FINE-STRUCTURAL IMMUNOCYTOCHEMISTRY OF CATENINS IN AMPHIBIAN AND MAMMALIAN MUSCLE

In adherens junctions, alpha- and beta-catenin serve to link cadherins to the cortical cytoskeleton. Immunofluorescence and immune-electron microscopy have been applied to elucidate the nature and localization of cadherin/catenin-mediated cell-cell adhesion sites in adult muscle tissues. Antibodies against alpha- and beta-catenin have been used as indicators of such sites in amphibian and mammalian muscle. Intercalat ed discs are prominent cell-cell adhesion sites in heart muscle. They contain large amounts of the two catenins, the distributions of which are disclosed. In addition and in contrast to their counterparts in gu inea pig, cardiomyocytes of Xenopus are also interconnected laterally by catenin-containing cell-cell junctions. These are doubler structure s that approach the intercellular contacts of two adjacent cells from both sides and occur in register with the Z-discs. We interpret these structures as catenin/cadherin-based costameres. Ultrastructural detai ls of these structures are described. In addition to its presence in c ell-cell adhesion sites, we have found beta-catenin, but not alpha-cat enin, in the Z-discs of heart and skeletal striated muscles. In smooth muscle, actin filaments insert into the dense bodies, which are there fore regarded as functional equivalents of the Z-discs. Accordingly, b eta-catenin is also found in these structures, again in the absence of alpha-catenin. These non-peripheral intracellular localizations in th e Z-discs of striated muscles and the dense bodies of smooth muscle in dicate a hitherto unknown function of beta-catenin in these specialize d cells.