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Topic 6-2

Topic 6-2. 2. Microtubule-Organizing Centers

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Topic 6-2

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    1. Topic 6-2 1

    2. Topic 6-2 2 Microtubule-Organizing Centers – (MTOCs) Assembly – two phases Nucleation Elongation Best studied – Centrosome Two barrel shaped centrioles Pericentriolar material (PCM) Sites where microtubules converge

    3. Topic 6-2 3 Microtubule-Organizing Centers – (MTOCs) Centrioles PCM – initiates formation of microtubules Microtubule minus end in centriole Microtubules enlongated at opposite end Basal Bodies and other MTOCs Cilia & Flagella Identical in structure to centrioles

    4. Topic 6-2 4 Microtubule-Organizing Centers – (MTOCs) Microtubule nucleation All MTOCs – a common protein – g-tubulin

    5. Topic 6-2 5 Microtubules Dynamic Properties Mostly extremely labile Non-covalent bonds More stable forms Stabilized by MAPs Enzymatic modification

    6. Topic 6-2 6 Microtubules Dynamic Properties Mostly extremely labile GTP required for assembly GTP bound to b-tubulin GTP hydrolysis after incorporation After dimer is released from structure – GDP replaced by GTP A dimer with GTP bound has a different conformation from a GDP-bound dimer

    7. Topic 6-2 7 Microtubules Dynamic Properties Growing microtubule + end is an open sheet GTP dimers added Added more rapidly than GTP can be hydrolysed GTP ‘cap’ favors addition of more dimers Microtubules can shrink very rapidly If open end becomes ‘closed’ – the structure becomes unstable

    8. Topic 6-2 8 Microtubules – Cilia and Flagella – structure/function Cilia and Flagella – two versions of the same structure Patterns of movement Cilia – power stroke – rigid state - recovery stroke – flexible Occur in large numbers Beating is coordinated Flagella – longer Different waveform patterns

    9. Topic 6-2 9 Microtubules – Cilia and Flagella – structure Core – axoneme Microtubule array – 9 peripheral doublets + central pair + ends at tip & - ends at base Each doublet One complete (13 subunits) - A tubule One incomplete (10-11 subunits) – B tubule

    10. Topic 6-2 10 Microtubules – Cilia and Flagella – structure Central tubules Enclosed by projections - Central sheath Connected to A tubules of peripheral doublets by radial spokes Doublets connected to each other – interdoublet bridge Interdoublet bridge – an elastic protein – nexin Radial spokes in groups of three. Basal body – A, B and C tubules

    11. Topic 6-2 11 Microtubules – Cilia and Flagella – structure Dynein arms Swinging cross-bridges Project from one doublet ‘Walk’ along the next So doublets ‘slide’ relative to each other

    12. Topic 6-2 12 Intermediate Filaments Only in animal cells Interconnected by cross-bridges of plectin Plectin Different isoforms One end –binds IF Other end varies – isoforms Another IF Microtube Microfiber Heterogenous group > 50 genes 6 major classes

    13. Topic 6-2 13 Intermediate Filaments

    14. Topic 6-2 14 Intermediate Filaments All classes have Central, rod-shaped a-helical domain Flanked by variable globular domains rod-shaped a-helical domains Spontaneously form coiled coils Both with same polarity Dimer has polarity

    15. Topic 6-2 15 Intermediate Filaments Assembly Tetramer of 2 dimers Staggered Antiparallel Tetramers lack polarity Distinguishing characteristic

    16. Topic 6-2 16 Microfilaments Globular protein –actin ATP- actin polymerizes Two strands of actin Wound around each other Double helix Actin filament = F-actin = Microfilament F-actin – often for in vitro form Each actin unit has polarity All actin units in same orientation Whole filament has polarity

    17. Topic 6-2 17 Microfilaments Arrangement variable Highly ordered arrays Loose networks Well defined bundles

    18. Topic 6-2 18 Microfilaments A major contractile protein of muscle Occurs in every cell A major protein Interacts specifically with myosin

    19. Topic 6-2 19 Microfilament – assembly / disassembly Prior to incorporation Actin monomer binds to ATP Actin is an ATP-ase ATP hydrolyzed after incorporation During assembly (rapid) Filament has an actin-ATP cap Favors assembly + end is the fast-growing end - end Slow growing Site of preferential depolymerization

    20. Topic 6-2 20 Microfilament – assembly / disassembly Monomers tend to move down the filament ‘treadmilling’ Intracellular equilibrium – monomeric actin and polymer Cellular control of this equilibrium Localized protein interactive effects Dynamic reorganization Locomotion cytokinesis

    21. Topic 6-2 21 Microfilament – assembly / disassembly

    22. Topic 6-2 22 Myosin – the molecular motor for Actin Filaments Myosin superfamily Conventional (Type II) myosins

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