Biophysical EPR Studies Applied to Membrane Proteins

Abstract

Membrane proteins are very important in controlling bioenergetics, functional activity, and initializing signal pathways in a wide variety of complicated biological systems. They also represent approximately 50% of the potential drug targets. EPR spectroscopy is a very popular and powerful biophysical tool that is used to study the structural and dynamic properties of membrane proteins. In this article, a basic overview of the most commonly used EPR techniques and examples of recent applications to answer pertinent structural and dynamic related questions on membrane protein systems will be presented.

Keywords: DEER; Electron paramagnetic resonance spectroscopy; Membrane proteins; Site-directed spin labeling; Structural topology and dynamics.

Figure 1

Cartoon representation of a membrane peptide (acetylcholine receptor (AchR) M2δ, PDB entry 1EQ8) incorporated into lipid bilayers (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC)). Methanethiosulfonate spin label (MTSL) (orange color) has been attached at 11th position. Figure was prepared using visual molecular dynamics (VMD) and molecular modeling was performed using CHARMM-GUI (http://www.charmm-gui.org).

Figure 2

Structure of MTSL (Methanethiosulfonate spin label) and the resulting side-chain produced by reaction with the cysteine residue of the protein.

Figure 3

CW-EPR spectra of different spin label side chain motions. Spectra were simulated using EasySpin simulation software [41].

Figure 4

Four pulse Q-band DEER (double electron-electron resonance) data for peptide analogue N-terminal microdomain of HsDHODH peptide and predicted conformational states in micelles (top panel) and POPC liposomes (bottom panel) (Adapted from ref. [75] with permission).