The underglycosylation of plasma alpha 1-antitrypsin in congenital disorders of glycosylation type I is not random.
Conditions under which the glycosylation capacity of cells is limited provide an opportunity for studying the efficiency of site-specific glycosylation and the role of glycosylation in the maturation of glycoproteins. Congenital disorders of glycosylation type 1 (CDG-I) provide such a system. CDG-I is characterized by underglycosylation of glycoproteins due to defects in the assembly or transfer of the common dolichol-pyrophosphate-linked oligosaccharide precursor of asparagine-linked glycans. Human plasma alpha1-antitrypsin is normally fully glycosylated at three asparagine residues (46, 83, and 247), but un-, mono-, di-, and fully glycosylated forms of alpha1-antitrypsin were detected by 2D PAGE in the plasma from patients with CDG-I. The state of glycosylation of the three asparagine residues was analyzed in all the underglycosylated forms of alpha1-antitrypsin by peptide mass fingerprinting using matrix-assisted laser desorption ionization time-of-flight mass spectrometry. It was found that asparagine 46 was always glycosylated and that asparagine 83 was never glycosylated in the underglycosylated glycoforms of alpha1-antitrypsin. This showed that the asparagine residues are preferentially glycosylated in the order 46>247>83 in the mature underglycosylated forms of alpha1-antitrypsin found in plasma. It is concluded that the nonoccupancy of glycosylation sites is not random under conditions of decreased glycosylation capacity and that the efficiency of glycosylation site occupancy depends on structural features at each site. The implications of this observation for the intracellular transport and sorting of glycoproteins are discussed.
Mills
Kevin
K
Biochemistry Endocrinology and Metabolism Unit, Institute of Child Health At Great Ormond Street Hospital, University College London, 30 Guilford Street, London, WC1N 1EH, UK.
Mills
Philippa B
PB
Clayton
Peter T
PT
Mian
Nasi
N
Johnson
Andrew W
AW
Winchester
Bryan G
BG
D000595:Amino Acid Sequence; D001665:Binding Sites / Q000235:genetics; D002239:Carbohydrate Metabolism, Inborn Errors / Q000097:blood / Q000145:classification / Q000378:metabolism*; D002240:Carbohydrate Sequence; D016022:Case-Control Studies; D015180:Electrophoresis, Gel, Two-Dimensional / Q000379:methods; D006031:Glycosylation; D006801:Humans; D008969:Molecular Sequence Data; D020033:Protein Isoforms / Q000097:blood / Q000737:chemistry; D019032:Spectrometry, Mass, Matrix-Assisted Laser Desorption-Ionization; D000515:alpha 1-Antitrypsin / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*
D020033:Protein Isoforms; D000515:alpha 1-Antitrypsin