In vivo actin dynamics are modified by actin binding

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In vivo actin dynamics are modified by actin binding proteins Profilin Binds ADP-actin and enhances exchange for ATP Binds to proline-rich sequences Cofilin F-actin severing protein Leads to more free (-) end-- enhances disassembly Thymosin beta 4 Sequesters free ATP-actin Acts as a reservoir for ATP-actin Binding proteins important for dynamic actin changes 12
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Myosin motor proteins move along actin filaments Myosin Uses energy of ATP hydrolysis to power movements Structure: 3 domains Head motor binds to actin binds to ATP Neck lever arm length helps dictate walk Tail cargo binding 13
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Most Myosins move towards the (+) end of actin How to study myosin movement in vitro? Filament sliding assay Visualize the movement of actin when myosin is fixed 1) Tether myosin to glass slide 2) Add fluorescently-labelled F- actin 3) Add ATP 4) Visualize movement of actin by microscopy If myosin moves towards the (+) end, which end will lead the way in a sliding filament assay? 14
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Different classes of myosins serve distinct functions 15
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Microtubules serve different function than actin Differences Tube structure Sturdy- allows for pulling and pushing motions without buckling usually exists as a singlet Bound nucleotide Minimal spontaneous assembly In vivo assembly from few locations Temperature sensitive-unstable at 4°C Similarities Filament structure flexible networks Spontaneous assembly Regulated assembly from a large number of locations within the cell 16
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Basic unit of a microtubule is an α / β dimer 3 types of tubulin: α , β , and γ α and β tubulin monomers interact with one-to-one stoichiometry to form dimer Both subunits bind GTP α -tubulin-bound GTP is not exchanged or hydrolyzed (GTP is fixed) β -tubulin-bound GTP is hydrolyzable and exchangeable (GTP is dynamic) γ -tubulin regulates growth of tubulin 17
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Tubulin dimers interact to form microtubules 1. α / β dimerization 2. Protofilament formation- stacking of dimers; new α subunit interacts with old β - subunit; GTP hydrolysis accompanies binding, but is not required 3. Tube formation- lateral connection of protofilaments; lateral and vertical growth occurs at same time to produce a tube of 13 parallel protofilaments 4. Elongation- growth of tube 5. Steady state- equilibrium 18
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Microtubules have polarity α -subunits at (-) end β -subunits at (+) end; favors polymerization GDP/GTP levels mediate polarity; GTP at (+) end, GDP along most of length
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