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Role of Glycosyltransferase 25 domain 1 in Type I Collagen Glycosylation and Molecular Phenotypes.

Glycosylation in type I collagen occurs as O-linked galactosyl- (G-) lesser and glucosylgalactosyl-hydroxylysine (GG-Hyl), however, its biological significance is still not well understood. To investigate the function of this modification in bone, we have generated preosteoblast MC3T3-E1 (MC)-derived clones, short hairpin (Sh) clones, in which Glt25d1 gene expression was stably suppressed. In Sh clones, the GLT25D1 protein levels were markedly diminished in comparison to controls (MC and those transfected with empty vector). In Sh collagen, both G- and GG-Hyl were significantly diminished with a concomitant increase of free-Hyl. In addition, the immature divalent cross-links significantly diminished while the mature trivalent cross-link increased. By mass spectrometric analysis, seven glycosylation sites were identified in type I collagen and the most predominant site was at the helical cross-linking site, α1-87. At all of the glycosylation sites, the relative levels of G- and GG-Hyl were markedly diminished, i.e. by ~50-75%, in Sh collagen, and at five of these sites, Lys hydroxylation was significantly increased. The collagen fibrils in Sh clones were larger and mineralization was impaired. These results indicate that GLT25D1 catalyzes galactosylation of Hyl throughout the type I collagen molecule and that this modification may regulate maturation of collagen cross-linking, fibrillogenesis, and mineralization.