title: Post-translational maturation of dystroglycan is necessary for pikachurin binding and ribbon synaptic localization.
abstract: Pikachurin, the most recently identified ligand of dystroglycan, plays a crucial role in the formation of the photoreceptor ribbon synapse. It is known that glycosylation of dystroglycan is necessary for its ligand binding activity, and hypoglycosylation is associated with a group of muscular dystrophies that often involve eye abnormalities. Because little is known about the interaction between pikachurin and dystroglycan and its impact on molecular pathogenesis, here we characterize the interaction using deletion constructs and mouse models of muscular dystrophies with glycosylation defects (Large(myd) and POMGnT1-deficient mice). Pikachurin-dystroglycan binding is calcium-dependent and relatively less sensitive to inhibition by heparin and high NaCl concentration, as compared with other dystroglycan ligand proteins. Using deletion constructs of the laminin globular domains in the pikachurin C terminus, we show that a certain steric structure formed by the second and the third laminin globular domains is necessary for the pikachurin-dystroglycan interaction. Binding assays using dystroglycan deletion constructs and tissue samples from Large-deficient (Large(myd)) mice show that Large-dependent modification of dystroglycan is necessary for pikachurin binding. In addition, the ability of pikachurin to bind to dystroglycan prepared from POMGnT1-deficient mice is severely reduced, suggesting that modification of the GlcNAc-β1,2-branch on O-mannose is also necessary for the interaction. Immunofluorescence analysis reveals a disruption of pikachurin localization in the photoreceptor ribbon synapse of these model animals. Together, our data demonstrate that post-translational modification on O-mannose, which is mediated by Large and POMGnT1, is essential for pikachurin binding and proper localization, and suggest that their disruption underlies the molecular pathogenesis of eye abnormalities in a group of muscular dystrophies.
authors.affiliation: Division of Neurology/Molecular Brain Science, Kobe University Graduate School of Medicine, Kobe 650-0017, Japan.
authors.forename: Motoi
authors.initials: M
authors.lastname: Kanagawa
authors.forename: Yoshihiro
authors.initials: Y
authors.lastname: Omori
authors.forename: Shigeru
authors.initials: S
authors.lastname: Sato
authors.forename: Kazuhiro
authors.initials: K
authors.lastname: Kobayashi
authors.forename: Yuko
authors.initials: Y
authors.lastname: Miyagoe-Suzuki
authors.forename: Shin'ichi
authors.initials: S
authors.lastname: Takeda
authors.forename: Tamao
authors.initials: T
authors.lastname: Endo
authors.forename: Takahisa
authors.initials: T
authors.lastname: Furukawa
authors.forename: Tatsushi
authors.initials: T
authors.lastname: Toda
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D002118:Calcium / Q000378:metabolism; D002352:Carrier Proteins / Q000235:genetics / Q000378:metabolism*; D002460:Cell Line; D049030:Dystroglycans / Q000235:genetics / Q000378:metabolism*; D006031:Glycosylation; D006801:Humans; D051379:Mice; D008817:Mice, Mutant Strains; D009137:Muscular Dystrophy, Animal / Q000235:genetics / Q000378:metabolism / Q000473:pathology; D009419:Nerve Tissue Proteins / Q000235:genetics / Q000378:metabolism*; D020419:Photoreceptor Cells, Vertebrate / Q000378:metabolism* / Q000473:pathology; D011485:Protein Binding; D011499:Protein Processing, Post-Translational; D017434:Protein Structure, Tertiary; D017384:Sequence Deletion; D013569:Synapses / Q000235:genetics / Q000378:metabolism* / Q000473:pathology
chemical_list: D002352:Carrier Proteins; D009419:Nerve Tissue Proteins; C553232:pikachurin protein, mouse; D049030:Dystroglycans; D002118:Calcium