Structural basis of laminin binding to the LARGE glycans on dystroglycan

Dystroglycan is a highly glycosylated extracellular matrix receptor with essential functions in skeletal muscle and the nervous system. Reduced matrix binding by alpha-dystroglycan (alpha-DG) due to perturbed glycosylation is a pathological feature of several forms of muscular dystrophy. Like-acetylglucosaminyltransferase (LARGE) synthesizes the matrix-binding heteropolysaccharide [-glucuronic acid-beta 1,3-xylose-alpha 1,3-](n). Using a dual exoglycosidase digestion, we confirm that this polysaccharide is present on native alpha-DG from skeletal muscle. The atomic details of matrix binding were revealed by a high-resolution crystal structure of laminin-G-like (LG) domains 4 and 5 (LG4 and LG5) of laminin-alpha 2 bound to a LARGE-synthesized oligosaccharide. A single glucuronic acid-beta 1,3-xylose disaccharide repeat straddles a Ca2+ ion in the LG4 domain, with oxygen atoms from both sugars replacing Ca2+-bound water molecules. The chelating binding mode accounts for the high affinity of this protein-carbohydrate interaction. These results reveal a previously uncharacterized mechanism of carbohydrate recognition and provide a structural framework for elucidating the mechanisms underlying muscular dystrophy.