A SYNAPTIC LOCALIZATION DOMAIN IN THE SYNAPTIC CLEFT PROTEIN LAMININ BETA-2 (S-LAMININ)

The basal lamina that ensheaths skeletal muscle fibers traverses the s ynaptic cleft at the neuromuscular junction. Synaptic and extrasynapti c portions of the basal lamina contain different laminin beta chains: beta 2 (or s) at synapses and beta 1 (or beta 1) extrasynaptically. La minin beta 2 is also confined to synapselike parches on myotube surfac es in vitro, whereas beta 1 is present throughout the extracellular ma trix. This differential localization of laminin beta chains was analyz ed by expression of chimeric beta 1-beta 2 molecules in cultured mouse myotubes. A 16-amino acid carboxyl-terminal sequence in beta 2 was ne cessary for synaptic localization, and an amino-terminal domain in bet a 1 promoted association with extracellular fibrils. The synaptic targ eting sequence of beta 2 contains a site previously shown to be adhesi ve for motor neurons.