Cytoskeleton

Question 1

Cytoskeleton

Answer 1

• Cells are organized and held together by a set of structural proteins that are divided into 3 categories
  
  • Microtubules
  • Microfilaments
  • Intermediate Filaments
• Cytoskeleton is important for:
  
  • Cell Shape
  • Cell Motility
  • Cell Strength
  • Cell Processes
  • Cell Polarization
  • Cell Signaling
  • Organelle Organization
  • Movement of Macromolecules
  • Membrane Organization

Question 2

Microtubule Structure and Function

Answer 2

• Hollow tubes, ~24nm diameter (a-tubulin and B-tubulin)
• Often centered around nucleus radiating outward
• a- and B-tubulin
• Polarized tubes
  
  • • end = a exposed
  • • end = B exposed
  • +/- are NOT charges
• Found in:
  
  • General cytoplasmic organization
  • Mitotic spindle
  • Axon and dendrites
  • Cilia
• Primary Functions:
  
  • cell scaffolding and polarization
  • polarized movement of organelles, proteins, and DNA
• Resists compression

Question 3

Microtubule Transport

Answer 3

• Polarized tubules allow polarized movement
• 2 motor proteins
  
  • Kinesin (toward + end)
  • Dynein (toward - end)
  • Bind to cargo and ‘walk’ down the tubules

Question 4

Microtubule Regulation - Nucleation

Answer 4

• Microtubules are slowto assemble
• Introducing a template drastically increases MT assembly
• 2 major nucleation sites (MTOC)
  
  • Centrosomes
  • Basal bodies
  • Both contain gamma-tubulin
• Nucleation polarizes microtubule assembly
  
  • Minus ends are anchored in the centrosome
  • Positive ends grow toward periphery
  • Location and number of centrosomes determines polarization of MT

Question 5

Microtubules, Nucleation, and the Cell Cycle

Answer 5

• Interphase (G2)
  
  • 1 centrosome (2 joined centrioles) near nucleus
  • Centrosomes splits into 2 centrioles
• Prophase
  
  • Centrioles duplicate forming 2 centrosomes and MT polymerization causes them to push away from each other
• Prometaphase
  
  • Centrosomes reach opposite sides of the cell. New microtubules polymerize and attach to kinetochores on chromosomes
• Metaphase
  
  • MT dynamics and MAP motor proteins cause chromosomesto line up between the centromeres
• Anaphase
  
  • Bipolar MT push chromatids apart toward either centromere
• Telophase
  
  • Cell division restores 1 centromere per cell

Question 6

Microtubule Regulation - Dynamic instability

Answer 6

• Tuber dimers must be bound to GRP to bind to the + end of MT
• B-tubulin slowly hydrolyzes GTP to GDP
• High concentrations of GTP-dimers in solution promote MT growth, stability
• Low concentrations of GTP-bound dimers in solution promoe MT catastrophe and collapse
• Dynamic instability uses up energy, but allows rapid regulation of MT network

Question 7

Microtubule Regulation - Microtubule Associated Proteins (MAPs)

Answer 7

• Many different proteins bind to MT to regulate MT assembly and function
• MAPs can:
  
  • Stabilize the MT structure (XMAP215)
  • Destabilize the MT structure (catastrophins)
  • Cause bundling of the MT structure (tau, MAP2)
  • Cause branching of the MT structure (augmin, TPX2)
  • Cause interaction with other cytoskeleton fibers or cell junctions
  • Move prloteins or organelles along the MT (Kinesin, dynein)
• No need to memorize specific just have an idea what they can do

Question 8

Microfilament Structure and Function

Answer 8

• Double helical fiber (~7nm)
• Found throughout the cell, but especially toward periphery
• Only one subunit (g-actin)
• Actin assembles (F-actin) into a polarized strand
• Found in:
  
  • cell cortex
  • Microvilli and stereocilia
  • Lamellipodia, filopodia
  • Muscle fibers
• Primary functions:
  
  • Cel membrane organization
  • Ameboid movement
  • Muscle contractions
  • Cytokinesis
• Resists stretching

Question 9

Microfilament Contraction and Transport

Answer 9

• Polarizatino allows polarized transport
• MF motor proteinsL myosins
• Myosin head that binds to actin filaments and hydrolyzes ATP
• ATP hydrolysis causes a change in shape in the myosin molecule, pushing it along the microfilament
• This movement is used to move vesicles, protrude membranes, contract the cell, etc
• Muscle fibers are specialized and densely organized fibers of actin and myosin

Question 10

Microfilament Regulation

Answer 10

• Microfilaments are dynamic strutures regulated by nucleation, dynamic instability/treadmilling, and actin binding proteins
• Dynamic Instability/Treadmilling
  
  • Similar to MT, but with ATP
• Nucleation
  
  • Partially similar to MT, but many nucleation events can occur, usually near membrane
• Actin binding proteins:
  
  • Stabilize MF (tropomyosin)
  • Destabilize MT (Thymosin, cofilin)
  • Encourage MT polymerization (profilin)
  • Cap MF (gelsolin)
  • Cause bundling of MF (Fimbrin, villin)
  • Crosslink MF (filamin)
  • Nucleate/cause branching of MF (ARP2/3)
  • Bind MF to membrane/extracellular matrix (Band4.1, dystrophin)
  • Generate force along the MF (myosins)

Question 11

Intermediate Filament Structure and Function

Answer 11

• Intermediate filaments are fibrous proteins that twist into rope like filaments that confer mechanical strength and resistance to shearing forces
• IF are made up of fibrous (non-globular) proteins
• Long fibrous proteins bundle and twist together into rope like structures
• There is no distinct IF monomer: different tissues have different IF proteins
  
  • Epithelium - cytokeratin
  • Connective tissue (Fibroblasts) - vimentin
  • Muscle - desmin
  • Glial cells/astrocytes - GFA protein
  • Nerves - neurofilament proteins
  • Neuronal stem cells - nestin
  • Nucleus (all cells) - lamins
• Even though there are distrinctly different IF proteins, all IF have very similar structures
• Mechanical Strength (rigidity and flexibility)
  
  • Different tissues experience different forces
    
    • Ex: skin vs. liver
  • This partially explains why different tissues have different IF proteins
• Non-motile scaffolding network
  
  • IF may attach to organelles and hold them in place
• Resists shearing
  
  • IF are able to resist deformation
  • And when they do deform, they are able to spring back into postition without breaking

Question 12

Intermediate Filament Regulation - Assembly/Disassembly

Answer 12

• Though relatively permanent, IF still need to change in response to changing conditions
• A good example are nuclear lamins
• Nuclear lamins support the nuclear membrane
• Nuclear lamins are phophorylated by lamin kinases during entry into prophse which destabilize the IF
• The lamins are dephosphorylated on entry to telophase which allows reassembly