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A more recent version of this article appeared on April 1, 2006
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Submitted on December 7, 2005
Revised on January 24, 2006
Accepted on February 2, 2006
*Randall Division of Cell and Molecular Biophysics, GKT School of Biomedical Sciences, King’s College London, Guy’s Campus, London SE1 1UL, United Kingdom;
Department of Biosciences, University of Kent, Canterbury CT2 7NJ, United Kingdom
Monitoring Editor: Erika Holzbaur
We define here a previously unrecognised structural element close to the heart muscle plasma membrane at the intercalated disk where the myofibrils lead into the adherens junction. At this location the plasma membrane is extensively folded. Immunofluorescence and immunogold electron microscopy reveal a spectrin-rich domain at the apex of the folds. These domains occur at the axial level of what would be the final Z-disk of the terminal sarcomere in the myofibril, although there is no Z-disk-like structure there. However, a sharp transitional boundary lies between the myofibrillar I-band and intercalated disk thin filaments, identifiable by the presence of Z-disk proteins,
-actinin and N-terminal titin. This allows for the usual elastic positioning of the A-band in the final sarcomere, while the transduction of the contractile force normally associated with the Z-disk is transferred to the adherens junctions at the plasma membrane. The axial conjunction of the transitional junction with the spectrin-rich domains suggests a mechanism for direct communication between intercalated disk and contractile apparatus. In particular, it provides a means for sarcomeres to be added to the ends of the cells during growth. This is of particular relevance to understanding myocyte elongation in dilated cardiomyopathy.
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