A Glycan of Ψ-Factor from Dictyostelium discoideum Contains a Bisecting-GlcNAc, an Intersecting-GlcNAc, and a Core α-1,6-Fucose

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A secretory glycoprotein named Ψ-factor that we have purified and cloned from Dictyostelium discoideum is prespore cell-inducing factor. To address its functional significance, it is necessary to examine the attached sites and structures of its glycans as well as its protein structure. Here we identified and isolated a tryptic glycosylated peptide with the 71st to 89th amino acids of Ψ-factor that contained the consensus amino acid sequence for an N-linked glycan (N-T-T). MALDI-TOF mass spectrometry indicated that the major protonated molecular ions, [M+H]+, of the glycopeptide were present at m⁄z 3,806, the minor m⁄z 3,603 and 3,400 ions corresponding to the loss of one and two N-acetylhexosamines respectively. Digestion of it with N-glycosidase F gave a molecular mass of 1,766.9 for the whole glycan moiety, which accounts for its composition of five hexoses, four N-acetylhexosamines, and a deoxyhexose. Further digestion experiments on the basis of the substrate specificity of α-mannosidase and β-N-acetylhexosaminidase allowed us to elucidate the unique structure of the glycan, which contains a bisecting and an intersecting GlcNAc and a core α1,6-fucosyl moiety.
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