Biophysics 8 - Cytoskeleton

    Cards (41)

    • The cytoskeleton
      • Responsible for giving the cell its shape
      • Allowing the cell to move
      • Allowing the cell to transport molecules and objects around inside
    • The cytoskeleton
      • Responsible for structural rigidity
      • Force application
      • Transport
    • Main components of the cytoskeleton
      • Actin
      • Tubulin
      • DNA
      • Motor proteins
    • Actin
      A globular protein that exists in all cells, highly conserved in structure across organisms
    • Actin
      • Forms two types of fibre: microfilaments (part of the cytoskeleton) and parts of muscle fibres
      • As a monomer it is called Globular (G)-actin, and as a fibre it is called F-actin. F-actin (actin fibres) are polymers of G-actin
      • Actin polymerisation is an active process that uses ATP as a source of energy. Actin hydrolyses ATP into ADP - it is an ATPase. The actin cytoskeleton is highly dynamic
    • Actin monomer
      Contains a-helix and b-sheet motifs as well as an ATP binding site
    • Actin polymerisation
      1. Actin by itself is inactive
      2. Actin binds ATP and becomes primed
      3. Small scale spontaneous nucleation of ATP-actin
      4. Actin polymerises as ATP is hydrolysed to ADP +Pi
      5. ADP bound actin depolymerises
    • Actin monomers form a double helix fibre
    • Proteins regulating actin polymerisation and depolymerisation
      • Profilin
      • Cofilin
      • Arp2/3
    • Profilin
      Binds actin monomers and helps direct them to the barbed end, can also bind to other proteins via proline-rich domains and to some membrane lipids
    • Cofilin
      Binds to ADP-actin and causes depolymerisation
    • Arp2/3
      Binds F-actin in the middle of a fibre and can act as a nucleation site for a new fibre, causing actin branching with a characteristic branch angle of 70 degrees
    • Actin treadmilling
      If the rate of polymerisation and depolymerisation are equal the fibre stays the same length but moves in the direction of the barbed end. Depends on the concentration of actin monomers (G-actin) and the presence of actin binding proteins
    • There is a critical concentration of g-actin required to achieve treadmilling
    • Above the critical concentration
      The rate of addition of actin monomers is greater than the rate of disassembly and the fibre elongates in one direction, generating a force
    • Ccrit is very difficult to know as it depends on the state of all the actin regulating proteins present in the system
    • Actin treadmilling is critical for the movement of lymphocytes and epithelial cells, called cell migration
    • Estimates of the force generated per actin fibre during cell migration are in the 5 pN range
    • If C=Ccrit, disassembly and assembly are happening at equal rates and no force is generated
    • Microtubules
      • Polymers of tubulin
      • Generally longer, thicker and more stable than actin fibres
      • Can be several tens of microns long and have an outer diameter of 24 nm
    • Tubulin
      First forms a-b dimers which later polymerise into full microtubules
    • Microtubule polymerisation uses GTP (Guanosine Tri Phosphate) as an energy source, similar to ATP for actin
    • Microtubules
      • Have directionality: a + end (with b-subunits exposed) and a - end with a-subunits exposed
      • Polymerisation generates force
    • Microtubules
      The next most important part of the cytoskeleton. Like actin they have a structural role and generate force, but are also very important for transport of substances around the cell
    • Actin is also important for transport, with cargos able to move along fibres
    • Transport occurs on, not within the microtubules
    • Microtubules
      Polymers of tubulin. Tubulin first forms a-b dimers which later polymerise into full microtubules
    • Microtubules
      • Generally longer, thicker and more stable than actin fibres. Can be several tens of microns long and have an outer diameter of 24 nm
    • Microtubule organising centre (MTOC)

      A central organelle containing a pair of centrioles (also made of tubulin) that contain a special kind of tubulin called g-tubulin. The MTOC is usually located near the nucleus
    • Role of microtubules
      • One of the most important jobs is to separate out the pairs of chromosomes during cell division (mitosis)
    • Drugs that interfere with tubulin and microtubules are often used in chemotherapy to block cell division (mitosis). An example is the drug Taxol, purified from the Pacific Yew tree which blocks microtubule disassembly
    • Intermediate filaments
      A family of around 10 structurally related proteins which all serve a similar structural role but are not active - they do not contain ATP or GTP binding sites and do not undergo tread-milling. They include vimentin, spectrin, keratin and lamins
    • Motor proteins
      Proteins that move along actin or tubulin fibres. Their role is to generate force to either move cargo (e.g. a membranous vesicle) along the fibre or to move the fibre itself. Some common types are Myosin (which move along actin fibres) and Dynein and Kinesin which move along microtubules
    • Motor proteins
      • They all get their energy from the hydrolysis of ATP to ADP
    • Cargo transported by motor proteins
      • Vesicles (liposomes) generated from the Golgi apparatus carrying surrounded molecules or embedded membrane proteins
      • Organelles such as mitochondria
    • Myosin movement along actin
      1. In the absence of ATP, myosin binds tightly to actin
      2. ATP binds and Myosin is released
      3. ATP is hydrolysed to ADP, Myosin pivots and makes a 10 nm step, generating a force of 5pN
      4. Myosin rebinds and ADP is released
    • Hand-over-hand movement
      The stepwise motion of the motor proteins
    • Motor protein directionality
      • Myosin and Kinesin walk towards the + end. Dynein walks towards the - end
    • Myosin step size
      • 10 nm per ATP molecule consumed
    • Kinesin step size
      • 8 nm
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