STRUCTURES OF SIALYLATED O-LINKED OLIGOSACCHARIDES OF BOVINE PERIPHERAL-NERVE ALPHA-DYSTROGLYCAN - THE ROLE OF A NOVEL O-MANNOSYL-TYPE OLIGOSACCHARIDE IN THE BINDING OF ALPHA-DYSTROGLYCAN WITH LAMININ

alpha-Dystroglycan is a heavily glycosylated protein, which is localiz ed on the Schwann cell membrane as well as the sarcolemma, and links t he transmembrane protein beta-dystroglycan to laminin in the extracell ular matrix. We have shown previously that sialidase treatment, but no t N-glycanase treatment, of bovine peripheral nerve alpha-dystroglycan greatly reduces its binding activity to laminin, suggesting that the sialic acid of O-glycosidically-linked oligosaccharides may be essenti al for this binding. In this report, we analyzed the structures of the sialylated O-linked oligosaccharides of bovine peripheral nerve alpha -dystroglycan by two methods. O-Glycosidically-linked oligosaccharides were liberated by alkaline-borotritide treatment or by mild hydrazino lysis followed by 2-aminobenzamide-derivatization. Acidic fractions ob tained by anion exchange column chromatography that eluted at a positi on corresponding to monosialylated oligosaccharides were converted to neutral oligosaccharides by exhaustive sialidase digestion. The sialid ases from Arthrobacter ureafaciens and from Newcastle disease virus re sulted in the same degree of hydrolysis. The neutral oligosaccharide f raction, thus obtained, gave a major peak with a mobility of 3.8-3.9 g lucose units upon gel filtration, and its reducing terminus was identi fied as a mannose derivative. Based on the results of sequential exogl ycosidase digestion, lectin column chromatography, and reversed-phase high-performance liquid chromatography, we concluded that the major si alylated O-glycosidically-linked oligosaccharide of the alpha-dystrogl ycan was a novel O-mannosyl-type oligosaccharide, the structure of whi ch was Sia alpha 2-3Gal beta 1-4GlcNAc beta 1-2Man-Ser/Thr (where Sia is sialic acid). This oligosaccharide constituted at least 66% of the sialylated O-linked sugar chains. Furthermore, a laminin binding inhib ition study suggested that the sialyl N-acetyllactosamine moiety of th is sugar chain was involved in the interaction of the alpha-dystroglyc an with laminin.