BIOCELL GEN Lecture 9 cytoskeleton

• Cytoskeleton


  • Structure (Filament network)
  • Function (Cell shape, motility, organization)

  
• Microtubules


  • Structure (Tubulin polymers -organised in Centrosomes-)
  • Function [cell shape (maintenance); cell (cilia/flagella) & organelle motility; cell (chromosome) division]

  
• Microfilaments (Actin filaments)


  • Structure (Actin filaments)
  • Function [cell shape (change);cell(pseudopodia/amoeboid) & organelle motility; cell (cytoplasm) division]

  
• Intermediate filaments


  • Structure (various monomers, ie Keratin, Lamin)
  • Function [cell shape (maintenance); cell motility (compression/tension); organelles (anchorage)]

  
• Accessory proteins


  • Structure (various monomers, ie Dynein, Kinesin, Myosin)
  • Function [motor protein associated with microtubules (dynein & kinesin) or actin (myosin)]

  
• The cytoskeleton is a network of fibers extending through the cytoplasm with 4 components: (1) Microtubules, (2) Microfilaments, (3) Intermediate filaments, (4) Accessory proteins

  
• Cytoskeleton

  • Cell shape maintenance and change
  • Cell organization [anchorage for organelles]
  • Cell motility [interaction between cytoskeleton & motor accessory proteins]
  • Organelles motility [provides track along which vesicles and other organelles move through motor protein "feet"]

  
• The cytoskeleton is an extremely 'fast acting' & 'dynamic' system by assembling, disassembling & reassembling constitutive subunits
• Cilia and flagella


  • Microtubule-containing extensions that project from some cells
  • Many unicellular protists are propelled through water by cilia or flagella
  • Cilia and flagella differ in their beating patterns

  
• Cilia and flagella movement

  1. Dynein has two "feet" that "walk" along microtubules

  2. One foot maintains contact, while the other releases and reattaches one step farther along

  3. Movements of the feet cause the microtubules to bend, rather than slide, because the microtubules are held in place

  
• Microfilaments (Actin filaments)


  • Solid rods about 7 nm in diameter, built as a twisted double chain of actin subunits
  • A network of microfilaments helps support the cell's shape
  • Bundles of microfilaments make up the core of microvilli of intestinal cells
  • Microfilaments that function in muscle contraction contain the protein myosin in addition to actin
  • In ameboid movement, cells crawl along a surface by extending pseudopodia (cellular extensions) and moving toward them
  • Cytoplasmic streaming, in plant cells, is a circular flow of cytoplasm within cells, driven by actin-protein interactions

  
• Intermediate filaments


  • Rope-like fibres, made up of intermediate filament proteins (eg: lamins, vimentin, keratins), depending on the cell type
  • Provide cells with mechanical stability (cell shape maintenance)
  • Important at cell-cell contact sites (desmosomes)
  • Cell motility (compression/tension)
  • Organelles anchorage
  • Keratins are the most diverse family -over 50 members
  • Vimentin IFs are particularly prevalent in fibroblasts, extend across the cytoplasm providing mechanical strength
  • Lamin proteins form the nuclear lamina just beneath the inner nuclear membrane

  
• Intermediate filaments: keratins


  • Hemidesmosomes in the skin are junctions that link intermediate filament proteins (KERATIN) inside the cell to the underlying basal lamina
  • Mutations in keratin leads to skin blisters (epidermolysis bullosa simplex)

  
• Intermediate filaments: lamins


  • Lamins are another family of intermediate filament protein
  • They are found inside the cell nucleus, playing a role in structure / stability
  • Mutations in lamin ("laminopathies") also cause disease, particularly associated with premature ageing
  • Hutchinson-Gilford Progeria Syndrome (HGPS)

  
• Actin-associated accessory proteins


  • Myosin superfamily
  • Generates force for muscle contraction
  • Tail & head structure
  • Some myosins are formed from heavy and light chains, such as myosin II (skeletal muscle myosin)
  • Myosin head domains bind ATP, and use the energy from its hydrolysis to move towards the plus ends of actin filaments
• Muscle contraction


  1. Muscle cells – thousands of actin filaments arranged parallel to one another

  2. 'Walking' of myosin drives parallel myosin and actin past each other

  3. Actin filaments approach each other in the middle

  4. Muscle cell shortens

  5. Muscle contraction = simultaneous shortening of many muscle cell
• Microtubule-associated accessory proteins


  • Kinesins and Dyneins
  • Kinesin: head domain highly conserved, tail binds to either a membrane-enclosed organelle or another microtubule, carries membrane enclosed organelles by walking towards plus ends of microtubules, involved in mitotic and meiotic spindle formation, chromosome separation during cell division
  • Dynein: Dynein motors are 'minus-end directed', drives the bending movement of cilia and flagella

  
• The microtubule cytoskeleton network plays a key role in maintaining cellular architecture and events through its dynamic properties and regulation

  
• Co-ordinated regulation of the actin cytoskeleton network allows cells to rapidly adapt and move in response to external stimuli
• Cytoskeleton, Structure


  Network of fibers extending through the cytoplasm4 components:(1) Microtubules(2) Microfilaments(3) Intermediate filaments(4) Accessory proteins
• Cytoskeleton, Functions


  muscle and skeleton’ of the cell• Cell shape maintenance and change• Cell organization [anchorage for organelles]• Cell motility [interaction between cytoskeleton& motor accessory proteins]• Organelles motility
• In animal cells, microtubules grow out from a centrosome near the nucleus• In animal cells, the centrosome has a pair of centrioles, each with nine triplets of microtubules arranged in a ring• Other eukaryotic cells organize microtubules in the absence of centrosomes with centrioles
• Microtubules Functions


  Functions of microtubules:• Shaping the cell• Guiding movement of organelles• Separating chromosomes during celldivision
• Many unicellular protists are propelled through water by cilia or flagella
• Motile cilia are found in large numbers on a cell surface• Flagella are limited to one or a few per cell• Cilia and flagella differ in their beating patterns
• Cilia and flagella share a common structure• A group of microtubules sheathed in an extension of the plasma membrane• Nine doublets of microtubules are arranged in a ring with two single microtubules in the center• A basal body that anchors the cilium or flagellum• A motor protein called dynein, which drives the bending movements of a cilium or flagellum
• Microfilaments are solid rods about 7 nm in diameter, built as a twisted double chain of actin subunits• A network of microfilaments helps support the cell’s shape• They form a cortex just inside the plasma membrane to help support the cell’s shape• Bundles of microfilaments make up the core of microvilli of intestinal cells
• Microfilaments that function in muscle contraction contain the protein myosin in addition to actin• In ameboid movement, cells crawl along a surface by extendingpseudopodia (cellular extensions) and moving toward them• Cytoplasmic streaming, in plant cells, is a circular flow of cytoplasm within cells, driven by actin-protein interactions
• Keratins are the most diverse family -over 50 members• Vimentin IFs are particularly prevalent in fibroblasts, extend across the cytoplasm providing mechanical strength• Lamin proteins form the nuclear lamina just beneath the inner nuclear membrane