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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.

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...

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 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...

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.

Polarity of the Cytoskeleton

Polarity of the Cytoskeleton

The intrinsic polarity of cells can be primarily attributed to two factors- i) the asymmetric accumulation of mobile components such are regulatory molecules and subcellular components across the cell and ii) the orientation of polar cytoskeletal filaments that make up the cytoskeletal networks, specifically microfilaments, and microtubules arranged along the axis of polarity. Interactions between the cytoskeletal filaments are crucial for the establishment and maintenance of the polar nature...

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Related Experiment Video

Updated: Jun 12, 2026

Aip1p Dynamics Are Altered by the R256H Mutation in Actin

Aip1p Dynamics Are Altered by the R256H Mutation in Actin

Published on: July 30, 2014

The structure of native G-actin.

Hui Wang¹, Robert C Robinson, Leslie D Burtnick

¹Department of Chemistry and Centre for Blood Research, Life Sciences Institute, University of British Columbia, Vancouver, British Columbia, Canada.

Cytoskeleton (Hoboken, N.J.)

|June 12, 2010

Summary

Heat shock protein Hsp27 facilitates G-actin crystallization, revealing the native G-actin structure. This structure provides insights into actin polymerization mechanisms and cation-coordinated conformational changes.

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Last Updated: Jun 12, 2026

Aip1p Dynamics Are Altered by the R256H Mutation in Actin

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Published on: July 30, 2014

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

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
Structural Biology
Molecular Biology

Background:

Heat shock proteins (HSPs) function as cytoplasmic chaperones, crucial for protein folding and preventing aggregation.
Hsp27, a specific heat shock protein, influences actin's behavior in solution.
Understanding G-actin's native structure is key to elucidating actin polymerization.

Purpose of the Study:

To determine the crystal structure of unmodified G-actin.
To investigate the role of Hsp27 in G-actin crystallization.
To compare the native G-actin structure with the F-actin model and understand polymerization implications.

Main Methods:

Supersaturated solutions of unmodified G-actin were used.
Stoichiometric amounts of Hsp27 were added to induce crystallization.
X-ray crystallography was employed to determine the G-actin structure.

Main Results:

Hsp27 induced G-actin crystallization, with Hsp27 not being part of the final crystal structure.
The study presents the first structure of native G-actin, free from modifying agents.
The determined structure revealed a closed nucleotide-binding cleft containing a calcium ion and ATP, with a disordered D-loop.

Conclusions:

The native G-actin structure, elucidated via Hsp27-mediated crystallization, offers a baseline for polymerization studies.
The structure suggests a mechanism where bound cations coordinate conformational changes and ATP hydrolysis during polymerization.
This work provides a foundation for understanding the structural basis of actin dynamics and regulation.