In vitro filament formation of a new fiber-forming protein from Tetrahymena pyriformis

Using a 38,000-dalton protein (FFP-38) purified from Tetrahymena acetone powder, we have succeeded in the polymerization of this protein into 14-nm filaments. The polymerization was initiated by incubating the purified FFP-38 fraction in a buffer containing 5 mM Mes (2-(N-Morpholino)ethanesulfonic acid), 50 mM KCl, 1.2 mM CaCl2, 0.6 mM ATP, pH 6.6, and by shifting the incubation temperature from 0 degrees C to 37 degrees C. The 14-nm filament is considered to consist of 7-nm globular subunits regularly arranged into 2 start, helical strands with 4 subunits per turn. The subunit may correspond to 9S tetramer of FFP-38, a native form of FFP-38. Since the subunit arrangement and subunit protein component of this 14-nm filament obviously differ from those of actin filament, 10-nm intermediate filament and microtubule, the 14-nm filament appears to be a newly found intracellular filament. Concerning the FFP-38 polymerization, some polymorphism appeared: we found ring structures having the diameters of 0.3--3.7 micrometers and latticed sheet structure, besides typical straight filaments.