Biosynthesis and processing of fibronectin in NIL.8 hamster cells.

Fibronectin is synthesized as a monomeric polypeptide chain. As early as it can be detected inside the cell, it carries carbohydrate side chains. These chains are sensitive to endoglycosidase H, suggesting that they are asparagine linked and high mannose form. The monomeric chains quickly dimerize while still inside the cell. Newly synthesized fibronectin appears as a dimer, both at the cell surface and secreted into the culture medium, about 30 min after commencement of labeling. This exported fibronectin has endoglycosidase H-resistant carbohydrate side chains, indicating processing from the high mannose form to a complex form. Exit of fibronectin to the outside of the cell follows quickly on carbohydrate processing; no large pool of endoglycosidase H-resistant fibronectin exists inside the cell. The dimeric fibronectin at the cell surface is initially deoxycholate soluble but slowly becomes deoxycholate-insoluble and also slowly forms high molecular weight aggregates which require reduction of disulfide bonds for their dissociation.