Pikachurin, the most recently identified
ligand of
dystroglycan, plays a crucial role in the formation of the photoreceptor ribbon synapse. It is known that glycosylation of
dystroglycan is necessary for its
ligand binding activity, and hypoglycosylation is associated with a group of
muscular dystrophies that often involve
eye abnormalities. Because little is known about the interaction between pikachurin and
dystroglycan and its impact on molecular pathogenesis, here we characterize the interaction using deletion constructs and mouse models of
muscular dystrophies with glycosylation defects (Large(myd) and
POMGnT1-deficient mice). Pikachurin-
dystroglycan binding is
calcium-dependent and relatively less sensitive to inhibition by
heparin and high NaCl concentration, as compared with other
dystroglycan ligand proteins. Using deletion constructs of the
laminin globular domains in the pikachurin C terminus, we show that a certain steric structure formed by the second and the third
laminin globular domains is necessary for the pikachurin-
dystroglycan interaction. Binding assays using
dystroglycan deletion constructs and tissue samples from Large-deficient (Large(myd)) mice show that Large-dependent modification of
dystroglycan is necessary for pikachurin binding. In addition, the ability of pikachurin to bind to
dystroglycan prepared from POMGnT1-deficient mice is severely reduced, suggesting that modification of the GlcNAc-β1,2-branch on O-
mannose is also necessary for the interaction. Immunofluorescence analysis reveals a disruption of pikachurin localization in the photoreceptor ribbon synapse of these model animals. Together, our data demonstrate that post-translational modification on O-
mannose, which is mediated by Large and
POMGnT1, is essential for pikachurin binding and proper localization, and suggest that their disruption underlies the molecular pathogenesis of
eye abnormalities in a group of
muscular dystrophies.