Effect of Cell Cortex Based Transient Confinement on Epidermal Growth Factor Receptor Interactions in Intact Cells

Abstract

Modifying diffusion of transmembrane receptors transiently changes their local concentration, which affects their rate of interaction, hence the cell signal. Underneath the plasma membrane bilayer is the cell cortex, a meshwork of actin and related filaments, which actively and passively interact with proteins in the plasma membrane. Diffusing transmembrane receptors are temporarily corralled between actin filaments, and traverse across filaments via hop diffusion. Receptors localized within corrals may have transient high frequency interactions before diffusing out of the corral, which may be dependent on size and confinement strength of the meshwork. In particular, dimerization rates of transmembrane protein epidermal growth factor receptor (EGFR) could be affected by this corralling, which can regulate build-up of second messenger signaling molecules to initiate a signal. Here we demonstrate how binned-imaging fluorescence correlation spectroscopy (bimFCS) can quantify corralling of epidermal growth factor receptor (EGFR). A novel technique developed by our lab, bimFCS measures diffusive behavior at multiple length scales simultaneously. Coupling this technique with total internal reflection fluorescence (TIRF), excitation of fluorescently-labeled EGFR is restricted near the basal membrane. Autocorrelation data is analyzed with a two-component diffusion model to separate fast short range diffusors from slower long range diffusors interacting with actin filaments. FCS Law is used to derive average confinement strength and size of corrals. The dependency of expression level and meshwork characteristics on receptor interactions, and as how this affects regulation of downstream signaling via intracellular phosphatases, is quantified through simulation. Our simulation suggests that, while overall quantity of protein dimerization is independent on how strongly proteins are confined, the period of high frequency dimerization events is dependent on confinement strength. The contribution that confinement has on high frequency receptor interactions also varies with expression level.