Universal length fluctuations of actin structures found in cells

Area of Science:

• Cell Biology
• Biophysics
• Biochemistry

Background:

• Actin filaments are crucial cytoskeletal components involved in cell motility, signaling, and division.
• Maintaining specific lengths of actin structures is vital for their cellular functions.
• Current models often attribute length control to balanced assembly and disassembly rates.

Purpose of the Study:

• To investigate the mechanism controlling the length of bundled actin structures.
• To reconcile theoretical predictions of actin length distribution with experimental observations.
• To propose a new model for actin bundle length regulation.

Main Methods:

• Re-examination of experimental data on stereocilia, microvilli, actin cables, and filopodia lengths.
• Development of a theoretical model based on independent filament dynamics within bundles.
• Comparison of model predictions with experimental length distributions.

Main Results:

• Standard balance point models predict Gaussian length distributions with variance proportional to steady-state length.
• Experimental data show variance scaling with the square of steady-state length, contradicting balance models.
• The proposed model, where the longest filament sets bundle length, predicts the observed non-Gaussian distribution and variance scaling.

Conclusions:

• The length of actin bundles is determined by the dynamics of individual filaments, with the longest filament setting the overall length.
• This mechanism explains the observed length variance in various cellular actin structures.
• Crosslinking filaments into networks is critical for controlling the size of cytoskeletal structures.