6 StructureFunction of the Cytosol

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6 Structure/Function of the Cytosol Cytoskeleton
and Cell Motility (2)
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Cytoskeleton

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

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Cytoskeleton

• 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

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Cytoskeleton

• Microtubule-Organizing Centers (MTOCs)
• Microtubule nucleation
• All MTOCs a common protein g-tubulin

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Cytoskeleton

• Microtubules Dynamic Properties
• Mostly extremely labile
• Non-covalent bonds
• More stable forms
• Stabilized by
• MAPs
• Enzymatic modification

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Cytoskeleton

• 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

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Cytoskeleton

• 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

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Cytoskeleton

• 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

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Cytoskeleton

• 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

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Cytoskeleton

• 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

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Cytoskeleton

• 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

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Cytoskeleton

• 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
• gt 50 genes
• 6 major classes

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Cytoskeleton

• Intermediate Filaments

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Cytoskeleton

• 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

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Cytoskeleton

• Intermediate Filaments
• Assembly
• Tetramer of 2 dimers
• Staggered
• Antiparallel
• Tetramers lack polarity
• Distinguishing characteristic

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Cytoskeleton

• 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

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Cytoskeleton

• Microfilaments
• Arrangement variable
• Highly ordered arrays
• Loose networks
• Well defined bundles

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Cytoskeleton

• Microfilaments
• A major contractile protein of muscle
• Occurs in every cell
• A major protein
• Interacts specifically with myosin

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Cytoskeleton

• 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

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Cytoskeleton

• 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

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Cytoskeleton

• Microfilament assembly / disassembly

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Cytoskeleton

• Myosin the molecular motor for Actin Filaments
• Myosin superfamily
• Conventional (Type II) myosins