α7β1 integrin is a component of the myotendinous junction on skeletal muscle

Immunization against a 70 kDa band that co-purifies with skeletal muscle integrins has resulted in an anti-body directed against the avian 7 integrin subunit. The specificity of the antibody was established by patterns of tissue staining and cross-reactivity with antibodies directed against the cytoplasmic domain of the rat 7 cytoplasmic domain. On sections of adult skeletal muscle the 7 integrin was enriched in the myotendinous junc-tion (MTJ). This localization was unique as neither the 1, 3, 5, 6 and v subunit localizes in the myotendi-nous junction. The distribution of the 7 subunit in the MTJ was examined during embryonic development. 7 expression in the junction is first apparent around embryo day 14 and is almost exclusively at the devel-oping MTJ at this stage. 3 is expressed with distinctive punctate staining around the junctional area in earlier embryos (11-day). The time of appearance of the 7 sub-unit in the MTJ correlates with the insertion of myofib-rils into subsarcolemmal densities and folding of the junctional membrane, suggesting a role of the 7 inte-grin in this process. Vinculin is present throughout development of the myotendinous junction, suggesting that the 7 integrin recognizes a preformed cytoskele-tal structure. The presence of the 7 subunit in the myotendinous junction and the 5 subunit in the adhe-sion plaque demonstrates a molecular difference between these two adherens junctions. It also points to possible origins of junctional specificity on muscle. Dif-ferences between these two junctions were developed further using an antibody against phosphotyrosine (PY20). Phosphotyrosine is thought to participate in the organization and stabilization of adhesions. The focal adhesion and the neuromuscular junction, but not the MTJ, contained proteins phosphorylated on tyrosine.