The Muscular Dystrophy Gene TMEM5 Encodes a Ribitol β1,4-Xylosyltransferase Required for the Functional Glycosylation of Dystroglycan

A defect in O-mannosyl glycan is the cause of alpha-dystroglycanopathy, a group of congenital muscular dystrophies caused by aberrant a-dystroglycan (alpha-DG) glycosylation. Recently, the entire structure of O-mannosyl glycan, [3GlcA beta 1-3Xyl alpha 1](n)-3GlcA beta 1-4Xyl-Rbo5P-1Rbo5P-3GalNAc beta 1-3GlcNAc beta 1-4 (phospho-6)Man alpha 1-, which is required for the binding of alpha-DG to extracellular matrix ligands, has been proposed. However, the linkage of the first Xyl residue to ribitol 5-phosphate (Rbo5P) is not clear. TMEM5 is a gene product responsible for alpha-dystroglycanopathy and was reported as a potential enzyme involved in this linkage formation, although the experimental evidence is still incomplete. Here, we report that TMEM5 is a xylosyltransferase that forms the Xyl beta 1-4Rbo5P linkage on O-mannosyl glycan. The anomeric configuration and linkage position of the product (beta 1,4 linkage) was determined by NMR analysis. The introduction of two missense mutations in TMEM5 found in alpha-dystroglycanopathy patients impaired xylosyltransferase activity. Furthermore, the disruption of the TMEM5 gene by CRISPR/Cas9 abrogated the elongation of the (-3GlcA beta 1-3Xyl alpha 1-) unit on O-mannosyl glycan. Based on these results, we concluded that TMEM5 acts as a UDP-e-xylose:ribitol-5-phosphate beta 1,4-xylosyltransferase in the biosynthetic pathway of O-mannosyl glycan.