The phosphate content of human fibronectin.

The nature and distribution of the covalently bound phosphate of human fibronectin have been investigated. Phosphate was detected only as phosphoserine and is not present on asparagine-linked oligosaccharides. Fibroblast fibronectin labeled with 3H-amino acids and [32P]orthophosphate was subjected to limited tryptic digestion. Initial cleavages which reduced the molecule to nondisulfide-bonded polypeptides of Mr approximately 210,000-220,000 did not entail the loss of phosphate. Subsequent cleavage products included intermediate-sized 32P-phosphopeptides (43,000 and 37,000), but all 32P label became increasingly associated with lower molecular weight material (13,000 and 9,000) eventually terminating in a phosphopeptide (or peptides) of Mr approximately 6,000. The phosphorylated region(s) of human fibronectin are apparently not located at the extreme COOH-terminal region containing the interchain disulfide bond, but seem to be within a 40,000-50,000 segment near one end of the molecule. Cyanogen bromide cleavage of 32P-labeled fibronectin isolated from fibroblast culture medium produced one major phosphopeptide (Mr approximately 5,000) which co-purified through two chromatographic procedures with a CNBr phosphopeptide derived from plasma fibronectin. The phosphorylation of fibronectin is evidently restricted to a small region of the protein molecule, implying a very site-specific event. The phosphorylated region is present on human fibronectins obtained from plasma, fibroblast surfaces, and fibroblast culture medium, indicating that the role of the phosphate may be important to all forms of fibronectin.