Cloning and expression of a proteoglycan UDP-galactose:β-xylose β1,4-galactosyltransferase I -: A seventh member of the human β4-galactosyltransferase gene family

A seventh member of the human beta 4-galactosyltransferase family, beta 4Gal-T7, was identified by BLAST analysis of expressed sequence tags. The coding region of beta 4Gal-T7 depicts a type II transmembrane protein with sequence similarity to beta 4-galactosyltransferases, but the sequence was distinct in known motifs and did not contain the cysteine residues conserved in the other six members of the beta 4Gal-T family. The genomic organization of beta 4Gal-T7 was different from previous beta 4Gal-Ts, Expression of beta 4Gal-T7 in insect cells showed that the gene product had beta 1,4-galactosyltransferase activity with beta-xylosides, and the linkage formed was Gal beta 1-4Xyl, Thus, beta 4Gal-T7 represents galactosyltransferase I enzyme (xylosylprotein pl,4-galactosyltransferase; EC 2,4,1,133), which attaches the first galactose in the proteoglycan linkage region GlcA beta 1-3Gal beta 1-3Gal beta 1-4Xyl beta 1-O-Ser, Sequence analysis of beta 4Gal-T7 from a fibroblast cell line of a patient with a progeroid syndrome and signs of the Ehlers-Danlos syndrome, previously shown to exhibit reduced galactosyltransferase I activity (Quentin, E,, Gladen, A., Roden, L., and Kresse, H, (1990) Proc. Natl, Acad. Sci, U, S. A. 87, 1342-1346), revealed two inherited allelic variants, beta 4Gal-T7(186D) and beta 4Gal-T7(206P), each with a single missense substitution in the putative catalytic domain of the enzyme. beta 4Gal-T7(186D) exhibited a 4-fold elevated K-m for the donor substrate, whereas essentially no activity was demonstrated with beta 4Gal-T7(206P). Molecular cloning of beta 4Gal-T7 should facilitate general studies of its pathogenic role in progeroid syndromes and connective tissue disorders with affected proteoglycan biosynthesis.