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J. Cell Biol.,
Volume 145, Number 7, June 28, 1999 1483-1495
Department of Physiology, University of Michigan, Ann Arbor, Michigan 48109-0622
Sarcomere maintenance, the continual process of replacement of contractile proteins of the myofilament lattice with newly synthesized proteins, in fully
differentiated contractile cells is not well understood.
Adenoviral-mediated gene transfer of epitope-tagged tropomyosin (Tm) and troponin I (TnI) into adult cardiac myocytes in vitro along with confocal microscopy
was used to examine the incorporation of these newly
synthesized proteins into myofilaments of a fully differentiated contractile cell. The expression of epitope-tagged TnI resulted in greater replacement of the endogenous TnI than the replacement of the endogenous
Tm with the expressed epitope-tagged Tm suggesting
that the rates of myofilament replacement are limited
by the turnover of the myofilament bound protein. Interestingly, while TnI was first detected in cardiac sarcomeres along the entire length of the thin filament, the
epitope-tagged Tm preferentially replaced Tm at the
pointed end of the thin filament. These results support a
model for sarcomeric maintenance in fully differentiated cardiac myocytes where (a) as myofilament proteins turnover within the cell they are rapidly exchanged with newly synthesized proteins, and (b) the
nature of replacement of myofilament proteins (ordered or stochastic) is protein specific, primarily affected by the structural properties of the myofilament
proteins, and may have important functional consequences.
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