Primary structure of human plasma fibronectin. The 29,000-dalton NH2-terminal domain

The complete amino acid sequence of the NH2-terminal domain obtained after trypsin digestion of human plasma fibronectin has been determined. It contains residues 1-259 and has a Mr of 29,000. The 29-kDa fragment was isolated from the other major tryptic cleavage products of 200, 180, and 31 kDa by affinity chromatography on gelatin- and heparin-Sepharose columns. The two high Mr fragments bound to gelatin and were easily removed; the 31-kDa fragment failed to bind to heparin while the 29-kDa fragment did and was eluted with 0.15 M NaCl. The 29-kDa domain has a blocked NH2-terminal (pyrrolidone carboxylic acid) which was removed by digestion with pyroglutamate aminopeptidase, and the amino acid sequence of the first 36 residues was obtained. The sequence showed a glutamine residue at position 3 which is probably the acceptor site for transglutaminase as reported for bovine fibronectin. Extensive trypsin digestion of the completely reduced and alkylated 29-kDa fragment yielded twenty-four peptides which were separated and purified by high performance liquid chromatography; their amino acid composition and amino acid sequence has been determined and the arrangement of peptides was achieved by comparison with the sequence recently reported for bovine fibronectin. The sequences in human and bovine fibronectin were nearly identical with only nine amino acid differences which can all be explained by single base substitutions. Apparently this domain is highly conserved in the two species studied thus far.