The variable twist of actin and its modulation by actin-binding proteins

Previous studies demonstrated that actin filaments have variable twist in which the intersubunit angles vary by approximately +/- 10 degrees within a filament. In this work we show that this variability was unchanged when different methods were used to prepare filaments for electron microscopy. We also show that actin-binding proteins can modulate the variability in twist. Three preparations of actin filaments were photographed in the electron microscope: negatively stained filaments, replicas of rapidly frozen, etched filaments, and frozen hydrated filaments. In addition, micrographs of actin + tropomyosin + troponin (thin filaments), of actin + myosin S1 (decorated filaments), and of filaments frayed from the acrosomal process of Limulus sperm (Limulus filaments) were obtained. We used two independent methods to measure variable twist based on Fourier transforms of single filaments. The first involved measuring layer line intensity versus filament length and the second involved measuring layer line position. We measured a variability in the intersubunit angle of actin filaments of approximately 12 degrees independent of the method of preparation or of measurement. Thin filaments have 15 degrees of variability, but the increase over pure actin is not statistically significant. Decorated filaments and Limulus filaments, however, have significantly less variability (approximately 2 and 1 degree, respectively), indicating a torsional stiffening relative to actin. The results from actin alone using different preparative methods are evidence that variable twist is a property of actin in solution. The results from actin filaments in the presence of actin-binding proteins suggest that the angular variability can be modulated, depending on the biological function.