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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...

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...

The Contractile Ring

The Contractile Ring

Contractile rings are composed of microfilaments and are responsible for separating the daughter cells during cytokinesis. Contractile ring assembly proceeds along with other cell cycle events; however, very few mechanistic details are known about the timing and coordination of the contractile rings with the cell cycle.
A small GTPase, RhoA, controls the function and assembly of the contractile ring. RhoA belongs to the Ras superfamily of proteins. The activation of formins by RhoA promotes...

Mechanism of Filopodia Formation

Mechanism of Filopodia Formation

Filopodia are thin, actin-rich cellular protrusions that play an important role in many fundamental cellular functions. They vary in their occurrence, length, and positioning in different cell types, suggesting their diverse roles.
Their main function is to guide migrating cells during normal tissue morphogenesis or cancer metastasis by recognizing and making initial contacts with the extracellular matrix. However, they can also act as stationary cell anchors or help to establish communication...

The Contractile Ring

The Contractile Ring

Contractile rings are composed of microfilaments and are responsible for separating the daughter cells during cytokinesis. Contractile ring assembly proceeds along with other cell cycle events; however, very few mechanistic details are known about the timing and coordination of the contractile rings with the cell cycle.
A small GTPase, RhoA, controls the function and assembly of the contractile ring. RhoA belongs to the Ras superfamily of proteins. The activation of formins by RhoA promotes...

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Related Experiment Video

Updated: Jun 28, 2026

In Vitro Polymerization of F-actin on Early Endosomes

In Vitro Polymerization of F-actin on Early Endosomes

Published on: August 28, 2017

Counterion-induced actin ring formation.

J X Tang¹, J A Käs, J V Shah

¹Department of Physics, Indiana University, Bloomington 47405, USA. jxtang@indiana.edu

European Biophysics Journal : EBJ

|February 1, 2002

Summary

This summary is machine-generated.

Multivalent cations or streptavidin induce actin rings and loops from filamentous actin (F-actin). Ring formation reveals insights into actin bending stiffness and adhesion energies.

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Last Updated: Jun 28, 2026

In Vitro Polymerization of F-actin on Early Endosomes

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Published on: August 28, 2017

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Tuning the Contractility and Deformation Modes of Active Actin-Based Assemblies In Vitro: From Two-Dimensional Active Networks to Liquid Crystal Drops

Tuning the Contractility and Deformation Modes of Active Actin-Based Assemblies In Vitro: From Two-Dimensional Active Networks to Liquid Crystal Drops

Published on: July 11, 2025

Area of Science:

Biochemistry
Biophysics
Materials Science

Background:

Filamentous actin (F-actin) is a crucial cytoskeletal component.
Actin self-assembly dynamics are influenced by environmental factors.

Purpose of the Study:

To investigate the formation of actin rings and loops induced by multivalent cations and streptavidin.
To understand the physical principles governing the self-assembly of filamentous actin into ring structures.

Main Methods:

Addition of > 20 mM divalent cations to very dilute solutions of phalloidin-stabilized F-actin.
Addition of streptavidin to crosslink sparsely biotinylated F-actin.
Analysis of ring structures, including undulations and associated aggregates.

Main Results:

Actin filaments form rings and loops in the presence of > 20 mM divalent cations or streptavidin.
Observed ring structures vary from single, overlapping filaments to laterally associated filaments.
Undulations in actin rings exhibit dynamics similar to thermal motions of single filaments.
Lariat-shaped aggregates and rodlike bundles coexist with ring structures.

Conclusions:

Polyvalent cation-induced actin rings are analogous to DNA toroids, with larger diameters due to F-actin's greater bending stiffness.
Actin ring formation provides a method to estimate adhesion energy mediated by counterions or streptavidin-biotin bonds.
The study elucidates novel self-assembly pathways for filamentous actin, relevant to cytoskeletal organization and biomaterials.