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Section 4.docx

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Dalhousie University
BIOL 2020
Patrice Cote

Cell Bio-Feb 20 brownian rachet: process by which the chaperones pull the protein inside the matrix. Chaperones come one by one, slowly pulling it down. mitochondrial processing peptidase cuts off pre-sequence. And then the protein can fold into its native structure. SECTION 4 Intermediate filaments: cell specific neurons-- neurofilaments skin cells - keratin Cytoskeleton functions: Determine cell shape positions organelles in the cell directs the movement of materials and organelles in cells moves cells from place to place mitosis and cell division Microtubules BIG filament 25nm (bigger than plasma membrane) hollow tubes made up of 13 protofilaments protofilament: each individual line they don’t line up perfectly; makes a spiral = results in a seam where things are out of phase alpha-beta tubulin subunits tubulin heterodimer: alpha tubulin (subunit)- GTP - negative polarity end beta tubulin (subunit) - GDP is exchangeable - positive polarity end the whole tubule has polarity (because we can recognize one end from the other) minus (alpha) end is usually closer to the nucleus because of directionality the cell is able to know which direction to go; the cell can send things in the right direction. specific microtubule functions: mechanical support to cells (cell shape) maintains the internal organization of cells movement of intracellular components (because of polarity) ex: vesicles involved in the movement of cells (analogy: tubule is like skeleton; actin is like muscles) cell division - movement of chromosomes - mitotic spindle Microtubule-Associated Proteins (MAPs) structural MAPs stabilizing proteins; prevent microtubule from falling apart on its own MAP1, MAP2, MAP4, tau clamps onto the side of the microtubule and prevents the subunits from falling away Dynamic MAPs motor proteins Axonal Transport motor neurons- axons can go a very long distance. these cells have developed a network to transport material transport near the nucleus all the way to the pre-synapse or axon terminals movement down the microtubule “highway” anterograde: towards axon terminals retrograde: towards nucleus minus end towards nucleus, plus end towards terminals Microtubule-associated motor proteins kinesin goes in anterograde dynein retrograde Kinesin: smallest molecule (380 kDa) tetramer constructed from two identical heavy chains and two identical light chains globular head binds to microtubule tail binds to cargo to be hauled plus end directed microtubule motor single kinesin molecule moves along a single protofilament hand over hand movement 8 nm step long, 8 nm = 1 tubulin dimer requires hydrolysis of one ATP molecule hand over hand movement: when one head binds to the microtubule, the resulting conformational change in the neck region pushes the other head forward to the next binding site. Dynein: cytoplasmic dynein (no cilia or flagella) may not be present in plants only two types of cytoplasmic dynein huge protein (1.5 million daltons) two identical heavy chains, variety of intermediate and light chains heavy chains has a large globular head with an elongated projection (stalk) head = force generating engine the stalk contains the microtubule - binding site moves towards the (-) end, towards cell body, retrograde moves things towards the golgi dynactin: adaptor; can regulate activity and help binding Microtubule Organizing Centre (MTOC) -Centrosome two barrel-shaped centrioles surrounded by electron dense pericentriolar material (PCM) centriole: nine evenly spaced fibrils, contains A B C tubules only A tubule is complete; connected to center of centriole by a radial spoke pairs situated at right angles to one another major site of microtubule initiation; remains at the center of the cell’s microtubular network PCM initiates microtubule formation minus end close to centrosome Basal Body flagellum ?-tubulin 0.005 of cell’s tubulin -critical component in microtubule nucleation - PCM attaches to gamma-tubulin Dynamic Properties of Microtubules assembly and disassembly is temperature dependent GTP required for microtubule assembly must be bound to beta-subunit GTP is hydrolyzed after subunit is bound, then it remains on the structure when disassembly occurs and a dimer is released, the GDP is replaced by a new GTP dynamic instability growing and shrinking microtubules can exist in the same region they can switch back and forth shrinkage occurs more rapidly than elongation Intermediate filaments: 10-12nm connected to other parts of cytoskeleton via Plectin (plectin is not actually an intermediate filament) made up of a huge variety of protein strongest element of the cytoskeleton a lot of tensile strength, very rigid, don’t bend very much most common protein: keratin (there are many different types of keratin) desmosomes hemidesmosome ^^important for connecting cells to each other diseases if there are mutations in keratin: epidermolysis bullosa simplex keratin is soft, junctions aren’t
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