Human Xylosyltransferase II Is Involved in the Biosynthesis of the Uniform Tetrasaccharide Linkage Region in Chondroitin Sulfate and Heparan Sulfate Proteoglycans

Abstract

Human xylosyltransferase I (XT-I) initiates the biosynthesis of the glycosaminoglycan (GAG) linkage tetrasaccharide in proteoglycans. Xylosyltransferase II (XT-II) is a protein homologous to XT-I but with hitherto unknown activity or physiological function. Here, we report the enzymatic activity of XT-II and provide evidence that XT-II initiates the biosynthesis of both heparan sulfate and chondroitin sulfate GAGs. Transfection of the xylosyltransferase-deficient Chinese hamster ovary mutant pgsA-745 with XT-I or XT-II coding cDNA completely restored GAG biosynthesis. GAG disaccharide analysis revealed that XT-I-and XT-II-transfected pgsA-745 cells produced similar amounts of chondroitin sulfate and heparan sulfate. Furthermore, a high xylosyltransferase activity was measured after transfection with cDNAs encoding either isozyme. Analysis of the enzyme activity revealed that XT-II catalyzes the transfer of xylose to similar peptide acceptors as XT-I but with different efficiency. The optimal XT-II acceptor was observed using a bikunin-related peptide (K m 5.2 M). Analysis of XT-I and XT-II mRNA expression in murine tissues showed a differential expression pattern for both enzymes. In particular, XT-II is highly expressed in liver tissue, where XT-I transcripts were not detected. This is the first report on the enzyme activity of XT-II and its involvement in chondroitin sulfate and heparan sulfate biosynthesis.

Transforming Growth Factor β1-regulated Xylosyltransferase I Activity in Human Cardiac Fibroblasts and Its Impact for Myocardial Remodeling

“…Both enzymes are capable of initiating the biosynthesis of chondroitin sulfate, dermatan sulfate, and heparan sulfate glycosaminoglycan chains (17,30,31). Until now we and others have not found any differences of both xylosyltransferases in regards to acceptor specificity or a preference for the biosynthesis of either chondroitin sulfate or heparan sulfate proteoglycans.…”

“…The GAGs chondroitin sulfate, dermatan sulfate, heparan sulfate, and heparin are bound to the proteoglycan core protein by a xylose-galactose-galactose binding region. Xylosyltransferase I (XT-I) and xylosyltransferase II (XT-II) are the initiating and apparently rate-limiting enzymes involved in GAG biosynthesis catalyzing the transfer of UDP-xylose to specific serine residues of the core proteins (15)(16)(17). We have shown previously that the XT-I enzyme is secreted from the endoplasmic reticulum into the extracellular space together with chondroitin sulfate (CS) proteoglycans (18).…”

Human xylosyltransferases in health and disease

“…The enzyme transferred xylose from UDP-d-xylose to serine residues in bikunin, silk and other core proteins, forming a blinkage which was selectively labile to b-xylosidase treatment and alkaline b-elimination [18]. Full-length forms of human XT-I have also been successfully expressed in CHO [25], HEK293 and SaOS-2 cells [19]. During cloning the cDNA of human XT-I, another cDNA from human and rat sources was cloned which coded for a different protein with high homology to XT-I (Fig.…”

Polycystic disease caused by deficiency in xylosyltransferase 2, an initiating enzyme of glycosaminoglycan biosynthesis

“…We have investigated the role that PGs play in the development of human polycystic disease by inactivating xylosyltransferase 2 [(XylT2) Enzyme Commission no. 2.4.2.26], an enzyme shown to be involved in HS and CS biosynthesis (25)(26)(27). We have shown in an experimental model system that reduced PGs are cystogenic.…”