Context-dependent functional substitution of α-skeletal actin by γ-cytoplasmic actin

We generated transgenic mice that overexpressed γ-_cyto actin 2000-fold above wild-type levels in skeletal muscle. γ- _cyto actin comprised 40% of total actin in transgenic skeletal muscle, with a concomitant 40% decrease in •-actin. Surprisingly, transgenic muscle was histologically and ultrastructurally identical to wild-type muscle despite near-stoichiometric incorporation of γ- _cyto actin into sarcomeric thin filaments. Furthermore, several parameters of muscle physiological performance in the transgenic animals were not different from wild type. Given these surprising results, we tested whether overexpression of γ-_cyto actin could rescue the early postnatal lethality in α-_sk actin-null mice (Acta1^-/-). By quantitative Western blot analysis, we found total actin levels were decreased by 35% in Acta1^-/- muscle. Although transgenic overexpression of γ-_cyto actin on the Acta1^-/- background restored total actin levels to wild type, resulting in thin filaments composed of 60% γ-_cyto actin and a 40% mixture of cardiac and vascular actin, the life span of transgenic Acta1^-/- mice was not extended. These results indicate that sarcomeric thin filaments can accommodate substantial incorporation of γ-_cyto actin without functional consequences, yet γ-_cyto actin cannot fully substitute for α- _sk actin.