BIO345 Chapter 8

• Cytoskeleton


  A system of filaments that provides spatial and mechanical functions for proper cell functioning

  
• Cytoskeleton filaments


  • Actin filaments
  • Microtubules
  • Intermediate filaments

  
• Actin filaments


  • Determine the shape of the cell's surface
  • Necessary for whole-cell locomotion
  • Drive the pinching of one cell into two

  
• Microtubules


  • Determine the positions of membrane-enclosed organelles
  • Direct intracellular transport
  • Form the mitotic spindle that segregates chromosomes during cell division

  
• Intermediate filaments


  • Provide mechanical strength
  • Line the inner face of the nuclear envelope, protecting the DNA
  • In the cytosol, they are twisted into strong cables that can hold epithelial cell sheets together
  • Help nerve cells to extend long and robust axons
  • Allow the formation of tough appendages (hair and fingernails)
• The 3 types of filaments are formed through self association of subunits as helices

  
• Accessory proteins modulate the dynamics and organization of these filaments in complex events such as cell division or migration
• Differences in the subunits structures and how they assemble give them different mechanical properties
• Actin and tubulin (the subunits of actin filaments and microtubules, respectively) bind and hydrolyze nucleoside triphosphates (ATP and GTP, respectively), and assemble head-to-tail to generate polarized filaments capable of generating force

  
• Bacterial cells contain homologs of actin, tubulin, and intermediate filaments that form dynamic structures that control cell shape and division

  
• Actin subunit


  Also called globular or G-actin, is a 375 a.a. polypeptide carrying a nucleotide (either ATP or ADP) bound in a deep cleft in the center of the molecule

  
• Formation of new actin filaments

  1. Nucleation

  2. Growth phase

  3. Equilibrium (steady) phase

  
• Nucleation


  Subunits assemble into an initial aggregate that is stabilized by multiple subunit-subunit contacts. It can elongate rapidly by addition of more subunits
• Growth phase


  Monomers are added to the exposed ends of the growing filament, causing filament elongation
• Equilibrium (steady) phase


  When the rate of addition of new subunits to the filament ends is exactly balanced by the rate of subunit dissociation from the ends

  
• In a test tube, polymerization of actin is controlled by its concentration, pH, salts concentrations, and ATP

  
• Within a cell, actin polymerization is also regulated by hundreds of accessory proteins that bind actin monomers or filaments

  
• Actin-binding proteins


  • Profilin
  • Thymosin
  • PM crosslinkers

  
• Profilin / Thymosin


  Actin filament dynamics are regulated by these accessory proteins that keep half of the actin in a monomeric form through association

  
• PM crosslinkers


  Connections between actin and the plasma membrane give the cell mechanical strength and permit the formation of cellular structures such as microvilli, lamellipodia, filopodia, and stereocilia

  
• Tubulin subunit


  A heterodimer formed of 2 globular proteins (α-tubulin and β-tubulin), each comprising 445-450 a.a., tightly bound by noncovalent bonds
• Many human neurological diseases have been linked to mutations in tubulin genes. Mutation in β-tubulin gene lead to a paralytic eye-movement disorder due to loss of ocular nerve function
• Microtubule


  A hollow cylindrical structure built from 13 parallel protofilaments, each composed of αβ-tubulin heterodimers stacked head to tail
• Microtubule dynamics are influenced by the binding and hydrolysis of GTP, with GTP-tubulin tending to polymerize and GDP-tubulin to depolymerize
• Microtubule-associated proteins (MAPs)


  • Stabilize microtubules against disassembly
  • Mediate microtubules interaction with other cell components
• Stathmin


  Binds along the side of two tubulin heterodimers, sequestering tubulin and regulating microtubule assembly
• Anterograde transport


  Transport of cargo from the center of the cell to the periphery
• Retrograde transport


  Transport of cargo from the periphery to the center of the cell
• Microtubule-associated proteins (MAPs)


  Proteins that bind to microtubules

  
• Microtubule-associated proteins (MAPs)


  • Mediate different functions, such as:
  • Stabilize microtubules against disassembly
  • Mediate microtubules interaction with other cell components

  
• Stathmin


  Binds along the side of two tubulin heterodimers
• Microtubules


  • Use motor proteins to transport cargo and perform a variety of functions
• Kinesin


  Motor proteins generally associated with anterograde transport
• Dynein


  Motor proteins more likely to perform retrograde transport
• Kinesin and Dynein


  • Dependent on energy derived from ATP hydrolysis for stepping motion to transport cargo
• Dynein


  • Has a larger step size than that of kinesin, making dynein a faster motor than kinesin
• Kinesin


  • Capable of transporting larger payloads than dynein
• Kinesin superfamily


  • More extensive and diverse group of motor proteins, while dynein is relatively limited
• Kinesin motor domain


  Undergoes a consistent ATP-binding and microtubule binding sequence to allow for locomotion

  
• Kinesin


  • Most kinesin families include two heavy chains, two light chains, and an elongated coiled coil