Distribution of resting and ligand-bound ErbB1 and ErbB2 receptor tyrosine kinases in living cells using number and brightness analysis

Ligand-driven dimerizations of ErbB receptor subunits fulfill a fundamental role in their activation. We have used the number and brightness analysis technique to investigate the existence of preformed ligand-independent dimers and clusters and to characterize the initial steps in the activation of ErbB1 and ErbB2. In cells expressing 50,000-200,000 receptors, ErbB1 was monomeric in the absence of ligand stimulation, whereas in CHO cells with receptor levels > 500,000 as much as 30% of ErbB1 was present as preformed dimers. EGF induced the formation of ErbB1 dimers as well as larger clusters (up to pentamers) that colocalized with clathrin-coated pits. The distribution of unstimulated ErbB2 in cells expressing 3.10(5) - 10(6) receptors was fundamentally different, in that this receptor was present in preformed homoassociated aggregates containing 5-10 molecules. These constitutive ErbB2 homoclusters colocalized with caveolae, increased in size at subphysiological temperatures, but decreased in size upon EGF stimulation. We conclude that these ErbB2 clusters are promoted primarily by membrane-mediated interactions and are dispersed upon ligand stimulation.