Multidrug resistance transporter EmrE is only moderately sensitive to lipid composition

Area of Science:

• Biochemistry
• Structural Biology
• Membrane Protein Dynamics

Background:

• Integral membrane proteins (IMPs) are embedded in lipid bilayers and their function is influenced by surrounding lipids.
• Lipid variations affect hydrophobic matching, membrane pressure, and electrostatics, potentially disrupting IMP folding and function.
• The small multidrug resistance (SMR) transporter EmrE expels toxic compounds from bacterial cytoplasm and its function may be lipid-dependent.

Purpose of the Study:

• To investigate the lipid-dependent behavior of the EmrE transporter.
• To determine the impact of various lipids on EmrE structure, substrate binding, and transport kinetics.
• To elucidate how environmental lipids modulate the function of this key bacterial transporter.

Main Methods:

• Utilized nuclear magnetic resonance (NMR) spectroscopy to assess structural integrity.
• Employed isothermal titration calorimetry (ITC) to quantify substrate binding thermodynamics.
• Investigated EmrE in a diverse set of lipids varying in saturation, length, headgroup, and charge.

Main Results:

• EmrE maintained consistent structure, binding affinity, and alternating access rates across tested lipid environments.
• Observed changes in binding enthalpy and entropy were counteracted by enthalpy-entropy compensation, minimizing overall affinity changes.
• Substrate binding and transport were primarily influenced by the substrate itself, not the lipid environment.

Conclusions:

• EmrE's core transport functions are robust and only modestly affected by lipid environment when substrate is bound.
• Lipid composition has a limited impact on EmrE's structure and substrate affinity in the presence of substrates.
• Further research is needed to explore lipid effects on EmrE in the absence of substrates.