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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...

Overview of Myosin Structure and Function

Overview of Myosin Structure and Function

Myosins are a family of molecular motor proteins, first identified in the skeletal muscles, where they are responsible for muscle contraction. Along with their role in muscle contraction, these proteins also play a role in the intracellular transport of molecules and vesicles. There are twenty-four classes of myosins based on their domain sequence and organization. Of the twenty-four, six classes (Myosin I, Myosin II, Myosin V, Myosin VI, Myosin VII, and Myosin X) have been well characterized.

Actin and Myosin in Muscle Contraction

Actin and Myosin in Muscle Contraction

Actin and myosin are contractile proteins that form the sarcomere found in skeletal muscle tissues for regulating muscle contraction. Actin, a globular contractile protein, interacts with myosin for muscle contraction. The skeletal tissue appears striped or striated under a microscope due to the repeated arrangement of contractile proteins actin and myosin along the length of myofibrils. Dark A bands and light I bands repeat along myofibrils, and the alignment of myofibrils in the cell causes...

The Role of Actin and Myosin in Non-muscle Cells

The Role of Actin and Myosin in Non-muscle Cells

Actin and myosin or actomyosin filaments also play a significant role in cells other than those involved in muscle contraction (which occurs within the sarcomere of muscle cells). The mechanism of non-muscle cell contractile bundles was first observed in Dictyostelium and Acanthamoeba. In non-muscle cells, two bundles are commonly found: stress fibers and actomyosin adherence belts. These contractile bundles are smaller and less organized than the ones found in muscle cells. They are held...

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Related Experiment Video

Updated: Jun 29, 2026

Dissecting Mechanoenzymatic Properties of Processive Myosins with Ultrafast Force-Clamp Spectroscopy

Dissecting Mechanoenzymatic Properties of Processive Myosins with Ultrafast Force-Clamp Spectroscopy

Published on: July 1, 2021

Two-headed binding of a processive myosin to F-actin.

M L Walker¹, S A Burgess, J R Sellers

¹Astbury Centre for Structural Molecular Biology, School of Biomedical Sciences, University of Leeds, UK.

|June 24, 2000

Summary

Myosin V motor proteins move along actin filaments in a straight line by spanning the actin helical repeat. Electron microscopy reveals the curved leading head of myosin V initiates this directed movement.

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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

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Last Updated: Jun 29, 2026

Dissecting Mechanoenzymatic Properties of Processive Myosins with Ultrafast Force-Clamp Spectroscopy

Dissecting Mechanoenzymatic Properties of Processive Myosins with Ultrafast Force-Clamp Spectroscopy

Published on: July 1, 2021

Probing Myosin Ensemble Mechanics in Actin Filament Bundles Using Optical Tweezers

Probing Myosin Ensemble Mechanics in Actin Filament Bundles Using Optical Tweezers

Published on: May 4, 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:

Cellular Biology
Biochemistry
Biophysics

Background:

Myosins are essential motor proteins that generate force and movement within cells by interacting with actin filaments.
The helical structure of actin typically leads to a helical path for myosin movement, but longer myosin heads could enable linear motion.
Myosin V, with its longer heads, is hypothesized to achieve linear cargo transport.

Purpose of the Study:

To investigate the working mechanism of Myosin V and determine if it spans the actin helical repeat during movement.
To elucidate the structural basis for Myosin V's ability to move linearly along actin filaments.
To identify the role of Myosin V head conformation in directed motion.

Main Methods:

High-resolution electron microscopy was employed to visualize Myosin V interacting with actin filaments.
Single particle processing techniques were utilized to analyze the structure and polarity of the actin-myosin V complex.
Structural analysis focused on the distance between Myosin V heads and their conformation.

Main Results:

Electron microscopy confirmed that working Myosin V molecules span the actin helical repeat, with head separations of approximately 13 actin subunits (range 11-15).
The study revealed a polar structure with one curved and one straighter Myosin V head, indicating directionality.
Single particle analysis identified the curved head as the leading head, suggesting its role in initiating the working stroke.

Conclusions:

Myosin V's ability to span the actin helical repeat enables linear motion and efficient cargo transport.
The distinct conformations of the two heads, particularly the curved leading head, are crucial for directed movement along actin filaments.
These findings provide structural insights into the processive, straight-line motility of Myosin V.