An emerging consensus for the structure of EmrE

Area of Science:

• Biochemistry
• Structural Biology
• Molecular Biology

Background:

• EmrE is a key multidrug transporter in the small multidrug-resistance family.
• It extrudes toxic polyaromatic cations from cells via proton symport.
• Understanding EmrE's structure is crucial for deciphering its transport mechanism.

Purpose of the Study:

• To determine the accurate structure of the EmrE multidrug transporter.
• To reconcile conflicting structural data from different experimental methods.
• To elucidate the monomer orientation within the EmrE dimer.

Main Methods:

• Cryo-electron microscopy (cryo-EM) of two-dimensional crystals.
• Analysis of evolutionary constraints.
• Construction of structural models.
• X-ray crystallography.
• Mutagenesis and biochemical assays.
• Cross-linking studies and EPR spectroscopy.

Main Results:

• Cryo-EM revealed an asymmetric EmrE dimer composed of eight alpha-helices.
• Initial interpretation suggested antiparallel monomer orientation.
• Revised X-ray structures, after retraction of erroneous data, closely matched the cryo-EM model.
• The antiparallel orientation is supported by extensive mutagenic and biochemical data.
• Conflicting data from cross-linking and EPR studies suggest parallel dimer models.

Conclusions:

• The antiparallel orientation of EmrE monomers represents a novel structural paradigm for membrane proteins.
• Revised structural data strongly support the antiparallel model, aligning with most experimental evidence.
• Despite some conflicting data, the antiparallel dimer model provides the most consistent explanation for EmrE function.