LARGE AND SMALL SPLICE VARIANTS OF COLLAGEN-XII - DIFFERENTIAL EXPRESSION AND LIGAND-BINDING

Collagen XII has a short collagenous tail and a very large, three-arme d NC3 domain consisting primarily of fibronectin type III repeats. Dif ferential splicing within this domain gives rise to a large (320 kD) a nd a small (220 kD) subunit; the large but not the small can carry gly cosaminoglycan. To investigate whether collagen XII variants have dist inct expression patterns and functions, we generated antibody and cDNA probes specific for the alternatively spliced domain. We report here that the large variant has a more restricted expression in embryonic t issue than the small. For example, whereas the small variant is widesp read in the dermis, the large is limited to the base of feather buds, Distinct proportions of mRNA for the two variants were detected depend ing on the tissue. Monoclonal antibodies allowed us to separate collag en XII variants, and to show that homo- and heterotrimers exist. Colla gen XII variants differ in ligand binding. Small subunits interact wea kly with heparin via their COOH-terminal domain. Large subunits have a dditional, stronger heparin-binding site(s) in their NH2-terminal extr a domain. In vivo, both large and small collagen XII are associated wi th interstitial collagen. Here we show biochemically and ultrastructur ally that collagen XII can be incorporated into collagen I fibrils whe n it is present during, but not after, fibril formation. Removal of th e collagenous domain of collagen XII reduces its coprecipitation with collagen I. Our results indicate that collagen XII is specifically ass ociated with fibrillar collagen, and that the large variant has bindin g sites for extracellular ligands not present in the small variant.