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Related Concept Videos

Generation of Straight or Branched Actin Filaments01:14

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 Filaments01:11

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...
Introduction to Actin01:26

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 Polymerization01:42

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 Motility01:13

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.
Actin Filament Depolymerization01:19

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 4, 2026

Tuning the Contractility and Deformation Modes of Active Actin-Based Assemblies In Vitro: From Two-Dimensional Active Networks to Liquid Crystal Drops
06:48

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

Structural dynamics of an actin spring.

L Mahadevan1, C S Riera, Jennifer H Shin

  • 1School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts, USA. lm@seas.harvard.edu

Biophysical Journal
|February 16, 2011
PubMed
Summary
This summary is machine-generated.

Horseshoe crab sperm use a unique actin spring mechanism for motility, distinct from typical actin dynamics. This active actin spring powers the acrosomal reaction, enabling egg penetration.

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The Mechanics of (Poro-)Elastic Contractile Actomyosin Networks As a Model System of the Cell Cytoskeleton
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The Mechanics of (Poro-)Elastic Contractile Actomyosin Networks As a Model System of the Cell Cytoskeleton
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The Mechanics of (Poro-)Elastic Contractile Actomyosin Networks As a Model System of the Cell Cytoskeleton

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Area of Science:

  • Cellular and Molecular Biology
  • Biophysics
  • Reproductive Biology

Background:

  • Actin-based motility typically involves polymerization/depolymerization or myosin motor contractility.
  • A third mechanism, utilizing an active actin spring, is observed in horseshoe crab sperm.

Purpose of the Study:

  • To investigate the dynamics of the active actin spring in horseshoe crab sperm.
  • To elucidate the mechanochemical principles underlying the acrosomal reaction.

Main Methods:

  • Ultrastructural analysis of the actin bundle.
  • Kinetic and energetic measurements.
  • Imaging of calcium ion (Ca2+) binding.

Main Results:

  • A 60-μm bent and twisted actin bundle rapidly straightens in the presence of Ca2+.
  • This straightening occurs at a constant velocity, driving acrosomal penetration.
  • A dynamical theory consistent with experimental observations was developed.

Conclusions:

  • The active actin spring represents a novel mechanism of cellular motility.
  • Energy is stored in conformational changes within the actin assembly and released cooperatively.
  • This mechanism provides a model for energy storage and release in macromolecular assemblies.