Surface iodination of epidermal growth factor receptors in intact cells

The membranes from epidermoid carcinoma cells (A-431) that were surface iodinated while intact using catalysis by lactoperoxidase and 125I as iodide contain one major labeled protein of Mr = 180,000. This protein is clearly iodinated on the outside of the intact cell because it is not the major protein labeled when isolated membranes are iodinated. This major surface-iodinated protein is almost certainly the epidermal growth factor (EGF) receptor, since both have the same Mr and have identical sensitivity to proteases. Both are nearly quantitatively converted from an Mr = 180,000 form to an Mr = 160,000 form by an endogenous calcium-activated neutral protease when cells are broken in the presence of calcium. Both are degraded by trypsin only if trypsin has access to the inside of the cell. This latter finding implies that the surface-iodinated EGF receptor spans the plasma membrane. Since the EGF receptor is an autophosphorylating kinase whose activity is enhanced in the presence of EGF, the receptor was labeled and identified using [gamma-32P] ATP. While both iodination and EGF-enhanced phosphorylation occur on tyrosine residues, peptide mapping of the iodinated or phosphorylated Mr = 180,000 band showed that different peptides were being labeled. Since the EGF receptor-kinase spans the plasma membrane, the peptide iodinated on the surface of the intact cell must be different from the peptides that are probably autophosphorylated on the cytoplasmic side of the membrane.