Isolation and partial characterization of a new human collagen with an extended triple-helical structural domain

The collagens are a family of major connective tissue proteins that are known to be the products of at least 11 distinct genes. Primary structural differences between the individual collagen types are thought to reflect functional diversity. We have isolated a previously unknown collagen gene product, termed "long-chain" (LC) collagen, from human chorioamniotic membranes by limited pepsin digestion. Comparison of the isolated alpha-chain subunit to the alpha chains of other collagen types by amino acid composition, peptide mapping with either cyanogen bromide fragmentation or staphylococcal V8 protease digestion, chromatographic elution position, and relative molecular weight indicates that this protein is a product of a previously unrecognized gene. We report structural studies indicating that this molecule contains three identical alpha-chain subunits that are each approximately molecular weight 170,000. The amino acid composition of LC alpha chains suggests that they are about 90% triple helical. Comparisons of the length of segment-long-spacing (SLS) crystallites made from LC molecules with those from types I and V collagens indicate that the LC molecule is substantially longer than these collagens and somewhat longer than the reported length of type IV collagen. This finding suggests that LC collagen represents an additional class of collagen molecules. We suggest that these molecules be referred to as type VII collagen.