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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...

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.

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...

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...

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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

A nucleotide state-sensing region on actin.

Dmitri S Kudryashov¹, Elena E Grintsevich, Peter A Rubenstein

¹Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90095, USA. dkudryas@ucla.edu

The Journal of Biological Chemistry

|June 10, 2010

Summary

Actin's W-loop acts as a nucleotide sensor, influencing actin's interactions. This loop's conformation changes with nucleotide state, affecting actin polymerization and binding protein interactions.

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Last Updated: Jun 12, 2026

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A Time-Efficient Fluorescence Spectroscopy-Based Assay for Evaluating Actin Polymerization Status in Rodent and Human Brain Tissues

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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

Area of Science:

Biochemistry
Molecular Biology
Cell Biology

Background:

Actin nucleotide state (ATP, ADP-P(i), ADP) affects its polymerization and interactions with binding proteins.
Molecular dynamics simulations suggested the W-loop (residues 165-172) changes conformation upon ATP hydrolysis, impacting actin subunit and protein binding.

Purpose of the Study:

To investigate the reciprocal communication between actin's W-loop and its nucleotide binding cleft.
To determine if the W-loop acts as a nucleotide sensor and influences actin-binding protein interactions.

Main Methods:

Site-specific labeling and point mutagenesis of W-loop residues (167, 169, 170).
Fluorescence spectroscopy to monitor W-loop conformation changes.
Assessing nucleotide release kinetics.
Studying the effect of latrunculin A and ADP-ribosylation on W-loop conformation.

Main Results:

Mutagenesis and labeling of W-loop residues affected nucleotide release.
Nucleotide state (ATP/ADP switch) altered W-loop-bound probe fluorescence.
The W-loop adopted an ATP-like conformation in the ADP-P(i) state.
Latrunculin A binding favored an ATP-like W-loop conformation; Arg-177 ADP-ribosylation induced a distinct W-loop conformation.

Conclusions:

The W-loop of actin functions as a nucleotide sensor.
This sensing mechanism likely contributes to nucleotide state-dependent changes in F-actin.
The W-loop may mediate nucleotide state-modulated interactions of G- and F-actin with actin-binding proteins.