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Microtubule Assembly and Disassembly

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					Microtubule Assembly and
       Disassembly



     By: Cherish Harmon
    Temperature Dependency

• Assembly and stability of microtubules are
  temperature dependent.
• If they are cooled to 4ºC, they
  depolymerize into αβ-tubulin dimers.
• When they are warmed to 37ºC in the
  presence of GTP, the tubulin dimers
  polymerize into microtubules.
• Change in temperature is used to purify.
Temperature Affects
         Tubulin Polymerization
• At αβ-tubulin concentrations above the critical
    concentration (Cc), the dimers polymerize into
    microtubules, whereas at concentrations below
    the Cc, microtubules depolymerize.
•   The nucleotide bound to the β-tubulin causes
    the Cc for assembly at the + and – ends of the
    microtubule to differ. Preferred assembly is at
    the + end.
    Tubulin Polymerization Cont.
• At αβ-tubulin concentration higher than the Cc
    for polymerization, dimers add to the + end.
•   When the αβ-tubulin concentration is higher that
    the Cc at the + end but lower than the Cc at the
    – end, microtubules can treadmill by adding
    subunits to one end and dissociating subunits
    for the opposite end.
  Tubulin Polymerization Cont.

• Because the intracellular concentration of
  the assembly-competent tubulin is much
  higher that the Cc of assembly,
  polymerization is highly favored in a cell.
• The initial rate of tubulin polymerization is
  accelerated in the presence of nuclei
  (microtubule-based structures of
  fragments).
Addition of Microtubule Fragments
       Microtubule Assembly
• Three steps:
• 1: Protofilaments assemble from αβ-
  tubulin subunits.
• 2: Protofilaments associate to form the
  wall of the microtubule.
• 3: The addition of more subunits to the
  ends of the protofilaments elongates the
  microtubule.
Stages in Assembly of Microtubules
     Disassembly of Microtubules
• Microtubule shortening:
• Under shortening conditions, the microtubule
    ends are splayed, as if the lateral interactions
    between protofilaments may have been broken.
•   When frayed apart and freed from lateral
    stabilizing interactions, the protofilaments may
    depolymerize by endwise dissociation of tubulin
    subunits.
•   This provides clues about the instability of a
    microtubule.
Observation of Disassembled
       Microtubules
The End!!!

				
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posted:7/23/2012
language:English
pages:12