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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.
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Cytoskeletal filaments are polymeric forms of smaller protein subunits. However, individual cytoskeletal filaments may easily disassemble or associate with other similar filaments to form rigid structures. Microfilaments, made of actin monomers, rely on actin-binding proteins to form bundles and create networks of individual actin filaments. Microtubules rely on microtubule-associated proteins (MAPs) to form sturdy cylindrical structures. However, the proteins involved in forming complex...
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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...
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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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Cations Stiffen Actin Filaments by Adhering a Key Structural Element to Adjacent Subunits.

Glen M Hocky1, Joseph L Baker2, Michael J Bradley3

  • 1Department of Chemistry, James Franck Institute, Institute for Biophysical Dynamics, and Computation Institute, The University of Chicago , Chicago, Illinois 60637, United States.

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Divalent cations bind to actin filaments at two specific sites, influencing polymerization and filament stiffness. Molecular dynamics simulations reveal how cation binding at the "stiffness site" alters actin mechanics by restricting D-loop movement.

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Area of Science:

  • Biochemistry
  • Structural Biology
  • Biophysics

Background:

  • Actin filaments are crucial cytoskeletal components regulated by ions.
  • Two cation binding sites on actin filaments are proposed to control polymerization and mechanical properties.

Purpose of the Study:

  • To identify candidate binding pocket geometries for divalent cations on actin filaments.
  • To elucidate the mechanism by which cation binding affects filament mechanical properties, particularly stiffness.

Main Methods:

  • Molecular dynamics simulations were employed to investigate cation binding sites.
  • Analysis focused on conformational changes and accessibility of actin subunit regions.

Main Results:

  • A magnesium ion in the "polymerization site" minimally affects actin conformation.
  • Binding of a magnesium ion in the "stiffness site" restricts the actin DNase-binding loop (D-loop), increasing filament torsional stiffness and bending persistence length.
  • Cation binding limits D-loop conformational flexibility and buries conserved residues, potentially affecting protein interactions.

Conclusions:

  • Molecular dynamics simulations provide insights into actin filament cation binding sites and their mechanical consequences.
  • Cation binding at the "stiffness site" stiffens actin filaments by modulating D-loop dynamics and accessibility.