Effects of Exogenous 0-Actinin (CapZ) on Actin Filamentous Structures in Cultured Muscle Cells

β-Actinin (CapZ) is a heterodimeric actin-binding protein which is localized in the Z-bands of myofibril. It caps the barbed end of actin filaments and nucleates actin-polymerization in a Ca2 -independent manner. As judged by these properties of β-actinin, it is conceivable that β-actinin is involved in the regulation of actin assembly, especially in the formation of I-Z-I complex during myofibrillogenesis. In this study, to examine the function of β-actinin in myofibrillogenesis, recombinant β-actinin (r-β-actinin) produced in an E. coli expression system was introduced into cultured myogenic cells by a microinjection method. Stress fibers in C2 myoblasts were disrupted soon after microinjection of recombinant β-actinin, but nascent as well as well-organized myofibrils were scarcely affected by exogenous β-actinin. Based on these observations, we suggest that in myoblasts where actin filaments are dynamically reorganized, reassembly process of actin filaments may be affected by the exogenous β-actinin, whereas actin filaments become more stable and less sensitive to exogenous β-actinin, when they are organized into myofibrillar structures and anchored to Z-lines in myotubes.