Microfilaments

Microfilaments, also known as actin filaments, are the thinnest filaments of the eukaryotic cytoskeleton, composed primarily of actin, a globular protein. They are involved in cell shape, movement, muscle contraction, intracellular transport, and division.

Structure:

  • Made of actin monomers (G-actin) that polymerize to form filamentous actin (F-actin).
  • Filaments are 7 nm in diameter, making them the smallest of the three main cytoskeletal elements.
  • Exhibit polarity:
    • Plus (+) end: Fast-growing
    • Minus (−) end: Slow-growing

Functions:

FunctionDescription
Cell shape maintenanceForms a supportive network under the plasma membrane (cortex)
Cell movementEnables amoeboid movement, lamellipodia, and filopodia in migrating cells
Muscle contractionInteracts with myosin to produce contraction in muscle cells
CytokinesisForms the contractile ring that pinches the cell in two during cell division
Intracellular transportMoves vesicles, organelles, and RNA within the cell
Endocytosis and exocytosisFacilitates vesicle budding and fusion with the plasma membrane

Microfilaments vs. Other Cytoskeletal Elements:

FeatureMicrofilamentsIntermediate FilamentsMicrotubules
Diameter~7 nm~10 nm~25 nm
SubunitsActinVarious (e.g., keratin)α- and β-tubulin dimers
FunctionsShape, motility, divisionStrength, structureTransport, division, cilia/flagella
PolarityYesNoYes
Dynamic InstabilityYes (treadmilling)NoYes

Dynamic Behavior:

  • Treadmilling: Actin subunits are added at the plus end and removed from the minus end, maintaining filament length while allowing turnover.
  • Remodeled constantly in response to signaling pathways, enabling rapid changes in cell shape and behavior.

Associated Proteins:

  • Formins and Arp2/3 complex: Promote actin nucleation and branching.
  • Cofilin: Promotes actin filament disassembly.
  • Myosin: Motor protein that interacts with actin for movement and contraction.

Clinical Relevance:

  • Cancer: Actin remodeling is critical for metastasis and invasion.
  • Pathogen infection: Some bacteria (e.g., Listeria) hijack actin to move inside host cells.
  • Genetic disorders: Mutations in actin or actin-regulating proteins affect immune cells, platelets, and neurons.
  • Drugs:
    • Cytochalasins disrupt actin polymerization.
    • Phalloidin binds and stabilizes actin filaments (used in microscopy).

Summary Table:

PropertyDescription
Protein SubunitActin (G-actin polymerizes into F-actin)
Diameter~7 nm
PolarityYes (+ and − ends)
Main FunctionsCell shape, motility, cytokinesis, muscle contraction
DynamicsRapid assembly/disassembly; treadmilling
Found InAll eukaryotic cells, especially near the cell membrane