Molecular Basis for Dystroglycan Binding to Laminin‐G Domain‐Containing Ligands

Abstract

Dystroglycan (DG) is a highly glycosylated extracellular matrix (ECM) receptor. The like‐acetylglucosaminyltransferase (LARGE) is responsible for posttranslational modification of alpha‐dystroglycan (α‐DG) which is required for its function. Reduced ligand binding activity of α‐DG due to perturbed glycosylation is a common pathologic feature among several different forms of muscular dystrophy (called dystroglycanopathies). We now report that LARGE is a bifunctional glycosyltransferase, with both xylosyltransferase (Xyl‐T) and glucuronyltransferase (GlcA‐T) activities. We examined the enzymatic activity of LARGE with respect to Xyl‐α‐pNP as well as GlcA‐β‐MU as the acceptors and UDP‐Xyl and UDP‐GlcA as donors, using a secreted form of LARGE that lacks its transmembrane domain (LARGEdTM). We have identified the enzymatic function of LARGE, which produces a polysaccharide with repeating units of [‐3Xyl‐α1,3GlcA‐β1‐] onto these acceptors. We further show that this modification confers binding ability on α‐DG to laminin‐G domain‐containing ECM ligands: laminin, agrin, and neurexin. α‐DG binds the LG4 and 5 domains of laminin alpha chains, and it is suggested that three basic patches contribute to these interactions. The LARGE‐synthesized, negatively charged glycan on α‐DG likely binds to laminin through electrostatic associations with these basic patches.