Polymerization of highly purified Tetrahymena 14-nm filament protein/citrate synthase into filaments and its possible role in regulation of enzymatic activity.

Tetrahymena 14-nm filament protein (49K protein) is a bifunctional protein with roles in the cytoskeleton and as citrate synthase. Previous studies in our laboratory showed that elongation factor 1 alpha (EF-1 alpha) copurifies with the 49K protein upon polymerization and depolymerization of the 49K protein. In this study, the 49K protein was isolated from partially purified 49K protein fraction containing EF-1 alpha. Using the purified 49K protein and/or purified EF-1 alpha, the interaction between 49K protein and EF-1 alpha in filament formation was investigated electronmicroscopically and it was demonstrated that purified 49K protein was capable of forming 14-nm filaments without EF-1 alpha. The 49K protein/citrate synthase has been suggested to form filaments in mitochondria. Here we show that the citrate synthase activity of 49K protein is decreased by polymerization and increased by depolymerization, suggesting a possible modulating mechanism of citrate synthase activity by monomer-polymer conversion in mitochondria in situ.