A new fiber-forming protein from Tetrahymena pyriformis

A new fiber-forming protein was isolated from the acetone powder of Tetrahymena pyriformis by co-precipitating with skeletal muscle myosin while trials were made to find actin or actin-like protein in Tetrahymena. It has a molecular weight of 38000 D and forms a tetramer (140000 D, 9 S) in physiological conditions. Its isoelectric point (pH 6.7), amino acid composition and antigenic determinant(s) differ significantly from those of non-muscle actin and skeletal muscle actin. It does not undergo G-F conversion while actin does, and does not activate Mg²⁺-ATPase of skeletal muscle myosin. The protein localizes in the oral apparatus and division furrow as revealed by fluorescent antibody method. The protein can be assembled into 14-nm filaments in a reassembly buffer. The in vitro filaments appear to correspond to some filaments included in the oral apparatus and the contractile ring. The fiber-forming protein from Tetrahymena may play important roles in cell motility including cell division.