Core Glycosylation of Collagen Is Initiated by Two β(1-O)Galactosyltransferases

Abstract

Collagen is a trimer of three left-handed alpha chains representing repeats of the motif Gly-X-Y, where (hydroxy)proline and (hydroxy)lysine residues are often found at positions X and Y. Selected hydroxylysines are further modified by the addition of galactose and glucose-galactose units. Collagen glycosylation takes place in the endoplasmic reticulum before triple-helix formation and is mediated by β(1-O)galactosyl- and α(1-2)glucosyltransferase enzymes. We have identified two collagen galactosyltransferases using affinity chromatography and tandem mass spectrometry protein sequencing. The two collagen β(1-O)galactosyltransferases corresponded to the GLT25D1 and GLT25D2 proteins. Recombinant GLT25D1 and GLT25D2 enzymes showed a strong galactosyltransferase activity toward various types of collagen and toward the serum mannose-binding lectin MBL, which contains a collagen domain. Amino acid analysis of the products of GLT25D1 and GLT25D2 reactions confirmed the transfer of galactose to hydroxylysine residues. The GLT25D1 gene is constitutively expressed in human tissues, whereas the GLT25D2 gene is expressed only at low levels in the nervous system. The GLT25D1 and GLT25D2 enzymes are similar to CEECAM1, to which we could not attribute any collagen galactosyltransferase activity. The GLT25D1 and GLT25D2 genes now allow addressing of the biological significance of collagen glycosylation and the importance of this posttranslational modification in the etiology of connective tissue disorders.

SUPPLEMENTAL MATERIAL

Supplemental material for this article may be found at http://mcb.asm.org/ .

ACKNOWLEDGMENTS

We thank Beat Steinmann (University Children's Hospital Zurich, Zurich, Switzerland) for helpful discussions, B. Roschitzki (Functional Genomic Center Zurich [FGCZ], Zurich, Switzerland) and the FGCZ for their support, and R. Tenni (University of Pavia, Pavia, Italy) for the GHyl and GGHyl amino acid standards.

This work was supported by Telethon Action Suisse, the Wolfermann-Nägeli Foundation, and grants from the Swiss National Science Foundation (PP00A-106756 and 3100A0-116039).