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Related Concept Videos

Actin Filament Depolymerization01:19

Actin Filament Depolymerization

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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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Generation of Straight or Branched Actin Filaments01:14

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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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Adaptability of Cytoskeletal Filaments01:12

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The cytoskeleton is a complex dynamic structure performing varied functions based on cellular requirements. The adaptability of the individual filaments in the cytoskeleton determines their ability to perform various functions within the cell. It can undergo rapid reorganization during processes like cell division or remain stable for several hours as in the interphase. The adaptability of these filaments depends on stringent regulatory mechanisms. The microfilament and microtubules of the...
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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.
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Disassembly of Intermediate Filaments01:35

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Intermediate filaments (IFs) do not undergo spontaneous disassembly. Enzymes, kinases, and phosphatases add and remove phosphates from specific sites to regulate their disassembly. The IF concentration in the cytoplasm also regulates the disassembly. If the concentration crosses a threshold, it activates the protein kinases in the vicinity, allowing the phosphorylation of IFs.
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Microtubule Instability02:17

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Microtubules are hollow cylindrical filaments having a diameter of approximately 25 nm and a length that varies from 200 nm to 25 μm. GTP-bound tubulin subunits form αβ-heterodimers for microtubule assembly. These core building blocks interact longitudinally, polymerizing into protofilaments. The protofilaments then interact with one another through lateral bonding forces to form stable cylindrical microtubules. These cylindrical filaments are dynamic as they undergo repeated...
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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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Broken Detailed Balance of Filament Dynamics in Active Networks.

J Gladrow1, N Fakhri2,3, F C MacKintosh3,4

  • 1Third Institute of Physics, Georg August University, 37077 Göttingen, Germany.

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Motor proteins create cell cytoskeleton fluctuations. We analytically found these fluctuations couple normal modes, violating detailed balance and creating conformational currents, consistent with experiments.

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Area of Science:

  • Biophysics
  • Soft Matter Physics
  • Cellular Mechanics

Background:

  • Cellular cytoskeletons exhibit dynamic fluctuations driven by motor proteins like myosin.
  • Semiflexible filaments, such as microtubules and carbon nanotubes, serve as effective probes for studying these biopolymer networks.
  • Understanding nonequilibrium fluctuations is crucial for cell mechanics and function.

Purpose of the Study:

  • To analytically calculate the shape fluctuations of semiflexible probe filaments in a viscoelastic, motor-driven cytoskeletal network.
  • To investigate the coupling between dynamic normal modes of filament fluctuations under nonequilibrium conditions.
  • To explore the implications of mode coupling for detailed balance and conformational dynamics.

Main Methods:

  • Analytical calculation of transverse bending fluctuations for semiflexible probe filaments.
  • Decomposition of fluctuations into dynamic normal modes.
  • Modeling of motor-driven nonequilibrium conditions in a viscoelastic environment.

Main Results:

  • Nonequilibrium driving leads to effective coupling between normal modes, disrupting their independent evolution.
  • This mode coupling results in nonzero circulatory currents in conformational phase space, indicating a violation of detailed balance.
  • Predictions for characteristic frequencies of these currents were derived and linked to motor activity temporal signatures.

Conclusions:

  • Motor-driven cytoskeletal activity induces coupled dynamics in probe filament fluctuations.
  • The violation of detailed balance and emergence of conformational currents are key signatures of nonequilibrium biopolymer networks.
  • Findings align with experimental observations in microtubule-cytoskeleton systems, validating the theoretical framework.