Microsecond Motion of the Bacterial Transporter EmrE in Lipid Bilayers

Area of Science:

• Biochemistry
• Structural Biology
• Membrane Transport

Background:

• EmrE is a bacterial transporter and a model for small multidrug resistance (SMR) proteins.
• It effluxes cationic substrates coupled to proton transport.
• Understanding EmrE's structure-dynamics relationship is key to elucidating SMR transporter mechanisms.

Purpose of the Study:

• To investigate the protein dynamics of EmrE in complex with a substrate.
• To correlate protein motions with substrate binding and release during transport.

Main Methods:

• Solid-state NMR spectroscopy was used on a substrate-bound S64V-EmrE mutant in lipid bilayers.
• ¹⁵N rotating-frame spin-lattice relaxation (R_1ρ) rates were measured under magic-angle spinning (MAS).
• ¹H-detected ¹⁵N spin-lock experiments were performed under 55 kHz MAS.

Main Results:

• ¹⁵N R_1ρ relaxation rates were measured site-specifically for the substrate-bound protein.
• Spin-lock field-dependent relaxation indicated backbone motions at ~6000 s^-1 at 280 K.
• These motions occur at both acidic and basic pH, independent of protonation state.

Conclusions:

• Microsecond timescale backbone motions are present in substrate-bound EmrE.
• These motions are faster than the overall transport cycle but align with substrate binding/release timescales.
• EmrE likely utilizes these dynamic motions to sample conformations for efficient substrate transport.