Understanding the physico-chemical basics of protein-membrane interactions are of key importance in functional characterization of the cell membrane. In order to simplify this complex system the best membrane mimetics are bicelles, which at q=0.5 (long chain/short chain lipid) ratio are ideal for solution state NMR studies1,2.
We chose biologically relevant protein fragments: the lysine-rich segments of the plant stress protein ERD14 and the C-terminal domain of a small Ca2+-binding, metastasis associated protein S100A4 to follow changes in size and morphology occurring upon interaction with neutral DHPC/DMPC and negatively charged DHPC/DMPC/DMPG bicelles under physiological conditions (pH=7.4; 150 mM NaCl).
Local environmental changes for both partners (typically 1mM/2mM unlabeled peptide and 150mM/300 mM lipids) were followed by 2D 1H-13C spectra, peptide amide environment using 1H-15N HSQC spectra, lipid phosphate headgroup environment by 31P NMR.
Translational diffusion experiments enable determination of the diffusion coefficient D that permits calculation of an effective hydration radius rH. SAXS measurements were performed on the same sample under the same experimental conditions. Fitting of a lentil core-shell model (with axis a,b,ta,tb) on the scattering curve enables a shape prediction and determination of gyration radius rG.
For all studied systems significant line broadening of the DMPC peak in the 31P spectra was detected upon peptide interaction; while peptide HN resonances shifted or broadened below detection limit. Diffusion coefficients were the same for the bicelle (followed on the DMPC methyl group) and for the peptide, showing that a peptide-bicelle complex was formed. Corresponding changes in rH and b axis parameters enable elucidation of morphology variation. The structural changes of peptides were also followed by CD spectroscopy
1Vold et al. J. Biomol. NMR 9 (1997) 329–335
2Bodor et al.Biochim. Biophys. Acta 1848 (2015) 760–766