Mutational and functional analysis of Large in a novel CHO glycosylation mutant.
Inactivating mutations of Large reduce the functional glycosylation of alpha-dystroglycan (alpha-DG) and lead to muscular dystrophy in mouse and humans. The N-terminal domain of Large is most similar to UDP-glucose glucosyltransferases (UGGT), and the C-terminal domain is related to the human i blood group transferase beta1,3GlcNAcT-1. The amino acids at conserved motifs DQD+1 and DQD+3 in the UGGT domain are necessary for mammalian UGGT activity. When the corresponding residues were mutated to Ala in mouse Large, alpha-DG was not functionally glycosylated. A similar result was obtained when a DXD motif in the beta1,3GlcNAcT-1 domain was mutated to AIA. Therefore, the first putative glycosyltransferase domain of Large has properties of a UGGT and the second of a typical glycosyltransferase. Co-transfection of Large mutants affected in the different glycosyltransferase domains did not lead to complementation. While Large mutants were more localized to the endoplasmic reticulum than wild-type Large or revertants, all mutants were in the Golgi, and only very low levels of Golgi-localized Large were necessary to generate functional alpha-DG. When Large was overexpressed in ldlD.Lec1 mutant Chinese hamster ovary (CHO) cells which synthesize few, if any, mucin O-GalNAc glycans and no complex N-glycans, functional alpha-DG was produced, presumably by modifying O-mannose glycans. To investigate mucin O-GalNAc glycans as substrates of Large, a new CHO mutant Lec15.Lec1 that lacked O-mannose and complex N-glycans was isolated and characterized. Following transfection with Large, Lec15.Lec1 cells also generated functionally glycosylated alpha-DG. Thus, Large may act on the O-mannose, complex N-glycans and mucin O-GalNAc glycans of alpha-DG.
Aguilan
Jennifer T
JT
Department of Cell Biology, Albert Einstein College Medicine, New York, NY 10461, USA.
Sundaram
Subha
S
Nieves
Edward
E
Stanley
Pamela
P
D000818:Animals; D001483:Base Sequence; D016466:CHO Cells; D006224:Cricetinae; D003412:Cricetulus; D017931:DNA Primers; D014453:Glucuronosyltransferase / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D016297:Mutagenesis, Site-Directed; D016133:Polymerase Chain Reaction; D013347:Subcellular Fractions / Q000201:enzymology
D017931:DNA Primers; D014453:Glucuronosyltransferase