CONCENTRATION OF PP125 FOCAL ADHESION KINASE (FAK) AT THE MYOTENDINOUS JUNCTION

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 the regulation of the integrin-based si gnal transduction mechanism involved in the assembly of this membrane specialization. In this study, we examined the immunocytochemical dist ribution of focal adhesion kinase in Xenopus skeletal muscle and its r ole in the formation of two sarcolemmal specializations, the myotendin ous junction and the neuromusuclar junction, using a monoclonal antibo dy (2A7) against this protein. Immunoprecipitation of Xenopus embryoni c 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. Fo cal 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 X enopus myotomal muscle cells, although its distribution was not exactl y congruent with that of the receptors. In these cells, the accumulati on focal adhesion kinase was induced by polystyrene microbeads. In add ition, beads also induce the formation of acetylcholine receptor clust ers and myotendinous junction-like specializations. By following the a ppearance of the focal adhesion kinase relative to the formation of th ese sarcolemmal specializations at bead-muscle contacts in cultured mu scle cells, we conclude that the accumulation of this protein was in p ace with the development of the myotendinous junction, but occurred we ll after the clustering of acetylcholine receptors. These results sugg est 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.