title: Evidence for core 2 to core 1 O-glycan remodeling during the recycling of MUC1.
abstract: The apical transmembrane glycoprotein MUC1 is endocytosed to recycle through the trans-Golgi network (TGN) or Golgi complex to the plasma membrane. We followed the hypothesis that not only the known follow-up sialylation of MUC1 in the TGN is associated with this process, but also a remodeling of O-glycan core structures, which would explain the previously described differential core 2- vs core 1-based O-glycosylation of secreted, single Golgi passage and recycling membrane MUC1 isoforms (Engelmann K, Kinlough CL, Müller S, Razawi H, Baldus SE, Hughey RP, Hanisch F-G. 2005. Glycobiology. 15:1111-1124). Transmembrane and secreted MUC1 probes show trafficking-dependent changes in O-glycan core profiles. To address this novel observation, we used recombinant epitope-tagged MUC1 (MUC1-M) and mutant forms with abrogated clathrin-mediated endocytosis (MUC1-M-Y20,60N) or blocked recycling (palmitoylation-defective MUC1-M-CQC/AQA). We show that the CQC/AQA mutant transits the TGN at significantly lower levels, concomitant with a strongly reduced shedding from the plasma membrane and its accumulation in endosomal compartments. Intriguingly, the O-glycosylation of the shed MUC1 ectodomain subunit changes from preponderant sialylated core 1 (MUC1-M) to core 2 glycans on the non-recycling CQC/AQA mutant. The O-glycoprofile of the non-recycling CQC/AQA mutant resembles the core 2 glycoprofile on a secretory MUC1 probe that transits the Golgi complex only once. In contrast, the MUC1-M-Y20,60N mutant recycles via flotillin-dependent pathways and shows the wild-type phenotype with dominant core 1 expression. Differential radiolabeling of protein with [(35)S]Met/Cys or glycans with [(3)H]GlcNH2 in pulse-chase experiments of surface biotinylated MUC1 revealed a significantly shorter half-life of [(3)H]MUC1 when compared with [(35)S]MUC1, whereas the same ratio for the CQC/AQA mutant was close to one. This finding further supports the novel possibility of a recycling-associated O-glycan processing from Gal1-4GlcNAc1-6(Gal1-3)GalNAc (core 2) to Gal1-3GalNAc (core 1).
authors: Medical Faculty, Institute of Biochemistry II, University of Cologne, 50931 Köln, Germany.
authors: Hanieh
authors: H
authors: Razawi
authors: Carol L
authors: CL
authors: Kinlough
authors: Simon
authors: S
authors: Staubach
authors: Paul A
authors: PA
authors: Poland
authors: Youssef
authors: Y
authors: Rbaibi
authors: Ora A
authors: OA
authors: Weisz
authors: Rebecca P
authors: RP
authors: Hughey
authors: Franz-Georg
authors: FG
authors: Hanisch
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D002462:Cell Membrane / Q000378:metabolism; D004285:Dogs; D011992:Endosomes / Q000378:metabolism*; D006031:Glycosylation; D006056:Golgi Apparatus / Q000378:metabolism; D057809:HEK293 Cells; D006801:Humans; D061986:MCF-7 Cells; D061985:Madin Darby Canine Kidney Cells; D008969:Molecular Sequence Data; D018396:Mucin-1 / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D009154:Mutation; D011134:Polysaccharides / Q000378:metabolism*; D021381:Protein Transport
chemical_list: D018396:Mucin-1; D011134:Polysaccharides
keywords: MUC1; O-glycosylation; glycan processing; membrane trafficking; recycling
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