B4GAT1 is the priming enzyme for the LARGE-dependent functional glycosylation of α-dystroglycan
Jeremy L Praissman,
David H Live,
Shuo Wang,
Annapoorani Ramiah,
Zoeisha S Chinoy,
Geert-Jan Boons,
Kelley W Moremen,
Lance Wells
Affiliations
Jeremy L Praissman
Complex Carbohydrate Research Center, University of Georgia, Athens, United States; Department of Biochemistry and Molecular Biology, University of Georgia, Athens, United States
David H Live
Complex Carbohydrate Research Center, University of Georgia, Athens, United States
Shuo Wang
Complex Carbohydrate Research Center, University of Georgia, Athens, United States
Annapoorani Ramiah
Complex Carbohydrate Research Center, University of Georgia, Athens, United States
Zoeisha S Chinoy
Complex Carbohydrate Research Center, University of Georgia, Athens, United States
Geert-Jan Boons
Complex Carbohydrate Research Center, University of Georgia, Athens, United States
Kelley W Moremen
Complex Carbohydrate Research Center, University of Georgia, Athens, United States; Department of Biochemistry and Molecular Biology, University of Georgia, Athens, United States
Lance Wells
Complex Carbohydrate Research Center, University of Georgia, Athens, United States; Department of Biochemistry and Molecular Biology, University of Georgia, Athens, United States
Recent studies demonstrated that mutations in B3GNT1, an enzyme proposed to be involved in poly-N-acetyllactosamine synthesis, were causal for congenital muscular dystrophy with hypoglycosylation of α-dystroglycan (secondary dystroglycanopathies). Since defects in the O-mannosylation protein glycosylation pathway are primarily responsible for dystroglycanopathies and with no established O-mannose initiated structures containing a β3 linked GlcNAc known, we biochemically interrogated this human enzyme. Here we report this enzyme is not a β-1,3-N-acetylglucosaminyltransferase with catalytic activity towards β-galactose but rather a β-1,4-glucuronyltransferase, designated B4GAT1, towards both α- and β-anomers of xylose. The dual-activity LARGE enzyme is capable of extending products of B4GAT1 and we provide experimental evidence that B4GAT1 is the priming enzyme for LARGE. Our results further define the functional O-mannosylated glycan structure and indicate that B4GAT1 is involved in the initiation of the LARGE-dependent repeating disaccharide that is necessary for extracellular matrix protein binding to O-mannosylated α-dystroglycan that is lacking in secondary dystroglycanopathies.
