Structures of Sialylated O-Linked Oligosaccharides of Bovine Peripheral Nerve α-Dystroglycan: THE ROLE OF A NOVEL O-MANNOSYL-TYPE OLIGOSACCHARIDE IN THE BINDING OF α-DYSTROGLYCAN WITH LAMININ*
α-Dystroglycan is a heavily glycosylated protein, which is localized on the Schwann cell membrane as well as the sarcolemma, and links the transmembrane protein β-dystroglycan to laminin in the extracellular matrix. We have shown previously that sialidase treatment, but not N-glycanase treatment, of bovine peripheral nerve α-dystroglycan greatly reduces its binding activity to laminin, suggesting that the sialic acid of O-glycosidically-linked oligosaccharides may be essential for this binding. In this report, we analyzed the structures of the sialylated O-linked oligosaccharides of bovine peripheral nerve α-dystroglycan by two methods. O-Glycosidically-linked oligosaccharides were liberated by alkaline-borotritide treatment or by mild hydrazinolysis followed by 2-aminobenzamide-derivatization. Acidic fractions obtained by anion exchange column chromatography that eluted at a position corresponding to monosialylated oligosaccharides were converted to neutral oligosaccharides by exhaustive sialidase digestion. The sialidases from Arthrobacter ureafaciens and from Newcastle disease virus resulted in the same degree of hydrolysis. The neutral oligosaccharide fraction, thus obtained, gave a major peak with a mobility of 3.8-3.9 glucose units upon gel filtration, and its reducing terminus was identified as a mannose derivative. Based on the results of sequential exoglycosidase digestion, lectin column chromatography, and reversed-phase high-performance liquid chromatography, we concluded that the major sialylated O-glycosidically-linked oligosaccharide of the α-dystroglycan was a novel O-mannosyl-type oligosaccharide, the structure of which was Siaα2-3Galβ1-4GlcNAcβ1-2Man-Ser/Thr (where Sia is sialic acid). This oligosaccharide constituted at least 66% of the sialylated O-linked sugar chains. Furthermore, a laminin binding inhibition study suggested that the sialyl N-acetyllactosamine moiety of this sugar chain was involved in the interaction of the α-dystroglycan with laminin.
This work was supported by grants from the Kato Memorial Bioscience Foundation, the Cell Science Research Foundation, the Science Research Promotion Fund from the Japan Private School Promotion Foundation, Research Grants 8A-1 and 8A-2 for Nervous and Mental Disorders from the Ministry of Health and Welfare, and Research Grants 07264239, 06454280, 08457195, 06770463, 05274105, 08281105, and 05557037 from the Ministry of Education, Science, Sports and Culture. The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
