Local ligand concentration gradients induced by the plasma membrane

The plasma membrane is a dynamic structure surrounded by the extracellular matrix. Although stimulation of transmembrane proteins by soluble ligands takes place in this environment, its influence on receptor activation is usually overlooked. In the current manuscript, we quantitatively measured the concentration of epidermal growth factor (EGF) at the cell membrane and found that ligand distribution is non-homogeneous due to two concentration peaks causing significantly higher EGF concentrations at the membrane than in the bulk solution. Using experiments, calculations, and simulations, we show that the membrane-proximal peak is caused by membrane turnover, while the more distant one, only present in ErbB2-expressing cells, is generated by locally hindered diffusion possibly caused by the extracellular matrix. Both theory and experiments reveal that these phenomena increase the apparent affinity and decrease the apparent cooperativity of the receptor-ligand complex. Interpretation of the effects of soluble ligands interacting with receptors must involve this non-homogenous concentration profile.