ZFIN ID: ZDB-PUB-160501-5
The functional O-mannose glycan on α-dystroglycan contains a phospho-ribitol primed for matriglycan addition
Praissman, J.L., Willer, T., Sheikh, M.O., Toi, A., Chitayat, D., Lin, Y.Y., Lee, H., Stalnaker, S.H., Wang, S., Prabhakar, P.K., Nelson, S.F., Stemple, D.L., Moore, S.A., Moremen, K.W., Campbell, K.P., Wells, L.
Multiple glycosyltransferases are essential for the proper modification of alpha-dystroglycan, as mutations in the encoding genes cause congenital/limb-girdle muscular dystrophies. Here we elucidate further the structure of an O-mannose-initiated glycan on alpha-dystroglycan that is required to generate its extracellular matrix-binding polysaccharide. This functional glycan contains a novel ribitol structure that links a phosphotrisaccharide to xylose. ISPD is a CDP-ribitol (ribose) pyrophosphorylase that generates the reduced sugar nucleotide for the insertion of ribitol in a phosphodiester linkage to the glycoprotein. TMEM5 is a UDP-xylosyl transferase that elaborates the structure. We demonstrate in a zebrafish model as well as in a human patient that defects in TMEM5 result in muscular dystrophy in combination with abnormal brain development. Thus, we propose a novel structure - a ribitol in a phosphodiester linkage - for the moiety on which TMEM5, B4GAT1, and LARGE act to generate the functional receptor for ECM proteins having LG domains.