Biomedical Engineering Reference
In-Depth Information
previously not possible to detect due to the limited resolution with which
microtubule assembly can be followed using conventional light microscopy
techniques. This has allowed us to observe, for example, that microtubule
growth does not always occur through a smooth process but that sometimes
fast length excursions are observed that are larger than single tubulin subunits
(see Figure 4.9). Because the characteristic length scale of these excursions
does not seem to depend on how fast the microtubule grows on average, we
propose that growth may sometimes occur through the addition of small tubu-
lin oligomers. In addition, this technique has allowed us to show how, on a
molecular scale, tubulin assembly is altered by the presence of the microtubule
associated protein XMAP215. This protein was originally isolated from
Xeno-
pus
egg extracts but has homologues in many other systems. It has been shown
to be an elongated flexible protein of about 60 nm long that significantly en-
hances the growth rate of microtubules
in vitro
. When we add XMAP215
to our trap experiment, we frequently observe fast length excursions whose
characteristic size is remarkably similar to the size of the XMAP215 protein
itself. Again, we propose that XMAP215 may recruit tubulin subunits in solu-
tion and thus promote the addition of long tubulin oligomers to the growing
microtubule end. An alternative explanation for the fast length excursions
that we observe, both in the absence and presence of XMAP215, is that the
microtubule occasionally goes into a mode where it rapidly adds subsequent
subunits up to a characteristic length. Given the limited spatial and time res-
olution of our set-up, this would be indistinguishable from the addition of an
oligomer.
4.7 Dynamics and Forces of Microtubule Bundles
Up to this point, we have focused on the force-generating capabilities of single
growing microtubules. While this is relevant for single microtubules pushing
against the cell ends of fission yeast cells, there are other situations where
microtubules more likely operate in parallel growing bundles. For example,
it has been long known from electron microscopy studies that the ends of
multiple dynamic microtubules interact “head-on” with the kinetochores of
mitotic chromosomes (see for example [31]). In fact, the first speculations
about microtubule (dis)assembly-based force generation were put forward in
this context [5]. One natural question is whether arranging multiple grow-
ing microtubules in a parallel array allows for pushing forces to be created
that are larger than can be achieved with a single microtubule. Another ques-
tion is whether parallel growing bundles of microtubules can coordinate their
catastrophe behavior when in contact with a resisting barrier. This would
be relevant to understand how multiple microtubules can push and pull on
chromosomes in a coordinated fashion which is what they appear to do when
moving chromosomes away and towards the poles of the mitotic spindle.
