High level expression of transfected β- and γ-actin genes differentially impacts on myoblast cytoarchitecture

Abstract

The impact of the human β- and γ-actin genes on myoblast cytoarchitecture was examined by their stable transfection into mouse C2 myoblasts. Transfectant C2 clones expressing high levels of hu-man β-actin displayed increases in cell surface area. In contrast, C2 clones with high levels of human γ-actin expression showed decreases in cell surface area. The changes in cell morphology were accompanied by changes in actin stress-fiber organization. The β-actin transfectants displayed well-defined filamentous organization of actin; whereas the γ-actin transfectants displayed a more diffuse organization of the actin cables. The role of the β-actin protein in generating the enlarged cell phenotype was examined by transfecting a mutant form of the human β-actin gene. Transfectant cells were shown to incorporate the aberrant actin protein into stress-fiber-like structures. High level expression of the mutant β-actin produced decreases in cell surface area and disruption of the actin microfilament network similar to that seen with transfection of the γ-actin gene. In contrast, transfection of another mutant form of the β-actin gene which encodes an unstable protein had no impact on cell morphology or cytoarchitecture. These results strongly suggest that it is the nature of the encoded protein that determines the morphological response of the cell. We conclude that the relative gene expression of β- and γ-actin is of relevance to the control of myoblast cytoarchitecture. In particular, we conclude that the β- and γ-actin genes encode functionally distinct cytoarchitectural information.