Beta-actinin is equivalent to Cap Z protein.

Chicken skeletal muscle beta-actinin, previously reported to bind the slow-exchanging (pointed) ends of actin filaments was purified to homogeneity. By two dimensional gel electrophoresis, it consists of two subunits, beta I (35 kDa) and beta II (32 kDa), and each subunit has two isoforms. The amino acid sequences of V8 protease-digested peptides of beta I were nearly identical with those of portions of the muscle barbed end-blocking protein Cap Z alpha, although several amino acids were different from those deduced from cDNA sequences (Casella, J.F., Casella, S.J., Hollands, J.A., Caldwell, J.E., and Cooper, J.A. (1989) Proc. Natl. Acad. Sci. U.S.A. 86, 5800-5804). The amino acid sequences of two peptides from beta II were completely identical with portions of Cap Z beta deduced from cDNA sequences (Caldwell, J.E., Waddle, J.A., Cooper, J.A., Hollands, J.A., Casella, S.J., and Casella, J.F. (1989) J. Biol. Chem. 264, 12648-12652). beta-Actinin capped the barbed end of an actin filament as evidenced by actin assembly of myosin S1-decorated filaments and specifically its impairment of growth in the "barbed" direction. Thus it is concluded that highly purified beta-actinin is identical with the more recently described Cap Z, an actin barbed-end capping protein of chicken skeletal muscle.