Crystal structure of the solute-binding protein BxlE from Streptomyces thermoviolaceus OPC-520 complexed with xylobiose

J Biochem. 2017 Jun 1;161(6):493-501. doi: 10.1093/jb/mvw097.


BxlE from Streptomyces thermoviolaceus OPC-520 is a xylo-oligosaccharide (mainly xylobiose)-binding protein that serves as the initial receptor for the bacterial ABC-type xylo-oligosaccharide transport system. To determine the ligand-binding mechanism of BxlE, X-ray structures of ligand-free (open form) and ligand (xylobiose)-bound (closed form) BxlE were determined at 1.85 Å resolution. BxlE consists of two globular domains that are linked by two β-strands, with the cleft at the interface of the two domains creating the ligand-binding pocket. In the ligand-free open form, this pocket consists of a U-shaped and negatively charged groove located between the two domains. In the xylobiose-bound closed form of BxlE, both the N and C domains move to fold the ligand without conformational changes in either domain. Xylobiose is buried in the groove and wrapped by the N-domain mainly via hydrogen bond interactions and by the C-domain primarily via non-polar interactions with Trp side chains. In addition to the concave shape matching the binding of xylobiose, an inter-domain salt bridge between Asp-47 and Lys-294 limits the space in the ligand-binding site. This domain-stabilized mechanism of ligand binding to BxlE is a unique feature that is not observed with other solute-binding proteins.

Keywords: bacterial ABC transport; crystal structure; ligand-dependent conformational change; xylooligosaccharide-binding protein.

MeSH terms

  • Bacterial Proteins / chemistry*
  • Bacterial Proteins / isolation & purification
  • Bacterial Proteins / metabolism*
  • Calorimetry
  • Crystallography, X-Ray
  • Disaccharides / chemistry*
  • Disaccharides / metabolism*
  • Models, Molecular
  • Streptomyces / chemistry*


  • Bacterial Proteins
  • Disaccharides
  • xylobiose