The Cell: Cytosol and Cytoskeleton

A cell’s cytoskeleton is a network of intracellular protein fibers that provides structural support, anchors organelles, and aids intra- and extracellular movement. The cytosol is the liquid inside the cell membrane that surrounds the organelles and cytoskeleton. The cytosol is a complex solution where many biochemical processes take place.

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Cytoskeleton

The cytoskeleton is composed of different protein fibers that extend through the cytosol. Three different types of fiber proteins provide the framework along which molecular motor proteins move.

Protein fibers

  • Microfilaments (actin filaments): 
    • Structure:
      • Smallest
      • Most common intracellular protein
      • 2 filaments made of approximately 7 nm actin polymers (F-actin) coiled around each other (double helix)
      • Growth is directional (positive and negative end).
    • Function: 
      • Disassembles and reassembles rapidly (treadmilling), allowing cell to move
      • Provides track for motor protein myosin 
      • Provides some structural support to cell
    • Examples:
      • Muscle contractions
      • Assemble along the circumference of a cell during cytokinesis and tighten to pinch it into 2 daughter cells
      • Macrophage movement within body
      • Cytoplasmic streaming (movement of cytoplasm within the cell)
  • Intermediate filaments: 
    • Structure:
      • Medium
      • 10 nm in diameter
    • Composed of several different fibrous proteins
    • Function:
      • Maintenance of cell shape
      • Anchor organelles
      • Structural components of nuclear lamina
    • Examples:
      • Keratin protects against mechanical stress.
      • Cell–cell connection and intercellular communications
      • More stable than microfilaments
      • Stronger tensile strength
  • Microtubules
    • Structure:
      • Largest
      • 25–100 nm spiral made up of alpha and beta tubulin
    • Function:
      • Anchor on centriole near nucleus
      • Directional growth: addition and subtraction of tubulin subunits
      • Highways for transportation within cell by molecular motors
    • Examples:
      • Assemble and provide framework for chromosome movement during cell division
      • Form cilia and flagella, allowing movement through extracellular matrix

Molecular motors

Specialized proteins “walk” along the microtubule highways, transporting organelles, substrates, or chromosomes within the cell. 

  • Structure:
    • Protein complexes:
      • Transport vesicle
      • Connector protein
      • Motor protein
    • Different complex based on direction of transportation:
      • Dynein (connector dynactin) moves from the periphery toward the nucleus.
      • Kinesin moves from the nucleus toward the periphery.
  • Function:
    • Convert chemical energy into kinetic energy
    • Transport substances along microtubule “highways”
    • Assembly walks down microtubule by ATP hydrolysis.
  • Examples:
    • Walk along mitotic spindle to separate sister chromatids during cell division
    • Transport neurotransmitter-filled vesicles down neuron axons 
Molecular motor moving along microtubule

Molecular motor moving along microtubule

Image by Lecturio.

Cytosol

  • Cytosol is a liquid found inside cells (fluid matrix):
    • Water (70%)
    • Dissolved ions (pH 7.0–7.4)
    • Small proteins
    • Large, water-soluble macromolecules
  • Surrounded by cell membrane in prokaryotic cells
  • Part of cytoplasm (all material within cell membrane except nucleus)
  • Site of eukaryotic cellular processes:
    • Signal transduction
    • Cytokinesis
    • Glycolysis
    • Pentose phosphate pathway
    • Protein biosynthesis
    • Gluconeogenesis
    • Transport of metabolites and vesicles
  • Organizational areas within cytosol (components do not mix randomly):
    • Concentration gradients: (e.g., calcium) can be created near open channel
    • Protein complexes: form to pass substrate along metabolic pathway
    • Protein compartments: form to separate contents from cytosol (e.g., proteasome enzymes within proteasome)
    • Microdomains created by cytoskeleton: can prevent larger proteins from accessing areas of cytosol (sieving)

References

  1. Alberts B, Johnson A, Lewis J, et al. (2002). Molecular Biology of the Cell. 4th edition. New York: Garland Science. How Cells Regulate Their Cytoskeletal Filaments. https://www.ncbi.nlm.nih.gov/books/NBK26809/
  2. Alberts B, Johnson A, Lewis J, et al. (2002). Molecular Biology of the Cell. 4th edition. New York: Garland Science. The Cytoskeleton and Cell Behavior. https://www.ncbi.nlm.nih.gov/books/NBK26930/
  3. Lodish H, Berk A, Zipursky SL, et al. (2000). Molecular Cell Biology. 4th edition. New York: W. H. Freeman. Section 18.1, The Actin Cytoskeleton. https://www.ncbi.nlm.nih.gov/books/NBK21493/

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