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

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