Differential distribution of subsets of myofibrillar proteins in cardiac nonstriated and striated myofibrils.

Cultured cardiac myocytes were stained with antibodies to sarcomeric alpha-actinin, troponin-I, alpha-actin, myosin heavy chain (MHC), titin, myomesin, C-protein, and vinculin. Attention was focused on the distribution of these proteins with respect to nonstriated myofibrils (NSMFs) and striated myofibrils (SMFs). In NSMFs, alpha-actinin is found as longitudinally aligned, irregular approximately 0.3-microns aggregates. Such aggregates are associated with alpha-actin, troponin-I, and titin. These I-Z-I-like complexes are also found as ectopic patches outside the domain of myofibrils in close apposition to the ventral surface of the cell. MHC is found outside of SMFs in the form of discrete fibrils. The temporal-spatial distribution and accumulation of the MHC-fibrils with respect to the I-Z-I-like complexes varies greatly along the length of the NSMFs. There are numerous instances of I-Z-I-like complexes without associated MHC-fibrils, and also cases of MHC-fibrils located many microns from I-Z-I-like complexes. The transition between the terminal approximately 1.7-microns sarcomere of any given SMF and its distal NSMF-tip is abrupt and is marked by a characteristic narrow alpha-actinin Z-band and vinculin positive adhesion plaque. A titin antibody T20, which localizes to an epitope at the Z-band in SMFs, precisely costains the 0.3-microns alpha-actinin aggregates in ectopic patches and NSMFs. Another titin antibody T1, which in SMFs localizes to an epitope at the A-I junction, typically does not stain ectopic patches and NSMFs. Where detectable, the T1-positive material is adjacent to rather than part of the 0.3-microns alpha-actinin aggregates. Myomesin and C-protein are found only in their characteristic sarcomeric locations (even in just perceptible SMFs). These A-band-associated proteins appear to be absent in ectopic patches and NSMFs.