ch 9 pt1

• The monomers of microtubules are a-tubulins and b-tubulins
• The monomers of microfilaments are g-actins
• Unlike microtubules and microfilaments, intermediate filaments are not polar
• Functions of microtubules include support, intracellular transport, and cell organization
• Motor units of microtubules are kinesins and dyneins
• Motor proteins of microfilaments, specifically actin filaments, are myosins.
• External stimuli are converted to signal transduction pathways that convey information to the cytoskeleton, resulting in the reorganization and movement of organelles or the change of cell shape, movement, and contraction
• There are two types of microtubules: cytoplasmic and axonemal microtubules
• Cytoplasmic microtubules are found in the cytosol and are responsible for

  • maintaining axons
  • formation of mitotic and meiotic spindles
  • placement and movement of vesicles
  • maintaining or altering cell shape
• Axonemal microtubules include the organized and stable microtubules found in structures specialized for movement such as

  • cilia
  • flagella
  • basal bodies to which cilia and flagella attach
• Microtubules are composed of typically 13 protofilaments, whose subunits are a heterodimer of tublin, consisting of an a-tubulin and a b-tubulin, both being globular proteins
• a-tubulin and b-tubulin bind non-covalently to form an ab-heterodimer
• GTP in a tubulin is physically trapped at the dimer interface and is never hydrolyzed or exchanged
• The GTP in the b tubulin can be hydrolyzed, exchanged, and converted to GDP
• All the dimers in microtubules are oriented in the same way, resulting in protofilaments having polarized minus ends and plus ends
• Microtubules resist forces that might compress or bend the fiber for mechanical support
• Nocodazole or colchicine promotes microtubule assembly, resulting in the dispersal of golgi elements into separate golgi stacks scattered throughout the cytoplasm
• Kinesin is an anterograde motor, heading towards the plus end of the microtubule.
• Dynein is a retrograde motor, heading towards the minus end of a microtubule
• Kinesin and dynein move along microtubule tracks
• Myosin moves along microfilament tracks
• Molecular motors convert energy from ATP into mechanical energy
• Molecular motors move unidirectionally along their cytoskeletal track in a stepwise manner

  
• Each kinesin step is approximately 8 nm, the length of one tubulin dimer, and requires the hydrolysis of a single ATP molecule
• Kinesin movement

  • proportional to the ATP concentration
  • moves in a "hand-over-hand" mechanism
  • is high processive
• Dynamic instability: process of alternating between growing and shrinking
• The dissociation rate of a GDP tubulin dimer is much more rapid than the dissociation of a GTP tubulin dimer
• Nucleation of microtubules occur at microtubule-organizing centers
• Microtubule-organizing centers (MTOCs) control the number of microtubules, their polarity, the number of protofilaments that make up their walls, and the time and location of their assembly
• Centrioles: short cylinders of modified microtubules
• Y turc: tubulin ring complex where nucleation occurs
• The growth of microtubules occurs by addition of subunits at the plus end of the polymer away from the centrosome, while the minus end is anchored at the centrosome

  
• Centrioles: short cylinders of modified microtubules
• Nucleation of a microtubule begins with y-tubulin at the minus end and is initiated by the yturcs: tubulin ring complex
• Y-turc: helical array of y-tubulin subunits where ab-tubulin dimers assemble
• Astral microtubules: project towards the cell cortex and interact with it thereby orienting the spindle of division
• Kinetochore microtubules: connected to chromosomes
• Polar microtubules: interact with microtubules from the opposite pole of the cell
• At prophase, the nuclear lamina (intermediate filaments) is phosphorylated and de-polymerized
• At prometaphase, the plus end of the kinetochore microtubules attach to the kinetochore belonging to the centromere of a chromosome