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

Area of Science:

• Biochemistry
• Structural Biology
• Biophysics

Background:

• Actin filaments are crucial cytoskeletal components.
• Previous research indicated variable twist in actin filaments, with intersubunit angles fluctuating by ~10 degrees.
• The structural basis and regulation of this twist variability remain incompletely understood.

Purpose of the Study:

• To investigate whether actin filament twist variability is an intrinsic property independent of preparation methods.
• To determine if actin-binding proteins modulate the twist variability of actin filaments.
• To quantify the twist variability in different actin-based filament systems.

Main Methods:

• Electron microscopy was employed on three preparations of actin filaments: negatively stained, cryo-etched, and cryo-hydrated.
• Micrographs of actin alone, thin filaments (actin + tropomyosin + troponin), decorated filaments (actin + myosin S1), and Limulus sperm acrosomal filaments were analyzed.
• Two Fourier transform-based methods (layer line intensity and position) were used to measure intersubunit angle variability.

Main Results:

• Actin filaments consistently showed a twist variability of approximately 12 degrees, irrespective of the preparation or measurement technique.
• Thin filaments exhibited a non-significant increase in variability (15 degrees).
• Decorated filaments and Limulus filaments displayed significantly reduced variability (approx. 2 and 1 degree), indicating torsional stiffening.

Conclusions:

• Variable twist is an intrinsic property of actin filaments in solution.
• Actin-binding proteins can modulate twist variability, suggesting functional regulation.
• Torsional stiffening by myosin S1 and structures in Limulus sperm filaments highlights the role of associated proteins in controlling actin filament mechanics.