Role of tropomyosin in actin filament formation in embryonic salamander heart cells.

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Abstract
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[sodium dodecylsulfate]-polyacrylamide gel electrophoresis shows that actin (43,000 daltons) is present in almost normal amounts, while myosin H 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. EM studies of normal hearts show numerous well-organized myofibrils. Although mutant cardiomyocytes contain a few 60- and 150-.ANG. filaments, organized sarcomeres 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). 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 (60-80.ANG.) filaments are seen in their place. Negative staining experiments corroborate this observation. The nonfilamentous actin located in the amorphous collections of mutant heart cells is apparently induced to form into filaments with the addition of tropomyosin.