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Introduction to Actin

Introduction to Actin

Actin is a highly conserved cytoskeletal protein found abundantly in eukaryotic cells. It constitutes 10% weight of the total cellular protein in muscle cells, while in non-muscle cells, it is lower and makes up around 1–5 percent of the total cell protein. Actin found in the unicellular amoebae and complex multicellular animals is around 80% similar, demonstrating their conservation over a billion years of evolution. Actin coding genes are conserved within species and across different species.

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 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.

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Related Experiment Video

Updated: Jul 21, 2026

Reconstitution of Membrane-Tethered Minimal Actin Cortices on Supported Lipid Bilayers

Reconstitution of Membrane-Tethered Minimal Actin Cortices on Supported Lipid Bilayers

Published on: July 12, 2022

Actin cytoskeleton: putting a CAP on actin polymerization.

V A Stevenson¹, W E Theurkauf

¹Program in Molecular Medicine and the Department of Molecular Genetics and Microbiology, University of Massachusetts Medical Center, 373 Plantation Street, Worcester, Massachusetts 01605, USA.

Current Biology : CB

|October 26, 2000

Summary

Cyclase-associated protein (CAP) in Drosophila acts as a key regulator, inhibiting actin polymerization during development. This protein may also play a direct role in cellular signal transduction pathways.

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Last Updated: Jul 21, 2026

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Published on: July 12, 2022

Reconstitution of Actin-Based Motility with Commercially Available Proteins

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Published on: October 28, 2022

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

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Published on: July 11, 2025

Area of Science:

Cell Biology
Developmental Biology
Molecular Biology

Background:

Actin polymerization is crucial for cellular processes.
Cyclase-associated proteins (CAPs) are known regulators of the actin cytoskeleton.
The precise roles of CAPs in development and signal transduction are under investigation.

Purpose of the Study:

To investigate the function of a Drosophila homolog of cyclase-associated protein (CAP).
To determine the role of Drosophila CAP in actin polymerization.
To explore the potential involvement of Drosophila CAP in signal transduction.

Main Methods:

Utilizing genetic analysis in Drosophila melanogaster.
Investigating protein interactions related to the actin cytoskeleton.
Employing molecular biology techniques to study signal transduction pathways.

Main Results:

A Drosophila homolog of cyclase-associated protein (CAP) was identified.
Drosophila CAP functions as a negative regulator of actin polymerization.
Evidence suggests Drosophila CAP may directly mediate signal transduction.

Conclusions:

Drosophila CAP is developmentally important.
CAP negatively regulates actin dynamics in Drosophila.
CAP has a potential dual role in cytoskeletal regulation and signal transduction.