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Related Concept Videos

Generation of Straight or Branched Actin Filaments01:14

Generation of Straight or Branched Actin Filaments

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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...
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Mechanism of Filopodia Formation01:39

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Filopodia are thin, actin-rich cellular protrusions that play an important role in many fundamental cellular functions. They vary in their occurrence, length, and positioning in different cell types, suggesting their diverse roles.
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Actin Polymerization01:42

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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.
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Mechanism of Lamellipodia Formation01:31

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Cells migrating in response to external stimuli form lamellipodia, which are thin membrane protrusions supported by a mesh of linked, branched, or unbranched actin filaments. These actin filaments interact with myosin motor proteins, creating the dynamic actomyosin complex within the cytoskeleton. Contractility, or the ability to generate contractile stress, is inherent to the actomyosin complex. It helps cells detect the stiffness of the surrounding ECM and exert contractile force for...
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Formation of Higher-order Actin Filaments01:11

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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...
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Actin Filament Depolymerization01:19

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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).
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Arp2/3-mediated bidirectional actin assembly by SPIN90 dimers.

Tianyang Liu1, Luyan Cao2,3, Miroslav Mladenov2

  • 1Institute of Structural and Molecular Biology, Birkbeck College, London, UK.

Nature Structural & Molecular Biology
|September 15, 2025
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Summary

Human SPIN90 protein acts as a dimer to nucleate bidirectional actin filaments, revealing a new mechanism for generating antiparallel actin structures. This finding offers insights into cellular processes like migration and transport.

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Area of Science:

  • Cell Biology
  • Molecular Biology
  • Biochemistry

Background:

  • The Arp2/3 complex is crucial for forming branched actin networks, essential for cellular functions like migration.
  • Certain proteins, including the WISH/DIP/SPIN90 family, can alter Arp2/3 complex activity to nucleate linear actin filaments.

Purpose of the Study:

  • To investigate the mechanism by which human SPIN90 nucleates actin filaments.
  • To determine the structural basis for SPIN90-mediated bidirectional actin nucleation by the Arp2/3 complex.

Main Methods:

  • Determined the 3-Å-resolution structure of the human SPIN90-Arp2/3 complex.
  • Analyzed the dimerization and interaction interfaces of SPIN90 with the Arp2/3 complex.

Main Results:

  • Human SPIN90 functions as a dimer, nucleating bidirectional actin filaments.
  • The structure revealed that SPIN90 dimerizes via a three-helix bundle and interacts with two Arp2/3 complexes.
  • Each SPIN90 molecule activates both bound Arp2/3 complexes, promoting bidirectional nucleation.

Conclusions:

  • SPIN90-mediated bidirectional nucleation is mechanistically similar to branched filament formation.
  • The conserved dimerization domain in SPIN90 orthologs suggests this is a common strategy in metazoans for generating antiparallel actin filaments.