Characterisation of Conformational and Ligand Binding Properties of Membrane Proteins Using Synchrotron Radiation Circular Dichroism (SRCD)

Membrane proteins are notoriously difficult to crystallise for use in X-ray crystallographic structural determination, or too complex for NMR structural studies. Circular dichroism (CD) is a fast and relatively easy spectroscopic technique to study protein conformational behaviour in solution. The advantage of synchrotron radiation circular dichroism (SRCD) measured with synchrotron beamlines compared to the CD from benchtop instruments is the extended spectral far-UV region that increases the accuracy of secondary structure estimations, in particular under high ionic strength conditions. Membrane proteins are often available in small quantities, and for this SRCD measured at the Diamond B23 beamline has successfully facilitated molecular recognition studies. This was done by probing the local tertiary structure of aromatic amino acid residues upon addition of chiral or non-chiral ligands using long pathlength cells (1–5 cm) of small volume capacity (70 μl–350 μl). In this chapter we describe the use of SRCD to qualitatively and quantitatively screen ligand binding interactions (exemplified by Sbma, Ace1 and FsrC proteins); to distinguish between functionally similar drugs that exhibit different mechanisms of action towards membrane proteins (exemplified by FsrC); and to identify suitable detergent conditions to observe membrane protein-ligand interactions using stabilised proteins (exemplified by inositol transporters) as well as the stability of membrane proteins (exemplified by GalP, Ace1). The importance of the in solution characterisation of the conformational behaviour and ligand binding properties of proteins in both far- andnear-UV regions and the use of high-throughput CD (HT-CD) using 96- and 384-well multiplates to study the folding effects in various protein crystallisation buffers are also discussed.