Structure of an EIIC sugar transporter trapped in an inward-facing conformation

Proc Natl Acad Sci U S A. 2018 Jun 5;115(23):5962-5967. doi: 10.1073/pnas.1800647115. Epub 2018 May 21.


The phosphoenolpyruvate-dependent phosphotransferase system (PTS) transports sugar into bacteria and phosphorylates the sugar for metabolic consumption. The PTS is important for the survival of bacteria and thus a potential target for antibiotics, but its mechanism of sugar uptake and phosphorylation remains unclear. The PTS is composed of multiple proteins, and the membrane-embedded Enzyme IIC (EIIC) component transports sugars across the membrane. Crystal structures of two members of the glucose superfamily of EIICs, bcChbC and bcMalT, were solved in the inward-facing and outward-facing conformations, and the structures suggest that sugar translocation could be achieved by movement of a structured domain that contains the sugar-binding site. However, different conformations have not been captured on the same transporter to allow precise description of the conformational changes. Here we present a crystal structure of bcMalT trapped in an inward-facing conformation by a mercury ion that bridges two strategically placed cysteine residues. The structure allows direct comparison of the outward- and inward-facing conformations and reveals a large rigid-body motion of the sugar-binding domain and other conformational changes that accompany the rigid-body motion. All-atom molecular dynamics simulations show that the inward-facing structure is stable with or without the cross-linking. The conformational changes were further validated by single-molecule Föster resonance energy transfer (smFRET). Combined, these results establish the elevator-type mechanism of transport in the glucose superfamily of EIIC transporters.

Keywords: bcMalT; double-cysteine cross-linking; elevator-type mechanism of transport; inward-facing conformation; single-molecule FRET.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, Non-P.H.S.

MeSH terms

  • Bacillus cereus / enzymology
  • Bacterial Proteins* / chemistry
  • Bacterial Proteins* / metabolism
  • Bacterial Proteins* / ultrastructure
  • Biological Transport
  • Cysteine / chemistry
  • Cysteine / metabolism
  • Fluorescence Resonance Energy Transfer
  • Molecular Dynamics Simulation
  • Phosphoenolpyruvate Sugar Phosphotransferase System* / chemistry
  • Phosphoenolpyruvate Sugar Phosphotransferase System* / metabolism
  • Phosphoenolpyruvate Sugar Phosphotransferase System* / ultrastructure
  • Phosphorylation
  • Protein Conformation


  • Bacterial Proteins
  • Phosphoenolpyruvate Sugar Phosphotransferase System
  • Cysteine

Associated data

  • PDB/6BVG