Pepsin-generated type VI collagen is a degradation product of GP140

A major extracellular matrix glycoprotein, GP140 , synthesized by WI-38 human lung fibroblasts has previously been shown to be collagen-like. A form of GP140 that is related to extracellular matrix GP140 both antigenically and in apparent molecular mass was isolated from human placenta. Types I-VI collagen were isolated from human tissues by limited pepsin digestion, selective salt precipitation, and chromatography. Immunoblot analysis of the collagens and GP140 utilizing affinity-purified polyclonal antiserum directed against extracellular matrix GP140 demonstrated cross-reactivity of antibodies with type VI collagen. Both type VI collagen and matrix GP140 could be digested with bacterial collagenase following reduction with dithiothreitol but were collagenase insensitive under nonreducing conditions, unlike types I-V collagen. Placental and matrix GP140 and type VI collagen were shown to have receptors for 125I-labeled Lens culinaris lectin. Pepsin digestion of WI-38 extracellular matrix GP140 yielded a 64,000-dalton band which co-migrated with subunits of reduced type VI collagen on Coomassie-stained sodium dodecyl sulfate-polyacrylamide gel electrophoresis gels, reacted with anti- GP140 antiserum and 125I-labeled L. culinaris lectin, and was collagenase-sensitive only under reducing conditions. CNBr fragmentation of extracellular matrix GP140 , the 64,000-dalton pepsin-resistant peptide of GP140 and type VI collagen followed by immunoblot analysis using anti- GP140 revealed similarities in peptide maps of GP140 and type VI collagen. Our data strongly suggest that GP140 and type VI collagen share characteristics that differ from those of other collagen types and that intermolecular disulfide bonding appears to stabilize these molecules in their native unreduced form, thus conferring collagenase resistance. Finally, the SC1 and SC2 subunits of type VI collagen appear to be generated by pepsin digestion of GP140 .