RT Journal Article
SR Electronic
T1 Measuring Protein Binding to Lipid Vesicles by Fluorescence Cross-Correlation Spectroscopy
JF bioRxiv
FD Cold Spring Harbor Laboratory
SP 146464
DO 10.1101/146464
A1 Daniela Krüger
A1 Jan Ebenhan
A1 Kirsten Bacia
YR 2017
UL http://biorxiv.org/content/early/2017/06/08/146464.abstract
AB Fluorescence Correlation Spectroscopy (FCS) has been previously used to investigate peptide and protein binding to lipid membranes, as it allows for very low amounts of sample, short measurement times and equilibrium binding conditions. Labeling only one of the binding partners however comes with certain drawbacks, as it relies on identifying binding events by a change in diffusion coefficient. Since peptide and protein aggregation can obscure specific binding and since non-stoichiometric binding necessitates the explicit choice of a statistical distribution for the number of bound ligands, we additionally label the liposomes and perform dual-color Fluorescence Cross-Correlation Spectroscopy (dcFCCS). We develop a theoretical framework showing that dcFCCS amplitudes allow calculation of the degree of ligand binding and the concentration of unbound ligand, leading to a binding model-independent binding curve. As the degree of labeling of the ligands does not factor into the measured quantities, it is permissible to mix labeled and unlabeled ligand, thereby extending the range of usable protein concentrations and accessible dissociation constants KD. The total protein concentration, but not the fraction of labeled protein needs to be known. In this work, we apply our dcFCCS analysis scheme to Sar1p, a protein of the COPII complex, which binds ‘major-minor-mix’ liposomes. A Langmuir isotherm model yields KD = (2.1±1.1)μM as the single site dissociation constant. The dual-color FCCS framework presented here is highly versatile for biophysical analysis of binding interactions. It may be applied to many types of fluorescently labeled ligands and small diffusing particles, including nanodiscs and liposomes containing membrane protein receptors.