Partial primary structure of bovine plasma fibronectin: Three types of internal homology (plasmin digestion/gelatin and heparin binding/interchain disulfide bridge/phosphorylation/gene multiplication)

Approximately one-half of the amino acid se- quence (911 amino acid residues out of 1,880 expected) for bovine plasma fibronectin (cold-insoluble globulin) has been determined. Three types of internal homology were identified, showing that a number of partial gene duplications (multiplications) have oc- curred during the evolution of this protein. Digestion of fibro- nectin with plasmin results in major fragments with molecular masses of 29, 170, 23, and 6 kilodaltons (kDal). The NH2-terminal 29-kDal fragment consists of 259 residues ordered as five mutually homologous domains (type I homology) with two disulfide bonds in each domain. The 170-kDal fragment shows two to three bands after NaDodSO4 gel electrophoresis, indicating heterogeneity. This fragment contains the gelatin binding site and the strong hep- arin binding site present in fibronectin. Digestion of the 170-kDal fragment with chymotrypsin liberates a 45-kDal fragment that also binds to gelatin. This fragment contains at least one domain of type I homology and two domains of type II homology. Further diges- tion of the 170-kDal fragment with chymotrypsin results in the formation of a 30-kDal fragment that retains the heparin binding activity. This fragment contains sequences constituting type III homology. The 23-kDal fragment consists of 178 residues having three domains of type I homology. The 6-kDal fragment consists of two identical peptides of 26 residues, and these two peptides are linked to each other by two disulfide bonds that form the inter- chain bridges. Another one of the peptides for which the sequence was determined links the COOH-terminus of the 29-kDal frag- ment to the NH2-terminus of the 170-kDal fragment. This and the fact that the COOH-terminal residue of the 6-kDal fragment is a glutamic acid residue order the four plasmin-digestion fragments as 29-, 170-, 23-, and 6-kDal in the intact fibronectin molecule.