Focal adhesion kinase is a recently characterized tyrosine kinase that is concentrated at focal contacts in cultured cells. It is thought to play an important role in the regulation of the integrin-based signal transduction mechanism involved in the assembly of this membrane specialization. In this study, we examined the immunocytochemical distribution of focal adhesion kinase in Xenopus skeletal muscle and its role in the formation of two sarcolemmal specializations, the myotendinous junction and the neuromuscular junction, using a monoclonal antibody (2A7) against this protein. Immunoprecipitation of Xenopus embryonic tissues with this antibody demonstrated a single band at a relative molecular mass of 116 kDa. A distinct concentration of immunolabeling for focal adhesion kinase was observed at the myotendinous junction of muscle fibers in vivo. At this site, the labeling for this protein is correlated with an accumulation of phosphotyrosine immunolabeling. Focal adhesion kinase was not concentrated at the neuromuscular junction in muscle cells either in vivo or in vitro. However, it was localized at spontaneously formed acetylcholine receptor clusters in cultured Xenopus myotomal muscle cells, although its distribution was not exactly congruent with that of the receptors. In these cells, the accumulation focal adhesion kinase was induced by polystyrene microbeads. In addition, beads also induce the formation of acetylcholine receptor clusters and myotendinous junction-like specializations. By following the appearance of the focal adhesion kinase relative to the formation of these sarcolemmal specializations at bead-muscle contacts in cultured muscle cells, we conclude that the accumulation of this protein was in pace with the development of the myotendinous junction, but occurred well after the clustering of acetylcholine receptors. These results suggest that focal adhesion kinase may be involved in the development and/or maintenance of the myotendinous junction through an integrin-based signaling system. Although it can accumulate at acetylcholine receptor clusters formed in culture, it does not appear to be involved in the development of the neuromuscular junction.
Might prepatterned acetylcholine-receptor clusters on surface myotubes be a sign of neuromuscular-junction maturation failure?