Association of the dystroglycan complex isolated from bovine brain synaptosomes with proteins involved in signal transduction

Dystroglycan is a transmembrane heterodimeric complex of alpha and beta sub units that links the extracellular matrix to the cell cytoskeleton. It was originally identified in skeletal muscle, where it anchors dystrophin to th e sarcolemma. Dystroglycan is also highly expressed in nonmuscle tissues, i ncluding brain. To investigate the molecular interactions of dystroglycan i n the CNS, we fractionated a digitonin-soluble extract from bovine brain sy naptosomes by laminin-affinity chromatography and characterized the protein components. The 120-kDa alpha-dystroglycan was the major I-125-laminin-lab eled protein detected by overlay assay. This complex, in addition to beta-d ystroglycan, was also found to contain Grb2 and focal adhesion kinase p125( FAK) (FAK). Anti-FAK antibodies co-immunoprecipitated Grb2 with FAK. Howeve r, no direct interaction between beta-dystroglycan and FAK was detected by co-precipitation assay. Grb2, an adaptor protein involved in signal transdu ction and cytoskeleton organization, has been shown to bind beta-dystroglyc an. We isolated both FAK and Grb2 from synaptosomal extracts by chromatogra phy on immobilized recombinant beta-dystroglycan. In the CNS, FAK phosphory lation has been linked to membrane depolarization and neurotransmitter rece ptor activation. At the synapses, the adaptor protein Grb2 may mediate FAK- beta-dystroglycan interaction, and it may play a role in transferring infor mation between the dystroglycan complex and other signaling pathways.