title: Glycosylation of α-dystroglycan: O-mannosylation influences the subsequent addition of GalNAc by UDP-GalNAc polypeptide N-acetylgalactosaminyltransferases.
abstract: O-Linked glycosylation is a functionally and structurally diverse type of protein modification present in many tissues and across many species. α-Dystroglycan (α-DG), a protein linked to the extracellular matrix, whose glycosylation status is associated with human muscular dystrophies, displays two predominant types of O-glycosylation, O-linked mannose (O-Man) and O-linked N-acetylgalactosamine (O-GalNAc), in its highly conserved mucin-like domain. The O-Man is installed by an enzyme complex present in the endoplasmic reticulum. O-GalNAc modifications are initiated subsequently in the Golgi apparatus by the UDP-GalNAc polypeptide N-acetylgalactosaminyltransferase (ppGalNAc-T) enzymes. How the presence and position of O-Man influences the action of the ppGalNAc-Ts on α-DG and the distribution of the two forms of glycosylation in this domain is not known. Here, we investigated the interplay between O-Man and the addition of O-GalNAc by examining the activity of the ppGalNAc-Ts on peptides and O-Man-containing glycopeptides mimicking those found in native α-DG. These synthetic glycopeptides emulate intermediate structures, not otherwise readily available from natural sources. Through enzymatic and mass spectrometric methods, we demonstrate that the presence and specific location of O-Man can impact either the regional exclusion or the site of O-GalNAc addition on α-DG, elucidating the factors contributing to the glycosylation patterns observed in vivo. These results provide evidence that one form of glycosylation can influence another form of glycosylation in α-DG and suggest that in the absence of proper O-mannosylation, as is associated with certain forms of muscular dystrophy, aberrant O-GalNAc modifications may occur and could play a role in disease presentation.
authors.affiliation: Developmental Glycobiology Unit, NIDCR, National Institutes of Health, Bethesda, Maryland 20892, USA.
authors.forename: Duy T
authors.initials: DT
authors.lastname: Tran
authors.forename: Jae-Min
authors.initials: JM
authors.lastname: Lim
authors.forename: Mian
authors.initials: M
authors.lastname: Liu
authors.forename: Stephanie H
authors.initials: SH
authors.lastname: Stalnaker
authors.forename: Lance
authors.initials: L
authors.lastname: Wells
authors.forename: Kelly G
authors.initials: KG
authors.lastname: Ten Hagen
authors.forename: David
authors.initials: D
authors.lastname: Live
mesh_terms: D000116:Acetylgalactosamine / Q000235:genetics / Q000378:metabolism*; D000818:Animals; D002460:Cell Line; D049030:Dystroglycans / Q000235:genetics / Q000378:metabolism*; D004721:Endoplasmic Reticulum / Q000235:genetics / Q000378:metabolism; D006031:Glycosylation; D006801:Humans; D008358:Mannose / Q000235:genetics / Q000378:metabolism*; D051379:Mice; D009097:Multienzyme Complexes / Q000235:genetics / Q000378:metabolism*; D009136:Muscular Dystrophies / Q000235:genetics / Q000378:metabolism; D017350:N-Acetylgalactosaminyltransferases / Q000235:genetics / Q000378:metabolism*; D014536:Uridine Diphosphate N-Acetylgalactosamine / Q000235:genetics / Q000378:metabolism*
chemical_list: D009097:Multienzyme Complexes; D049030:Dystroglycans; D014536:Uridine Diphosphate N-Acetylgalactosamine; D017350:N-Acetylgalactosaminyltransferases; D000116:Acetylgalactosamine; D008358:Mannose