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Intracellular Signaling Affects Focal Adhesions01:17

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Integrins act both as extracellular input receivers and as intracellular processing activators. As their name suggests, integrins are entirely integrated into the membrane structure. Their hydrophobic membrane-spanning regions interact with the phospholipid bilayer's hydrophobic region. These membrane receptors provide extracellular attachment sites for effectors like hormones and growth factors. They activate intracellular response cascades when their effectors are bound and active.
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When a ligand binds to a cell-surface receptor, the receptor's intracellular domain changes shape, which may either activate its enzyme function or allow its binding to other molecules. The initial signal is amplified by most signal transduction pathways. This means that a single ligand molecule can activate multiple molecules of a downstream target. Proteins that relay a signal are most commonly phosphorylated at one or more sites, activating or inactivating the protein. Kinases catalyze...
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Proteins undergo chemical modifications that trigger changes in the charge, structure, and conformation of the proteins. Phosphorylation, acetylation, glycosylation, nitrosylation, ubiquitination, lipidation, methylation, and proteolysis are various protein modifications that regulate protein activity. Such modifications are usually enzyme-driven.
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Oligopeptide Competition Assay for Phosphorylation Site Determination
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Cas phosphorylation regulates focal adhesion assembly.

Saurav Kumar1, Amanda Stainer1, Julien Dubrulle1

  • 1Fred Hutchinson Cancer Center, Seattle, United States.

Elife
|July 25, 2023
PubMed
Summary

Cell attachment triggers signaling, but its role in focal adhesion assembly remained unclear. We found that Cas phosphorylation precedes integrin activation, revealing a two-step model for focal adhesion formation.

Keywords:
cell biologycell migrationcell spreadingepithelial cellshuman foreskin fibroblastintegrinp130Cas

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Area of Science:

  • Cell biology
  • Molecular signaling
  • Biochemistry

Background:

  • Integrin-mediated cell attachment initiates rapid tyrosine kinase signaling, crucial for cellular processes.
  • The precise role of this signaling in integrin activation and focal adhesion assembly is not fully understood.
  • Crk-associated substrate (Cas) is a known substrate of Src-family kinases (SFKs) and a key player in signaling pathways.

Purpose of the Study:

  • To elucidate the role of Cas phosphorylation and its effectors in the early stages of integrin-mediated cell attachment.
  • To investigate the sequential events leading to focal adhesion assembly.
  • To propose a model for focal adhesion formation based on the observed signaling dynamics.

Main Methods:

  • Immunofluorescence microscopy to visualize phospho-Cas clusters and protein localization.
  • Biochemical assays to assess protein phosphorylation and association.
  • Cell spreading assays on different extracellular matrix proteins (collagen, fibronectin).
  • Genetic manipulation (e.g., using siRNA) to determine the requirement of specific proteins (Cas, Crk/CrkL, SFKs, Rac1, vinculin).

Main Results:

  • Phosphorylated Cas (phospho-Cas) and its effectors (Crk/CrkL) form clusters that precede focal adhesion assembly and contain inactive integrin β1.
  • Cas is essential for cell spreading and focal adhesion assembly in various cell types on collagen and fibronectin.
  • Cas cluster formation depends on Cas, Crk/CrkL, SFKs, and Rac1, but not vinculin.
  • Rac1 signaling, modulated by reactive oxygen species and the ubiquitin proteasome system, provides feedback on Cas phosphorylation.

Conclusions:

  • A two-step model for focal adhesion assembly is proposed, initiated by phospho-Cas clusters containing inactive integrin β1.
  • Positive feedback mechanisms involving Rac1 drive the growth of these initial clusters.
  • Integrin activation and recruitment of core focal adhesion proteins like vinculin occur after the formation of these initial Cas-based signaling hubs.