Native collagen molecules containing A and B chains were isolated from pepsin-solubilised human chorionic and amniotic membrane extracts by fractional salt precipitation and DEAEcellulose chromatography. They exhibited a circular dichroism spectrum, and a melting curve, characteristic for a triple-helical structure. Electron microscopical investigations of their segmentlong-spacing crystallites revealed a molecule similar to those of the interstitial types I, I1 and I11 collagens. After denaturation, the A and B chains were separated by DEAE-cellulose chromatography and were consistently recovered in a ratio of 1 :2. Renaturation experiments indicated that only the B chains are able to reform triple-helical molecules which are stable under conditions in vivo. The data support a molecular formula A(B)2 for the native collagen molecule.

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 a-chain subunit to the a 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 a-chain subunits that are each approximately molecular weight 170,000. The amino acid composition of LC a 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.Human connective tissues are known to contain multiple collagen types (1, 2). Documented structural differences between the collagen types suggest that they serve unique functions and are, in part, responsible for the variations in biochemical and physiological properties observed between different connective tissues (2). For example, the interstitial collagens (types I, II, and III) all have the same helical length (3), are ultimately post-translationally processed to a similar degree (4), are susceptible to the same degradative enzyme systems (5), and by aligning in D-staggered array, form banded fibers that are stabilized by lysine and hydroxylysine-derived crosslinks (6). In contrast, type IV collagen in basement membranes is not proteolytically processed extracellularly (7)(8)(9), has regions of increased flexibility and susceptibility to various proteases (10-12), has a triple-helical domain 1.3 times the length of type I collagen (13)(14)(15), and has been shown to form a fiber system dependent upon the presence of molecular domains that are analogous to the interstitial collagen propeptides (16). This fiber system is believed to be unique to basement membrane collagen and to underlie the distinct function of that specialized connective tissue structure.Type V procollagen has been shown to undergo only a limited degree of proteolytic processing (17-19). The structure of the matrix form of these molecules is unknown, but one would predict that its fiber form might be different from that of the interstitial collagens or of basement membrane collagen.Recently, several additional collagenous pro...

cDNA clones encoding osteoinductive factor (OIF) have been isolated from a bovine osteoblast library. Sequence analysis of these clones indicated that the 105-amino-acid OIF is synthesized as a larger 299-amino-acid precursor, the carboxyl terminus of which is cleaved to yield the mature protein. Northern blot analysis of bovine osteoblast mRNA revealed two OIF-specific transcripts of 1.9 and 2.4 kb. The polymerase chain reaction was used to obtain clones coding for human OIF from the osteosarcoma cell line, MG-63. The human OIF cDNA encodes a precursor of 298 amino acids that exhibits 94% identity to the bovine protein. Northern blot analysis of various cell lines and tissues indicated that expression of OIF transcripts is limited and may be restricted to cells of bone lineage.

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