Intracellular processing of epidermal growth factor. I. Acidification of ¹²⁵I-epidermal growth factor in intracellular organelles

We previously reported that ¹²⁵I-labeled epidermal growth factor is processed intracellularly to acidic macromolecules in Rat-1 fibroblasts. The present study defines the precursor-product relationship and localization of the processing steps to subcellular organelles by the use of a single isoelectric species of ¹²⁵I-epidermal growth factor and Percoll gradient fractionation. The native pI 4.55 ¹²⁵I-epidermal growth factor was rapidly processed to a pI 4.2 species on or near the cell surface and in organelles corresponding to clathrin-coated vesicles, Golgi, and endoplasmic reticulum. This species was then processed to a pI 4.35 species in similar organelles. The pI 4.2 and 4.35 species were converted to a pI 4.0 species in dense, lysosome-like organelles. This species was ultimately degraded and exocytosed from the cell as low molecular weight products.