Characterization of cardiotin, a structural component in the myocard.

The characterization and subcellular distribution of cardiotin, a structural component of striated muscle, is described using a monoclonal antibody. This high molecular mass component (> 300 kDa) is expressed in the myocard of several species and to a lesser extent also in skeletal muscle. Cardiotin is not found in smooth muscle tissues, other mesenchymal or epithelial tissues. The cardiotin distribution pattern is independent of other sarcomeric components, such as desmin, myosin, actin, titin, nebulin, and desmoplakin, and shows a longitudinal filamentous localization between myofibrils. The average distance between parallel running cardiotin filaments is approximately 2.3 microns, as concluded from confocal scanning laser microscopic analysis of double-immunolabeled muscle preparations. The cardiotin filamentous staining reaction is oriented perpendicularly to the typical cross-striations observed with antibodies to desmin, spanning several sarcomeres and showing a length between 12 to 80 microns in frozen sections. Its localization pattern suggests a possible link with the sarcoplasmic reticulum. We have never observed cardiotin filaments to cross the intercalated disks, stained by antibodies to desmoplakins or desmin. Cardiotin cannot be solubilized from cardiac muscle by nonionic detergents of high concentrations of KCl or KI, suggesting a structural role in the myocard. The protein could so far not be detected in developing embryonic heart, but expression seems to be initiated after birth, depending on the species examined.