McCormick D, van der Rest M, Goodship J, Lozano G, Ninomiya Y, Olsen B R
Proc Natl Acad Sci U S A. 1987 Jun;84(12):4044-8. doi: 10.1073/pnas.84.12.4044.
Type IX collagen represents 5-20% of the total collagen in hyaline cartilage. The molecules of this collagen are composed of three genetically distinct polypeptide subunits. One of these subunits, alpha 2(IX), contains covalently bound glycosaminoglycan (chondroitin sulfate or dermatan sulfate). We report here on the structure of the glycosaminoglycan attachment site of type IX collagen-proteoglycan. We show, by a combination of cDNA and peptide sequencing, that the attachment region contains the sequence Gly-Ser-Ala-Asp, located within the noncollagenous domain NC3 of the alpha 2(IX) chain. By comparing the exons encoding the NC3 domain in the alpha 2(IX) and alpha 1(IX) genes, we find that the exon coding for the glycosaminoglycan attachment site in the alpha 2(IX) gene is 48 base pairs long, whereas the homologous alpha 1(IX) exon is 33 base pairs. The NC3 domain is, therefore, five amino acid residues longer in alpha 2(IX) than in alpha 1(IX). The extra sequence in alpha 2(IX), Val-Glu-Gly-Ser-Ala, provides a simple explanation for the kink observed at the NC3 domain of type IX molecules when examined by electron microscopy. The inserted block of amino acid residues also provides the NC3 domain of alpha 2(IX) chains with a serine residue, not present in alpha 1(IX) that serves as attachment site for a glycosaminoglycan side chain. Our data show that the amino acid sequence that surrounds the glycosylated serine residue in type IX collagen-proteoglycan differs from glycosylated sequences in noncollagenous core proteins. The data also provide strong evidence that glycosylation of type IX collagen is not a chance glycosylation of a serine residue in a noncollagenous domain, but is a specific post-translational modification of this unusual collagen molecule.
IX型胶原蛋白占透明软骨中总胶原蛋白的5%-20%。这种胶原蛋白的分子由三个基因不同的多肽亚基组成。其中一个亚基，α2(IX)，含有共价结合的糖胺聚糖（硫酸软骨素或硫酸皮肤素）。我们在此报告IX型胶原蛋白蛋白聚糖的糖胺聚糖附着位点的结构。我们通过cDNA和肽测序相结合的方法表明，附着区域包含位于α2(IX)链非胶原结构域NC3内的Gly-Ser-Ala-Asp序列。通过比较α2(IX)和α1(IX)基因中编码NC3结构域的外显子，我们发现α2(IX)基因中编码糖胺聚糖附着位点的外显子长48个碱基对，而同源的α1(IX)外显子长33个碱基对。因此，α2(IX)中的NC3结构域比α(IX)中的长五个氨基酸残基。α2(IX)中的额外序列Val-Glu-Gly-Ser-Ala，为通过电子显微镜检查时在IX型分子的NC3结构域观察到的扭结提供了一个简单的解释。插入的氨基酸残基块还为α2(IX)链的NC3结构域提供了一个α1(IX)中不存在的丝氨酸残基，该丝氨酸残基作为糖胺聚糖侧链的附着位点。我们的数据表明，IX型胶原蛋白蛋白聚糖中围绕糖基化丝氨酸残基的氨基酸序列与非胶原核心蛋白中的糖基化序列不同。数据还提供了强有力的证据，表明IX型胶原蛋白的糖基化不是非胶原结构域中丝氨酸残基的随机糖基化，而是这种特殊胶原蛋白分子的一种特异性翻译后修饰。
