125I-fibronectin was partially digested by trypsin and the interaction of the labelled fragments produced with cultured cells and isolated nuclei was studied. Fragments not only became associated with the surface of cultured cells but a significant amount of radioactivity remained associated with the cell pellet following trypsin treatment, indicating that certain fragments may have been internalized. In addition to binding at the cell surface, fibronectin fragments also showed the ability to bind to isolated nuclei in a dose-dependent nonsaturable manner. Considerably reduced binding was observed at 4 degrees C compared to that at 25 degrees C. Cells allowed to grow and proliferate in the presence of 125I-fibronectin also incorporated the label onto their cell surface. A significant proportion of radioactivity again remained associated with the cell pellet after trypsin treatment and on subsequent fractionation approximately one-half of this radioactivity was found to co-purify with the isolated nuclear pellet in the form of low molecular weight peptides Mr 20,000 to 50,000.
Animals Azides