ALPHA-7-BETA-1 INTEGRIN IS A COMPONENT OF THE MYOTENDINOUS JUNCTION ON SKELETAL-MUSCLE

Immunization against a 70 kDa band that co-purifies with skeletal musc le integrins has resulted in an antibody directed against the avian al pha7 integrin subunit. The specificity of the antibody was established by patterns of tissue staining and cross-reactivity with antibodies d irected against the cytoplasmic domain of the rat alpha7 cytoplasmic d omain. On sections of adult skeletal muscle the alpha7 integrin was en riched in the myotendinous junction (MTJ). This localization was uniqu e as neither the alpha1, alpha3, alpha5, alpha6 and alpha(v), subunit localizes in the myotendinous junction. The distribution of the alpha7 subunit in the MTJ was examined during embryonic development. Alpha7 expression in the junction is first apparent around embryo day 14 and is almost exclusively at the developing MTJ at this stage. Alpha3 is e xpressed with distinctive punctate staining around the junctional area in earlier embryos (11-day). The time of appearance of the alpha7 sub unit in the MTJ correlates with the insertion of myofibrils into subsa rcolemmal densities and folding of the junctional membrane, suggesting a role of the alpha7 integrin in this process. Vinculin is present th roughout development of the myotendinous junction, suggesting that the alpha7 integrin recognizes a preformed cytoskeletal structure. The pr esence of the alpha7 subunit in the myotendinous junction and the alph a5 subunit in the adhesion plaque demonstrates a molecular difference between these two adherens junctions. It also points to possible origi ns of junctional specificity on muscle. Differences between these two junctions were developed further using an antibody against phosphotyro sine (PY20). Phosphotyrosine is thought to participate in the organiza tion and stabilization of adhesions. The focal adhesion and the neurom uscular junction, but not the MTJ, contained proteins phosphorylated o n tyrosine.