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31 Cards in this Set
- Front
- Back
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3 cytoskeletal filaments
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1) Actin filaments = Micro filaments/F-actin
= 8nm, subunits = G-actin 2) Intermediate filaments = 10nm, subunits = keratin, desmin, vimentin, etc. 3) Microtubles = 25nm, subunits = tubulin |
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Common characteristics of cytoskeletal filaments
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1) Self-assembly via subunits - no enzymes
2) Dynamic - can assemble/disassemble, sometimes quickly 3) Tightly controlled by accessory proteins (respond to signals) 4) Can also form STABLE structures |
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Actin structures
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1) Epithelial cell - microvilli (stable), cell cortex (membrane support), adherens belt (circumfrentially links cells @ lumenal edge)
2) Migrating cell - Filipoidia (lasts only seconds), Lamellipodium (leading edge), cell cortex (membrane support) 3) Adherent dividing cell - Stress fibers, contractile ring (@ middle to pinch off) |
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Acting monomers
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G-actin (globular)
- Bind ATP or ADP - Low-level ATP-ase activity |
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F-actin structure
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Polar, helical arrangement
- Filaments have + and - end - Subunits add faster to + end |
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Treadmilling
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More ATP-bound subunits at + end b/c faster rate of addition
- Assembly at + end, disassembly at - end |
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Why treadmilling enhancer proteins?
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Because intrinsic treadmilling rate = SLOW!
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Thymosin-β
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Binds G-actin, maintains local concentration near + end
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Profilin
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regenerates ATP monomers
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Cofilin
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breaks up monomers w/ ADP (near the - end)
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Arp2/3
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Initiates new branch from side of existing filament
- In response to WASP |
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Sequence of forming branched networks
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- Extracellular signal/stimuli
- Activated GTP-ases, PIP2 - Activate WASP - Activate Arp2/3 - new branched filament - "Barbed" (+) end elongates, push membrane forward - End capped - too floppy if too long - Aging - Cofilin depolymerizes - Regeneration - Profilin, etc. |
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Listeria
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In unpasteurized milk, cheese
- Has Act-A protein - Hijacks actin in cells, rockets around - Can move through tissue undetected |
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Endocytosis
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Via Arp2/3, polymerizes and assists with invaginations
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Actin binding proteins
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Organize F-actin into structures
- Fimbrin - Filamin - Spectrin - Dystrophin - Myosin |
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Fimbrin
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Tightly binds 2 microfilaments
- Support microvilli, filopodia |
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Filamin
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Loosely binds 2 microfilaments
- Loose gels, meshwork - Forms cortex that supports cell membrane |
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Spectrin
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Links F-actin filaments and membrane proteins
- Creates network underlying the cell membrane/structure |
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Dystrophin
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Links F-actin to membrane proteins
- Especially in muscle cells... |
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Wiscott-Aldrich
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Caused by WASP mutations in hematopoetic cells
- Poor Arp2/3 function - Platelets small, poor function - Immunodeficiency - T-cells, antigen presenting mechs don't work well |
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Filamin mutations
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Filamin helps form "leading edge" of migrating cells
- No filamin, no migrating - E.g. - Neurons don't migrate to cortex = late onset epilepsy |
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Hereditary spherocytosis
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Spectrin mutation
- RBC's are spherical, not discoid - Increased fragility, shorter life = variable anemia |
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Dystrophin mutation
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DMD or Becker's
- Membrane weakend, easily ruptures during contraction - Muscle cells rupture, die, replaced with fibrotic scar tissue |
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Myosin
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Actin binding protein and ATPase
- Convert ATP energy into force on actin filaments |
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Myosin II
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Common form for muscle contraction
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Myosin structure
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Hexamer
- 2 Heavy chains - bind actin, ATP - 4 Light chains - essential and regulatory |
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Myosin motor mechanism
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- Curled head tightly bound to actin
- ATP binds, head releases actin - ATP hydrolyzes - head straightens, re-binds actin - Pi released - head re-curls, applies force to actin - ADP released |
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Myosin regulation - Skeletal muscle
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Tropomyosin covers actin binding sites
- Ca2+ binds Troponin (part of tropomyosin complex) - Tropomyosin slides out of the way, exposing actin binding sites |
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Myosin regulation - Smooth muscle
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ca2+ indirectly activates
- Ca2+ binds Calmodulin - Activated Calmodulin binds, activates Myosin Light Chain Kinase (MLCK) - MLCK phosphorylates myosin light chains - Contraction! |
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Myosin II functions
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- Muscle contraction
- Cytokinesis - Cell motility - Actin pushes out at front, myosin contracts end of cell towards front |
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Familial Hypertrophic Cardiomyopathy
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Heavy chain mutation - less force per myosin
- Requires more muscle to provide same force - enlarged heart - Death in young athletes |
