Early and late cathepsin D-derived fragments of fibronectin containing the C-terminal interchain disulfide cross-link.
Fibronectin consists of two very similar subunits connected by disulfide bonds close to their C-terminal ends. After short digestion with cathepsin D a fragment containing an intact subunit linked to a C-terminal piece of the other chain was isolated. Both possible combinations were realized. The remnant chain, removed by reduction, had a relative molecular mass (Mr) of 65000 or 75000, respectively, whether it originated from the shorter or the longer fibronectin subunit. After prolonged cathepsin D digestion another disulfide-linked fragment of Mr 90000 containing peptide chains of Mr 65000 and Mr 36000 was found. The same fragment also formed during prolonged digestion of a previously described cathepsin D-derived heparin-binding piece of Mr 140000 which consisted of disulfide-linked chains of Mr 65000 and Mr 75000. The cut-off material emerged as a mixture of two similar heparin binding peptides of Mr 35000 and Mr 37000. The residual disulfide-linked peptide of Mr 90000 was retained by heparin-Sepharose, too, indicating that the non-degraded chain of Mr 65000 also contained a heparin-binding site. Plasminolysis of the Mr 140000 fragment first liberated the two individual chains and subsequently cleaved the longer one into two domains of nearly equal size. A rather basic one (pI 8.4-8.8) with low cystine content bound to heparin-Sepharose, while the other cystine-rich and more acidic domain (pI 5.0-5.8) was not retained. The shorter peptide chain, also showing affinity to heparin, was attacked by plasmin rather slowly. It, however, also appeared to be composed of two differently charged regions as it had an isoelectric point in the neutral range (pI 6.4-6.8) in spite of its heparin-binding properties. In the longer chain the heparin-binding domain was located in a distal position to the C-terminal disulfide link. The same is assumed for the shorter chain by reason of homology.