Unit 6

• 3 Types of filaments

  1. Actin
  2. Microtubules
  3. Intermediate filaments
• Actin


  Cell surface shape & movement. Actin filaments and microtubules are composed of globular subunits
• Microtubules

  Intracellular transport, positioning of organelles
• Intermediate filaments

  Mechanical strength. Composed of elongated and fibrous subunits. Resistant to tearing
• Protofilaments


  Long linear strings of filament subunits joined end to end
• Nucleation


  Multiple subunit-subunit contacts lead to rapid elongation by addition of more subunits
• Critical concentration


  Concentration at which monomeric subunits are in equilibrium with polymer. Here, rate of subunit addition=rate of subunit loss
• Treadmilling


  Polymer maintains a constant length, even though there is a net flux of subunits through the polymer. Happens at an intermediate subunit concetration, where growth of plus end is equal to shrinkage at minus end