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* Department of Physiology, McGill University, Montreal, Quebec, Canada H3G1Y6; The effect of laminin on the distribution of
dystroglycan (DG) and other surface proteins was examined by fluorescent staining in cultures of muscle
cells derived from Xenopus embryos. Western blotting
confirmed that previously characterized antibodies are reactive in Xenopus. In control cultures, Centre for Research in Neuroscience,
McGill University, Montreal General Hospital Research Institute, Montreal, Quebec, Canada H3G 1A4; § Department of
Pathology, Robert Wood Johnson Medical School, Piscataway, New Jersey 08854; and MRIC Biotechnology Group,
The North East Wales Institute, Plas Coch, Mold Road, Wrexam, United Kingdom LL11 2AW
DG, 
DG,
and laminin binding sites were distributed as microclusters (<1 µm2 in area) over the entire dorsal surface of
the muscle cells. Treatment with laminin induced the
formation of macroclusters (1-20 µm2), accompanied
by a corresponding decline in the density of the microclusters. With 6 nM laminin, clustering was apparent
within 150 min and near maximal within 1 d. Laminin
was effective at 30 pM, the lowest concentration tested.
The laminin fragment E3, which competes with laminin
for binding to DG, inhibited laminin-induced clustering but did not itself cluster DG, thereby indicating that other portions of the laminin molecule in addition to its
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, utrophin, or phosphotyrosine, and their formation was not inhibited by a tyrosine kinase inhibitor. The results reinforce the notion
that unclustered 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 molecular interactions. Laminin self binding may be the basis for
the laminin-induced clustering.
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