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Actin Polymerization and Cell Motility

Actin Polymerization and Cell Motility

Actin is a family of globular proteins that are highly abundant in eukaryotic cells. It makes up approximately 1-5% of total cell protein concentration. Actin monomers polymerize to form a complex network of polarized filaments, the actin cytoskeleton, that plays a crucial role in many cellular processes, including cell motility, division, endocytosis, and metastasis of cancer cells.
Actin cytoskeleton dynamics can produce pushing, pulling, and resistance forces that help the cell to migrate.

Actin Polymerization

Actin Polymerization

Actin polymerization occurs through the head-to-tail association of binding sites on monomeric actin or G-actin to form filamentous or F-actin. The polymerization can be divided into three phases ̶ nucleation, elongation, and steady-state phase.
The nucleation phase involves forming a stable nucleus consisting of three actin monomers to form a new actin filament. Actin-binding proteins such as formins and Arp2/3 complex help filament growth post-nucleation. The Formins form straight actin...

Generation of Straight or Branched Actin Filaments

Generation of Straight or Branched Actin Filaments

The straight or branched structure formation of actin filaments is controlled by nucleating proteins such as the formins and Arp2/3 complex. Formin-mediated assembly results in straight filaments, whereas Arp2/3 protein complex-mediated assembly results in branched actin filaments.
Arp2/3 Complex
Arp2/3 complex is a seven-subunit complex consisting of two proteins similar to actin- Arp2 and Arp3, and five other subunits that help keep Arp2 and Arp3 inactive. When required, the complex is...

Actin Filament Depolymerization

Actin Filament Depolymerization

Actin filaments (F-actin) are composed of actin subunits. The dissociation of actin monomers can occur from either end of F-actin. The rate of dissociation is faster from the minus-end or the pointed end, where the actin subunits exist with a bound ADP, together known as ADP-actin. The depolymerization of F-actin is aided by proteins, including the actin-depolymerizing factor (ADF) and cofilin family of proteins, gelsolin, and glia maturation factor (GMF).
In F-actin, the ADF/cofilin proteins...

Formation of Higher-order Actin Filaments

Formation of Higher-order Actin Filaments

The polymerization of G-actin monomers into filamentous F-actin is a multi-step process. Once the F-actins are formed, they can bundle together in different arrangements to form higher-order networks and regulate cellular functions. Common examples include the formation of lamellipodia and filopodia at the cell's leading edge by actin reorganization in a migrating cell. The microvilli on the brush border epithelial cells are also formed through the F-actin network.
The high-order actin networks...

Actin Treadmilling

Actin Treadmilling

Actin filaments undergo polymerization and depolymerization from either end. The polymerization and depolymerization rates depend on the cytosolic concentration of free G-actins. The polymerization rate is generally higher at the plus or barbed end, while the depolymerization rate is higher at the minus or pointed end. At a steady state, critical concentration describes the concentration of free G-actin monomers at which the polymerization rate at the plus end is equal to that of the...

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Related Experiment Video

Updated: Jun 20, 2026

Using Microfluidics and Fluorescence Microscopy to Study the Assembly Dynamics of Single Actin Filaments and Bundles

Using Microfluidics and Fluorescence Microscopy to Study the Assembly Dynamics of Single Actin Filaments and Bundles

Published on: May 5, 2022

Force-velocity relation for actin-polymerization-driven motility from Brownian dynamics simulations.

Kun-Chun Lee¹, Andrea J Liu

¹Department of Neurobiology, Physiology and Behavior, University of California, Davis, California, USA.

Biophysical Journal

|September 2, 2009

Summary

We simulated actin-polymerization-driven motility, finding that motor speed depends on load. At high loads, the stall pressure is determined by the actin network's mechanical properties, consistent with experiments.

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Reconstitution of Actin-Based Motility with Commercially Available Proteins

Reconstitution of Actin-Based Motility with Commercially Available Proteins

Published on: October 28, 2022

Area of Science:

Biophysics
Cellular Mechanics
Computational Biology

Background:

Actin-polymerization-driven motility is crucial for cellular processes.
Understanding the force-velocity relationship is key to elucidating motility mechanisms.
Existing models often simplify filament dynamics and network properties.

Purpose of the Study:

To numerically simulate the force-velocity relation for actin-polymerization-driven motility.
To investigate the influence of load on motility speed and stall pressure.
To connect simulation results with experimental observations and theoretical mechanisms.

Main Methods:

Brownian dynamics simulations were employed.
A physically consistent formulation of the dendritic nucleation model was used.
Semiflexible filaments self-assembling and pushing a disk were modeled.

Main Results:

At low loads, disk speed was independent of load.
At high loads, speed decreased, vanishing at a characteristic stall pressure.
Velocity at low loads is reaction-rate controlled; stall pressure at high loads is determined by the actin network's mechanical properties.

Conclusions:

The simulated force-velocity relation aligns with experimental data.
The results support a self-diffusiophoretic mechanism for actin-based motility.
New in vitro experiments are proposed to further validate these findings.