The relationship between talin and acetylcholine receptor clusters in Xenopus muscle cells.

Talin is involved in mediating the cytoskeleton-extracellular matrix interaction at focal contacts in cultured fibroblasts. Recently this protein has been localized at both the myotendinous junction (MTJ) and the neuromuscular junction (NMJ) in skeletal muscle. At the MTJ, talin may mediate the insertion of myofibrils into the plasma membrane, thus serving a function similar to that seen at focal contacts. However, the function of talin at the NMJ is unknown. In this study, we examined its distribution at both mature and developing acetylcholine receptor (AChR) clusters in Xenopus muscle cells both in vivo and in vitro with immunofluorescence. At the NMJs of both myotomal and submaxillaris muscles, talin was absent from the AChR clusters. In cultured myotomal muscle cells, it was absent from 40% of both the nerve-associated AChR patches and the spontaneously formed AChR clusters located on the top surface of the cells. We therefore conclude that this protein is not essential for maintenance of AChR clusters at the NMJ. In addition to MTJs, talin was invariably associated with AChR clusters induced by polyornithine-coated beads, and, to a large extent, with spontaneously formed clusters on the ventral side of cultured cells. A common feature of these talin-positive domains is the deep membrane infoldings, where bundles of actin filaments are inserted into the membrane. Thus, talin may be involved in the formation and maintenance of these structures. The deep membrane infoldings, though prominent at most NMJs, are absent from the two muscles under study in vivo. Our work thus suggests that the postjunctional membrane at the NMJ is heterogeneous, consisting of an AChR cluster domain and, often but not always, a domain for proteins involved in cytoskeleton-membrane linkage as exemplified by talin.