The molecular mechanism of load adaptation by branched actin networks

  1. Tai-De Li
  2. Peter Bieling  Is a corresponding author
  3. Julian Weichsel
  4. R Dyche Mullins  Is a corresponding author
  5. Daniel A Fletcher  Is a corresponding author
  1. City University of New York, United States
  2. Max Planck Institute of Molecular Physiology, Germany
  3. University of California, Berkeley, United States
  4. University of California, San Francisco, United States

Abstract

Branched actin networks are self-assembling molecular motors that move biological membranes and drive many important cellular processes, including phagocytosis, endocytosis, and pseudopod protrusion. When confronted with opposing forces, the growth rate of these networks slows and their density increases, but the stoichiometry of key components does not change. The molecular mechanisms governing this force response are not well understood, so we used single-molecule imaging and AFM cantilever deflection to measure how applied forces affect each step in branched actin network assembly. Although load forces are observed to increase the density of growing filaments, we find that they actually decrease the rate of filament nucleation due to inhibitory interactions between actin filament ends and nucleation promoting factors. The force-induced increase in network density turns out to result from an exponential drop in the rate constant that governs filament capping. The force dependence of filament capping matches that of filament elongation and can be explained by expanding Brownian Ratchet theory to cover both processes. We tested a key prediction of this expanded theory by measuring the force-dependent activity of engineered capping protein variants and found that increasing the size of the capping protein increases its sensitivity to applied forces. In summary, we find that Brownian Ratchets underlie not only the ability of growing actin filaments to generate force but also the ability of branched actin networks to adapt their architecture to changing loads.

Data availability

Source Data files have been provided for the top and bottom panels of Figure 1c. Movies 1-3 contain source data for Figures 1-4.

Article and author information

Author details

  1. Tai-De Li

    Advanced Science Research Center, City University of New York, New York, United States
    Competing interests
    The authors declare that no competing interests exist.
  2. Peter Bieling

    Department of Systemic Cell Biology, Max Planck Institute of Molecular Physiology, Dortmund, Germany
    For correspondence
    peter.bieling@mpi-dortmund.mpg.de
    Competing interests
    The authors declare that no competing interests exist.
    ORCID icon "This ORCID iD identifies the author of this article:" 0000-0002-7458-4358
  3. Julian Weichsel

    Department of Chemistry, University of California, Berkeley, Berkeley, United States
    Competing interests
    The authors declare that no competing interests exist.
  4. R Dyche Mullins

    Department of Cellular and Molecular Pharmacology, University of California, San Francisco, San Francisco, United States
    For correspondence
    Dyche.Mullins@ucsf.edu
    Competing interests
    The authors declare that no competing interests exist.
    ORCID icon "This ORCID iD identifies the author of this article:" 0000-0002-0871-5479
  5. Daniel A Fletcher

    Department of Bioengineering and Biophysics, University of California, Berkeley, Berkeley, United States
    For correspondence
    fletch@berkeley.edu
    Competing interests
    The authors declare that no competing interests exist.
    ORCID icon "This ORCID iD identifies the author of this article:" 0000-0002-1890-5364

Funding

National Institutes of Health (1R35 GM118119)

  • R Dyche Mullins

National Institutes of Health (R01 GM134137)

  • Daniel A Fletcher

Howard Hughes Medical Institute

  • R Dyche Mullins

Human Frontier Science Program (LT-000843/2010)

  • Peter Bieling

Human Frontier Science Program (CDA00070/2017-2)

  • Peter Bieling

European Molecular Biology Organization (ALTF 854-2009)

  • Peter Bieling

Chan Zuckerberg Initiative

  • Daniel A Fletcher

The funders had no role in study design, data collection and interpretation, or the decision to submit the work for publication.

Reviewing Editor

  1. Alphee Michelot, Institut de Biologie du Développement, France

Publication history

  1. Received: August 27, 2021
  2. Accepted: June 7, 2022
  3. Accepted Manuscript published: June 24, 2022 (version 1)

Copyright

This is an open-access article, free of all copyright, and may be freely reproduced, distributed, transmitted, modified, built upon, or otherwise used by anyone for any lawful purpose. The work is made available under the Creative Commons CC0 public domain dedication.

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  1. Tai-De Li
  2. Peter Bieling
  3. Julian Weichsel
  4. R Dyche Mullins
  5. Daniel A Fletcher
(2022)
The molecular mechanism of load adaptation by branched actin networks
eLife 11:e73145.
https://doi.org/10.7554/eLife.73145

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