Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

928 Chapter 16: The Cytoskeleton
rapid growth with GTP-capped end
random loss of GTP cap
CATASTROPHE
rapid shrinkage
regain of GTP cap
RESCUE
rapid growth with GTP-capped end
(A)
etc.
GTP-tubulin dimer
α
β
50 nm
GTP
exchangeable GTP
GTP
GTP
GTP
GTP cap
straight protofilament
GTP HYDROLYSIS CHANGES SUBUNIT CONFORMATION
AND WEAKENS BOND IN THE POLYMER
GDP
curved protofilament
DEPOLYMERIZATION
GDP
GDP
GDP
GDP
less stable
region of
microtubule
containing
GDP-tubulin
dimers
GDP-tubulin
dimer
GDP
GDP-GTP EXCHANGE
(B)
GTP
(C)
GROWING
SHRINKING
Figure 16–44 Dynamic instability due to the structural differences between a growing and a shrinking microtubule
end. (A) If the free tubulin concentration in solution is between the critical concentrations of the GTP- and GDP-bound forms,
a single microtubule end may undergo transitions between a growing state and a shrinking state. A growing microtubule has
GTP-containing subunits at its end, forming a GTP cap. If nucleotide hydrolysis proceeds more rapidly than subunit addition, the
cap is lost and the microtubule begins to shrink, an event called a “catastrophe.” But GTP-containing subunits may still add to
the shrinking end, and if enough MBoC6 add to form m16.16/16.44
a new cap, then microtubule growth resumes, an event called “rescue.” (B) Model
for the structural consequences of GTP hydrolysis in the microtubule lattice. The addition of GTP-containing tubulin subunits to
the end of a protofilament causes the end to grow in a linear conformation that can readily pack into the cylindrical wall of the
microtubule. Hydrolysis of GTP after assembly changes the conformation of the subunits and tends to force the protofilament
into a curved shape that is less able to pack into the microtubule wall. (C) In an intact microtubule, protofilaments made from
GDP-containing subunits are forced into a linear conformation by the many lateral bonds within the microtubule wall, given a
stable cap of GTP-containing subunits. Loss of the GTP cap, however, allows the GDP-containing protofilaments to relax into
their more curved conformation. This leads to a progressive disruption of the microtubule. Above the drawings of a growing
and a shrinking microtubule, electron micrographs show actual microtubules in each of these two states. Note particularly the
curling, disintegrating GDP-containing protofilaments at the end of the shrinking microtubule. (C, from E.M. Mandelkow,
E. Mandelkow and R.A. Milligan, J. Cell Biol. 114:977–991, 1991. With permission from The Rockefeller University Press.)