LAMININ-INDUCED CLUSTERING OF DYSTROGLYCAN ON EMBRYONIC MUSCLE-CELLS - COMPARISON WITH AGRIN-INDUCED CLUSTERING

The effect of laminin on the distribution of dystroglycan (DG) and oth er surface proteins was examined by fluorescent staining in cultures o f muscle cells derived from Xenopus embryos. Western blotting confirme d that previously characterized antibodies are reactive in Xenopus, In control cultures, alpha DG, beta DG, and laminin binding sites were d istributed as microclusters (<1 mu m(2) in area) over the entire dorsa l surface of the muscle cells. Treatment with laminin induced the form ation of macroclusters (1-20 mu m(2)), accompanied by a corresponding decline in the density of the microclusters. With 6 nM laminin, cluste ring was apparent within 150 min and near maximal within 1 d. Laminin was effective at 30 pM, the lowest concentration tested. The laminin f ragment E3, which competes with laminin for binding to alpha DG, inhib ited laminin-induced clustering but did not itself cluster DG, thereby indicating that other portions of the laminin molecule in addition to its alpha DG binding domain are required for its clustering activity. Laminin-induced clusters also contained dystrophin, but unlike agrin- induced clusters, they did not contain acetylcholine receptors, utroph in, or phosphotyrosine, and their formation was not inhibited by a tyr osine kinase inhibitor. The results reinforce the notion that uncluste red DG is mobile on the surface of embryonic muscle cells and suggest that this mobile DG can be trapped by at least two different sets of m olecular interactions. Laminin self binding may be the basis for the l aminin-induced clustering.