Epidermal growth factor receptor (EGFR, also known as ErbB-1 or HER-1) is a membrane bound protein that has been associated with a variety of solid tumors and the control of cell survival, proliferation and metabolism. Quantification of EGFR expression level in cells membrane and the interaction kinetics with drugs are thus important for cancer diagnosis and treatment. Here we report mapping of the distribution and interaction kinetics of EGFR in their native environment with a surface plasmon resonance imaging (SPRi) technique. Monoclonal anti-EGFR antibody was used as a model drug in this study. The binding of the antibody to EGFR overexpressed A431 cells was monitored in real time, which was found to follow the first-order kinetics with association rate constant (ka) and dissociation rate constant (kd) to be (2.7 ± 0.6)×105 M-1s-1 and (1.4 ± 0.5)×10-4 s-1, respectively. The dissociation constant (KD) was determined to be (0.53 ± 0.26) nM with up to seven folds variation among different individual A431 cells. In addition, the averaged A431 cell surface EGFR density was found to be 636/μm2 with an estimation of 5×105 EGFR per cell. Additional measurement also revealed different EGFR positive cell lines (A431, HeLa and A549) show receptor density dependent anti-EGFR binding kinetics. The results demonstrate that SPRi is a valuable tool for direct quantification of membrane protein expression level and ligand binding kinetics at single cell resolution. Our findings show that the local environment affects the drug-receptor interactions and in-situ measurement of membrane protein binding kinetics is important.

Fluorescent labeling is a mainstream technology for detecting molecular binding. Despite the importance, few studies have been devoted to quantitatively examine the effect of labeling on the molecular binding processes. Here we present a quantitative study on the binding kinetics of fluorescent-labeled and un-labeled molecules (lectin proteins) with glycoproteins on the membrane of cells using surface plasmon resonance imaging (SPRi) technique. The study shows that fluorescent labeling has a significant influence on the binding behaviors of proteins, especially the association processes, and the influence depends sensitively on the charge of fluorescent labels. It further shows that the labels also affect the local distribution of probe proteins, due to the inhomogeneous surface charge distribution of the cell membrane. Our work indicates that fluorescent labeling in general affects the binding behaviors, but proper design of the label will help to minimize its effect.