A receptor is normally triggered when, and only when, its ligand binds to the receptor. It's not so simple for the ErbB1 receptor, which responds to stimulation with epidermal growth factor (EGF) by dimerization, cross phosphorylation of the monomers, and triggering of downstream pathways. Verveer et al. have visualized phosphorylated ErbB1 in MCF7 cells with fluoresence lifetime imaging microscopy in conjunction with fluorescence resonance energy transfer, which was used to monitor the binding of an antibody to phosphotyrosine. Focal stimulation of the cell with EGF immobilized on beads resulted in a rapid propagation of receptor phosphorylation over the entire surface of the cell--even those receptors that had not been exposed to EGF. Verveer, PJ., Wouters, F.S., Reynolds, A.R., and Bastiaens, P.I.H. (2000) Quantitative imaging of lateral ErbB1 receptor signal propagation in the plasma membrane. Science 290 : 1567-1570. [Abstract] [Full Text]