DECORIN-BINDING SITES FOR COLLAGEN TYPE-I ARE MAINLY LOCATED IN LEUCINE-RICH REPEATS 4-5

Decorin and biglycan are structurally related interstitial proteoglyca ns synthesized in connective tissues like skin, tendon, and cartilage. Despite the conspicuous sequence similarities, where about 55% of the amino acid residues in decorin and biglycan are located in identical positions, the two proteoglycans show differences in their interaction with collagen. Decorin binds to collagen type I, whereas biglycan in several assay systems shows no affinity for this collagen type. Here w e have made use of these structural similarities and affinity differen ces in studies of the collagen binding properties of decorin. Recombin ant biglycan/decorin chimeras were produced in mammalian cells and ana lyzed for their capacity to bind collagen. In the chimeras, biglycan c ontributes sequences crucial for synthesis and export from the mammali an cells, and decorin provides potential collagen-binding properties. By using this approach we show that decorin binds to the collagen prim arily via leucine-rich repeats 4-5 composed of some 40 amino acid resi dues. Proteoglycan chimeras containing decorin sequences from the N te rminus to leucine-rich repeat 3 or sequences from leucine rich repeat 6 to the C terminus do not show any detectable binding to collagen. A proteoglycan chimera containing decorin leucine rich repeats 4-5 flank ed by biglycan sequences binds to collagen. However, this chimera bind s to collagen with somewhat lower affinity than wild type decorin, sug gesting that additional low affinity binding sites may be located in o ther parts of decorin. Alternatively, the conformation of the collagen binding leucine-rich repeats 4-5 are different in decorin and in the biglycan/decorin chimera, leading to a lower collagen affinity for the latter.