The Keratan Sulfate-enriched Region of Bovine Cartilage Proteoglycan Consists of a Consecutively Repeated Hexapeptide Motif

Abstract

We have determined the sequence of a cDNA clone encoding the keratan sulfate-rich domain of the large aggregating cartilage proteoglycan core protein. The C-terminal portion of the deduced amino acid sequence is homologous to the chondroitin sulfate-rich region (domain CS1) of the rat chondrosarcoma proteoglycan, and the N-terminal portion is homologous to the second globular domain (G2) of the rat proteoglycan (Doege, K., Sasaki, M., Horigan, E., Hassell, J. R., and Yamada, Y. (1987) J. Biol. Chem. 262, 17757-17767). We could identify, inserted between these regions, a region absent in the rat proteoglycan. This domain corresponds to the keratan sulfate-enriched region of the bovine proteoglycan. It consists of a highly conserved hexapeptide motif consecutively repeated 23 times. Transfer blot analysis of genomic DNA indicated a single gene. The coding region for the keratan sulfate-enriched region was present both in human and bovine DNA, whereas the coding region for this domain appears to be absent in the rat genome. Transfer blot analysis of RNA showed that the keratan sulfate-rich region is present in proteoglycans from fetal as well as adult sources. Furthermore, RNA protection assays of RNA isolated from adult and fetal bovine articular cartilage showed that no alternative splicing occurs within this keratan sulfate-enriched region. These experiments show that the fetal bovine cartilage proteoglycan contains the keratan sulfate attachment domain, although it lacks the keratan sulfate side chains.