7.6.3 Dynamic Instability: Different Behavior of Plus
and Minus Ends in Polymerization/Depolymerization
Episode of Microtubule
Observation of microtubules that grew from isolated centrosomes (serving as nuclei
for polymerization) demonstrated that those microtubules are divided into growing
and shortening populations, in the same solution [71]. Thus, although as a whole,
microtubules are either in a growing or a shortening phase, individual microtubules
independently grow or shorten. This remarkable phenomenon has been called
dynamic instability [72].
The dynamic instability results from the fact that the two ends of a microtubule
are not equivalent as in the case of actin. The two ends behave independently during
a polymerization/depolymerization episode. This was confirmed by the observation
of the polymerization/depolymerization events at each end simultaneously. The
nuclei prepared by binding dynein to microtubules, or a protein complex called
axoneme that had been isolated from sea urchin sperm were used. By dark field
microscopy [73] or differential interference contrast microscopy [65], researchers
could confirm the independent nature of the dynamics of the two ends of a single
microtubule.
Comparison of the elongation and shortening events at each end has shown that
the rate of elongation was several times slower than the rate of shortening. As
described above, this is because the shortening is accelerated by the elastic energy
stored in the microtubule. The association and dissociation rate constants at two ends
of microtubule as estimated by direct observations [65, 71, 74] and it has been
demonstrated that at one end the elongation occurs faster and over a longer distance
than the opposite end. The off-rate constant of the faster-growing end is smaller than
that of the slow-growing end. This makes the former end look much more active than
the opposite end; the more active end has been called “plus” end and the opposite
end “minus” end.
7.6.4 Catastrophe and Rescue
As described above, in the dynamic instability, an elongation phase is followed by a
shortening phase and vise-versa. The switch from the elongation to the shortening
phase is called “catastrophe”, and the switch from the shortening to the elongation
phase is called “rescue”. The catastrophe is thought to occur as a result of GTP
hydrolysis: once GTP in each tubulin is hydrolyzed, the inter-protofilament interaction becomes weakened and the microtubule lattice destabilized by the outward
curvature of protofilaments (see the previous section), leading to the catastrophe.
The frequencies of catastrophe and rescue at plus ends are on the order of 10
-3 /sec at
15–16 μM heterodimer [65, 74]; the catastrophe rate decreases and the rescue rate
increases with the increase in tubulin concentration as one would expect.
7.6 Microtubule
119
and Minus Ends in Polymerization/Depolymerization
Episode of Microtubule
Observation of microtubules that grew from isolated centrosomes (serving as nuclei
for polymerization) demonstrated that those microtubules are divided into growing
and shortening populations, in the same solution [71]. Thus, although as a whole,
microtubules are either in a growing or a shortening phase, individual microtubules
independently grow or shorten. This remarkable phenomenon has been called
dynamic instability [72].
The dynamic instability results from the fact that the two ends of a microtubule
are not equivalent as in the case of actin. The two ends behave independently during
a polymerization/depolymerization episode. This was confirmed by the observation
of the polymerization/depolymerization events at each end simultaneously. The
nuclei prepared by binding dynein to microtubules, or a protein complex called
axoneme that had been isolated from sea urchin sperm were used. By dark field
microscopy [73] or differential interference contrast microscopy [65], researchers
could confirm the independent nature of the dynamics of the two ends of a single
microtubule.
Comparison of the elongation and shortening events at each end has shown that
the rate of elongation was several times slower than the rate of shortening. As
described above, this is because the shortening is accelerated by the elastic energy
stored in the microtubule. The association and dissociation rate constants at two ends
of microtubule as estimated by direct observations [65, 71, 74] and it has been
demonstrated that at one end the elongation occurs faster and over a longer distance
than the opposite end. The off-rate constant of the faster-growing end is smaller than
that of the slow-growing end. This makes the former end look much more active than
the opposite end; the more active end has been called “plus” end and the opposite
end “minus” end.
7.6.4 Catastrophe and Rescue
As described above, in the dynamic instability, an elongation phase is followed by a
shortening phase and vise-versa. The switch from the elongation to the shortening
phase is called “catastrophe”, and the switch from the shortening to the elongation
phase is called “rescue”. The catastrophe is thought to occur as a result of GTP
hydrolysis: once GTP in each tubulin is hydrolyzed, the inter-protofilament interaction becomes weakened and the microtubule lattice destabilized by the outward
curvature of protofilaments (see the previous section), leading to the catastrophe.
The frequencies of catastrophe and rescue at plus ends are on the order of 10
-3 /sec at
15–16 μM heterodimer [65, 74]; the catastrophe rate decreases and the rescue rate
increases with the increase in tubulin concentration as one would expect.
7.6 Microtubule
119