βactinin is not distinguishable from an actin barbed-end capping protein in chicken breast muscle

β-Actinin is an actin-pointed end capping protein in skeletal muscle. Casella et al. have reported that a protein isolated from muscle acetone powder by procedures similar to those used for βactinin purification caps the barbed end of an actin filament (J. Biol. Chem. 261, 10915-10921 (1986)). We have confirmed the above results. However, it turned out that the two proteins were identical as to subunit sizes, peptide maps, and cross-reactivities with anti-β-actinin IgG. The binding of the two proteins to opposite ends of an actin filament remains unexplained.