Core 1 glycans on alpha-dystroglycan mediate laminin-induced acetylcholine receptor clustering but not laminin binding.

ABSTRACT

Although unique O-linked oligosaccharides on alpha-dystroglycan are important for binding to a variety of extracellular ligands, the function(s) of more generic carbohydrate structures on alpha-dystroglycan remain unclear. Recent studies suggest a role for glycoconjugates bearing the core 1 disaccharide Galbeta(1-3)GalNAc in acetylcholine receptor (AChR) clustering on the surface of muscle cells. Here, we report experiments demonstrating that the core 1-specific lectin jacalin almost completely abrogated laminin-induced AChR clustering in C2C12 myotubes and that alpha-dystroglycan was the predominant jacalin-binding protein detected in C2C12 myotube lysates. Although jacalin likely inhibited laminin-induced AChR clustering by directly binding to alpha-dystroglycan, jacalin had no effect on laminin binding to the myotube surface or to alpha-dystroglycan. Like jacalin, peanut agglutinin lectin also binds the core 1 disaccharide but not when it is terminally sialylated as expressed on alpha-dystroglycan. We show that C2C12 alpha-dystroglycan bound to peanut agglutinin only after digestion with neuraminidase. Simultaneous treatment of myotubes with neuraminidase and endo-O-glycosidase diminished alpha-dystroglycan binding to peanut agglutinin and inhibited neuraminidase-induced AChR clustering. We conclude that sialylated core 1 oligosaccharides of alpha-dystroglycan are important for laminin-induced AChR clustering and that their function in this process is distinct from the established role of alpha-dystroglycan oligosaccharides in laminin binding.