JP

Chapter 20 - Microtubules

20.1: Microtubule Organization and Dynamics

  • GTPases have a branch of tubulin that belong to the same family, and those tubulins polymerize into microtubules which are small hollow cylindrical structures which a diameter of 25 nm
  • Microtubules exhibit structural and functional polarity like actin microfilaments
  • Protofilaments are formed by dimeric aB-tubulin which interact with end to end, and protofilaments are laterally associated with microtubules
  • The (+) of microtubules lose and gain subunits, and microtubules have structural polarity
  • The critical concentration of aB- tubulin affects the assembly and disassembly of microtubules:
    • If the Cc is above, the microtubules assemble
    • If the Cc is below, the microtubules disassemble
  • There are two situations that are shown regarding tubulin concentrations:
    • Tread milling, subunits increasing and decreasing
    • Dynamic instability, lengthening, and shortening of the oscillation
  • GTP bound to B-tubulin on the (+) side or if the GTP has been hydrolyzed to GDP affects the balance in the growth and shrinkage of microtubules
  • MAPs (microtubule-associated proteins) affect the stability of and organize microtubules:
    • MAPs can control cytosolic microtubule depolymerization
    • They can also organize and crosslink microtubules into bundles and membranes for the mediation of filaments in the membrane
  • Drugs like colchicine and taxol can help with cancer as they disrupt microtubules and provide an antimitotic effect
  • Microtubule organizing centers (MTOCs) determine cell polarity

20.2: Kinesin and Dynein-Powered Movements

  • Kinesin and dynein, two different families of motor proteins, help with the transport of proteins, organelles, and vesicles along microtubules
  • Kinesins move cargo to the (+) side and dyneins move cargo to the (-) side
  • Kinesins work as dimers with head domains that bind microtubules and ATP and tail domains that bind other cargo or vesicles
  • Dynactin links dyneins to their cargoes which consist of vesicles and chromosomes
  • Dynactin is a multiprotein complex that also works to bind microtubules
  • A myosin motor probably powers the transport of vesicles along microfilaments in areas that don’t have a large amount of microtubule in the cell
  • The cell is moved forward by flagellar beating and materials are moved through the cell by ciliary beating

20.3: Microtubule Dynamics and Motor Proteins in Mitosis

  • Mitosis is the process of replicated chromosomes being separated and partitioned into two daughter cells
  • Mitosis consists of four sub-stages:
    • Prophase
    • Metaphase
    • Anaphase
    • Telophase
  • Mitotic apparatus of animal cells:
    • Astral microtubules (asters)
    • Polar and kinetochore microtubules (football-shaped spindle)
    • Spindle pores
    • Chromosomes attached to the microtubules of the kinetochore
  • Microproteins Bimc, CENP-E, and cytosolic dynein move into and stay in all spindles
  • Motor protein-mediated interactions help establish the orientation of spindles
  • The chromosomes are centered by opposing forces created by motor proteins
  • Both microtubule dynamics and microtubule motors are needed for chromosomal transport to the opposite poles at anaphase
  • Daughter cells are created during cytokinesis through cleavage