The plaque protein myozap defines unique junction strands in smooth muscle cell bundles

In bundles of smooth muscle cells associated with mammalian organs of the urogenital and alimentary tract we have discovered and partly characterized a novel kind of mirror image-symmetrically organized cell-cell junctions which are not based on direct transcellular contacts of transmembrane proteins or surface glycoproteins but represent extensive plasma membrane regions coated with cytoplasmic plaque ridges. The parallel, regularly spaced, rather thick (25-40 nm) and mostly very long (often for the entire length of the cell) myofilament-anchoring plaques of these junctions contain not only the focal adhesion-associated proteins vinculin, plectin and talin but also abundant amounts of protein myozap recently discovered as a major component of the plaques of adherens junctions in epithelia and endothelia as well as of the composite junctions connecting cardiomyocytes. α5- and β1-integrins are identified as the most prominent transmembrane molecules of these special smooth muscle junctions. In contrast, the tetraspanin CD151 co-distributes only in part with the myozap-positive junctions. These junctions are furthermore negative for proteins of adhering junctions. The myozap-containing junctional strands alternate with caveolin/dystrophin-positive regions. Characteristically, these novel cell-cell junctions are separated by extracellular matrix including the basal laminae of the adjacent cells which frequently fuse to a single electron-dense lamella forming the symmetry plane. We propose that the myozap-containing junctional strands may have important mechanical functions enabling and coordinating the contraction of closely-packed smooth muscle cell bundles throughout the body.