a novel fluorescence binding assay, we find that an aromatic residue at this position is essential for high-affinity substrate binding. Comparison with other S subunit structures suggests that TM5 and Loop5-6 contain a dynamic, conserved motif that plays a key role in gating substrate entry and release by S subunits of ECF transporters. An Aromatic Cap Seals the Substrate Binding Site in an ECF-Type S Subunit for Riboflavin.,Karpowich NK, Song J, Wang DN J Mol Biol. 2016 Jun 13. pii: S0022-2836(16)30211-X. doi:, 10.1016/j.jmb.2016.06.003. PMID:27312125[2] From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine. References
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