Intracellular localization of fibronectin by immunoelectron microscopy.

We have localized fibronectin, a major extracellular glycoprotein of cultured fibroblasts, in chick embryo fibroblasts at the ultrastructural level using affinity-purified antibodies to fibronectin. The use of a ferritin bridge procedure permitted quantitation of localization in various organelles. These results provide the first intracellular ultrastructural localization of fibronectin. Extracellular labeling was confined to aggregates and fibrils, with little or no labeling of the plasma membrane. The principal sites of intracellular localization were the rough endoplasmic reticulum and the Golgi apparatus. Treatment of cells with the protein synthesis inhibitors cycloheximide and pactamycin reduced fibronectin localization in the endoplasmic reticulum to 50% of normal levels. Removal of cycloheximide permitted recovery of labeling to 85% of control levels in the endoplasmic reticulum. Similar, but much reduced, changes also occurred in the Golgi apparatus.