Transcript Microtubule Assembly and Disassembly

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!!!