Distinct isoforms of chicken decorin contain either one or two dermatan sulfate chains

Decorin, a member of a family of proteins with leucine-rich repeat motifs, is a widely distributed extracellular matrix proteoglycan that is thought to be responsible for the structure, tissue organization, and surface properties of fibrils. In mammals, decorin carries one chondroitin/dermatan sulfate chain as a distinction from its homologue, biglycan, which contains two glycosaminoglycan chains. With the aim to study decorin-collagen interactions in chicken, where the fibrillar organization of cartilage collagens is best understood, we have isolated decorin-related proteoglycans from sternal cartilage of 40-day-old broiler chickens. Small chondroitin/dermatan sulfate proteoglycans were resolved by hydrophobic interaction chromatography into two fractions, DCN I and DCN II. Both forms contained dermatan sulfate and, in addition, keratan sulfate chains. Tryptic fingerprinting revealed that the core proteins of DCN I and DCN II were identical. The protein was identified as decorin by amino-terminal sequencing. DCN II was found to contain two dermatan sulfate chains, whereas DCN I had a single dermatan sulfate chain. The dermatan sulfate attachment sites are located near the NH2 terminus of the core protein, i.e. at Ser-4 and Ser-16 in DCN II and at Ser-4 in DCN I. The keratan sulfate attachment sites are located in the central portion of the core protein, at Asn-179 and Asn-230. The presence of two dermatan sulfate chains renders the chicken proteoglycan DCN II structurally similar to mammalian biglycan. Interestingly, biglycan has not been detected in chicken. Therefore, in birds, DCN II may function as a biglycan substitute.