Cooperativity and frustration in protein-mediated parallel actin bundles

Area of Science:

• Biophysics
• Cell Biology
• Structural Biology

Background:

• Cytoskeletal actin filaments form essential cellular structures.
• Actin bundling proteins, such as fascin and espin, regulate actin organization.
• Understanding protein-mediated actin bundling mechanisms is crucial for cell mechanics.

Purpose of the Study:

• To investigate the distinct mechanisms by which fascin and espin bundle actin filaments.
• To elucidate the role of linker binding geometry and flexibility in actin bundling.
• To correlate protein-specific bundling behaviors with linker bond stiffness.

Main Methods:

• Small-angle X-ray scattering (SAXS) studies to analyze actin filament structure.
• Development of a coarse-grained statistical model for protein-actin interactions.
• Comparative analysis of fascin and espin bundling characteristics.

Main Results:

• Increased linker binding induces systematic overtwist in actin filaments.
• Fascin facilitates a continuous range of actin twist states during bundling.
• Espin promotes either untwisted filaments or fully overtwisted bundles, exhibiting discontinuous bundling.
• Model demonstrates that linker geometry and flexibility mediate cooperative binding.

Conclusions:

• The distinct bundling mechanisms of fascin and espin arise from differences in their linker bond stiffness.
• Fascin's flexible linkers allow for continuous twist states, while espin's stiffer linkers lead to discontinuous bundling.
• Protein-linker interactions are key determinants of actin filament organization and mechanics.