The Journal of Cell Biology, Vol 82, 227-238, Copyright © 1979 by The Rockefeller University Press
Role of tropomyosin in actin filament formation in embryonic salamander heart cells
LF Lemanski
Recessive mutant gene c in Ambystoma mexicanum embryos causes a failure of
the heart to function even though initial heart development appears normal.
An analysis of the constituent proteins of normal and mutant hearts by
SDS-poly-acrylamide gel electrophoresis shows that actin (43,000 daltons)
is present in almost normal amounts, while myosin heavy chain (200,000
daltons) is somewhat reduced in mutants. Both SDS- polyacrylamide gel
electrophoresis and immunofluorescence studies reveal that tropomyosin is
abundant in normal hearts, but very much reduced in mutants. Electron
microscope studies of normal hearts show numerous well-organized
myofibrils. Although mutant cardiomyocytes contain a few 60- and 150-A
filaments, organized sacromeres are absent. Instead, amorphous
proteinaceous collections are prominent. Previously reported heavy
meromyosin (HMM)-binding experiments on glycerinated hearts demonstrate
that most of the actin is contained within the amorphous collections in a
nonfilamentous state, and the addition of HMM causes polymerization into F
actin (Lemanski et al., 1976, J. Cell. Biol. 68:375-388). In the present
study, glycerol-extracted hearts are incubated with tropomyosin, purified
from rabbit or chicken skeletal muscle. This treatment causes the amorphous
collections to disappear, and large numbers of distinct thin actin (60- to
80-A) filaments are seen in their place. Negative staining experiments
corroborate this observation. These results suggest that the nonfilamentous
actin located in the amorphous collections of mutant heart cells is induced
to form into filaments with the addition of tropomyosin.
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