121' StructureFunction of the Cytosol

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12-1. Structure/Function of the
Cytosol Cytoskeleton and Cell Motility
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Cytoskeleton

• Eukaryotic cell Skeletal System
• Three well defined filamentous structures
• Micotubules
• Microfilaments
• Intermediate filaments

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Cytoskeleton

• Eukaryotic cell Skeletal System
• Micotubules
• Rigid tubes
• Tubulin
• Microfilaments
• Solid / thinner
• Actin
• Intermediate filaments
• Tough ropelike fibres
• Many related proteins

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Cytoskeleton

• Functions of Cytoskeleton
• 1 Dynamic scaffold
• 2 Internal framework
• 3 Network of highways
• 4 Force generating apparatus cell movement
• 5 m-RNA anchoring
• 6 Cell division

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Cytoskeleton

• Microtubules Structure and Composition
• Components of a diverse array of structures
• Mitotic spindle
• Core of flagella and cilia
• Tubes of globular proteins
• Longitudinal rows
• Protofilaments
• Cross-section 13 rows of protofilaments
  circular
• Dimeric building blocks a-tubulin and b-tubulin
  

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Cytoskeleton

• Microtubules Structure and Composition
• a-tubulin and b-tubulin
• Similar 3-D structure
• Form dimers
• Fit together non-covalent bonds

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Cytoskeleton

• Microtubules Structure and Composition
• a-tubulin and b-tubulin
• Linear array
• Asymmetric
• a-tubulin at one end
• b-tubulin at other end
• Same polarity
• Plus end fast growing
• Minus end slow growing

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Cytoskeleton

• Microtubules Structure and Composition
• a-tubulin
• Bound GTP
• Not hydrolysed
• Non-exchangable
• b-tubulin
• Bound GDP
• Exchanged for GTP prior to assembly

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Cytoskeleton

• Microtubule Associate Proteins (MAPs)
• Mostly in brain
• Exception MAP4 many cell types (non-neuronal)
• Domain attaches to microtubule
• Domain extends out filament
• Various roles
• Cross-bridges connecting microtubules
• Increase microtubule stability
• Alter microtubule rigidity
• Alter microtubule rate of assembly
• Activity phosphatases and phosphokinases

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Cytoskeleton

• Microtubules structural roles
• Determine cell shape
• Axons of nerve cells
• Internal organization
• Axonal transportation
• Materials moved from cell body along axon
• Anteriograde
• From axon to cell body endocytosis
• Retrograde
• Axons have microfilaments, intermediate filaments
  and microtubules
• Interconnected

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Cytoskeleton

• Microtubules structural roles
• Passive
• Tracks for many motor proteins
• Motor proteins use ATP
• Move cellular cargo
• Vesicles, Mitochondria, Lysosomes, Chromosomes
• Motor proteins Three families
• Myosins
• Kinesins
• Dyneins
• Kinesins and Dyneins move on microtubules

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Cytoskeleton

• Motor proteins
• Move unidirectionally
• Stepwise
• Series of conformational changes
• A mechanical cycle
• Coupled to chemical cycle Energy
• Steps
• ATP binding to motor
• Hydrolysis of ATP
• Release of ADP and Pi
• Binding of new ATP

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Cytoskeleton

• Motor proteins
• Kinesin
• Tetramer
• 2 identical heavy and 2 identical light chains
• Functional domains
• Pair of globular heads
• Bind microtubule
• ATP-hydrolysing
• Neck / stem and tail
• Tail binds cargo
• Move toward plus end of microtubule
• Plus end directed

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Cytoskeleton

• Motor proteins
• Kinesin

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Cytoskeleton

• Motor proteins
• Kinesin
• Velocity proportional to ATP
• Move one heterodimer at a time (step)
• One head always attached
• Heads are coordinated
• Each at different stages of chemical and
  mechanical cycles
• When one head binds
• Conformational change in adjacent neck region
• Swings other head forward
• Kinesin walks along microtubule

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Cytoskeleton

• Motor proteins
• Kinesin
• One member of a superfamily of related proteins
• Kinesin related proteins KRPs
• Kinesin-like proteins KLPs
• gt 50
• Heads similar
• Tails heterogenous binding different cargoes

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Cytoskeleton

• Motor proteins
• Kinesin-mediated organelle transport
• Kinesins aligned with plus ends away from nucleus
• Tend to move organelles in anterograde direction

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Cytoskeleton

• Motor proteins
• Cytoplasmic Dynein
• Movement of cilia and flagella
• And ubiquitous motor protein in eukaryotic cells
• Huge - gt 1.5 Mda
• 2 identical heavy chains
• Many intermediate and light chains
• Heavy chain
• Large globular head
• Force generating engine
• Minus end directed

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Cytoskeleton

• Motor proteins
• Cytoplasmic Dynein Two roles
• Force generation spindle mitosis
• Minus-end directed motor for Golgi Complex and
  vesicles
• Requires a sub-unit complex dynactin