5) The cytoskeleton

• The cytoskeleton is a network of protein filaments in the cytoplasm of cells that provide structural support.
• The cytoplasm is highly dynamic. This allows it to respond to external and internal changes.
• The benefits of the cytoskeletons ability to adapt:

  1. Structural integrity
  2. Intracellular transport
  3. Cell movement
  4. Cell division.
  5. Cellular signalling
• The three protein filaments in the cytoskeleton are

  1. Intermediate filaments
  2. Microtubules (MTs)
  3. Actin filaments

  
• Functions of the different cytoskeleton filaments:

  • Intermediate: Mechanical strength
  • Microtubules: Organising the cytoplasm
  • Actin filaments: Maintaining cell structural integrity
• Intermediate filaments have an intertwined rope-like structure which provides tensile strength, allowing them to withstand mechanical stress and preventing cells rupturing

  
• What is the role of microtubules in eukaryotic cells?

  - Transporting and positioning membrane- bound organelles

  - Intracellular transport of cytosolic macromolecules

  - Cell division

  - Form structures in some cells, such as cilia and flagella
• Microtubules are key components in processes such as cell division, intracellular transport and maintaining cell shape
• Microtubules are the “scaffolding” of cells made up of tubulin proteins. They’re dynamic and change size to help facilitating the movement of organelles and macromolecules in the cytoskeleton
• The centrosome co-ordinates microtubule activity, within the cell.

  αβ-tubulin dimers and bind to specific sites of the centrosome (γ-tubulin rings) to assemble into linear chains of protofilaments.

  
• describe the ends of microtubules

  plus end = fast growing, beta tubulin

  minus end = slow growing, alpha tubulin
• MT size is influenced by polymerisation and depolymerisation of αβ tubulin dimers at y-tubulin rings.

  • Growth is a result of + end binding, shortening is a result of - end binding.
• Motor proteins (such as dyneins) use hydrolysis of ATP to cause a conformational change and move towards MT - end.

  • This active transport mechanism facilitates intracellular movement
• Actin filaments can be found in microvilli, cytoplasm, and cytoskeleton
• • Actin filaments are formed by G-actin monomers which carry ATP
  • G-actin binds to F-actin at the plus end, elongating the filament.
  • During depolymerisation, ATP is hydrolysed to destabilise the filament. The actin filament disassembles back into G-actin monomers, facilitating cell motility

  
• Actin-binding proteins bind and regulate the structure of actin filaments.
• Actin monomer-sequestering proteins regulate polymerisation by binding to G-actin monomers and prevent them from binding to filaments.
• Nucleating proteins provide a template to initiate actin polymerisation
• Actin bundling proteins crosslink actin filaments together into bundles and networks

  
• Actin filaments act as a scaffold for the plasma membrane at the cell cortex.
• Actin filaments play a role in cell migration.

  1. Edge of a migrating cell extends past the membrane through protrusions like lamellipodia formed by actin polymerisation (addition of G-actin at + end)
  2. The protrusion adheres to integrin receptors (Transmembrane proteins) in the ECM or other cells - known as focal adhesions
  3. Myosin motor proteins use ATP to slide along actin filaments, causing muscle contraction.
  4. The cell drags itself forward

  
• Actin-related proteins bind to actin filaments to form Lamellipodia.
• Actin filaments involvement in contraction:

  1. motor proteins such as myosin II interact with G-actin and F-actin
  2. Myosin II binds to F-actin using ATP hydrolysis, causing a conformational change allowing movement along the actin filament
  3. Myosin II heads migrate towards the positive end of actin
  4. Actin-Myosin cause shortening of actin filaments
• Cytoskeletal elements

  • Actin filaments
  • Microtubules
  • Intermediate filaments
  • Lamins (Nucleus)
• Actin filaments

  • Filament diameter ~7 nm
  • Subunit proteins: G-actin (monomeric), F-actin (polymer)
  • Arrangement: Two polymer strands in helical association
  • Polarity: '-'end & '+'end
• Microtubules

  • Filament diameter ~24 nm (outer diameter)
  • Subunit proteins: α-tubulin, β-tubulin
  • Arrangement: α-tubulin/β-tubulin dimers assemble into protofilaments that form hollow tubes
  • Polarity: 'minus'end & 'plus'end
• Intermediate filaments

  • Filament diameter ~10 nm
  • Subunit proteins: Keratins (~50 types), Vimentin, Desmin, GFAP, Neurofilament (NF)
  • Arrangement: Rope-like polymers with complex protofilament assembly
  • High mechanical stability
  • No polarity