to fine-tune microtubule growth rates in
Xenopus
spindles. To
implement this tool kit, we depleted the endogenous XMAP215
from
Xenopus
egg extracts (Figure 3d), added back the mutant
polymerases and assembled spindles (Figure 3e). At endogenous
concentration, all mutants promoted assembly of spindles at
lengths proportional to their polymerase activity (Figure 3f). An
alternative way to modified XMAP215 activity is adding back
recombinant wildtype XMAP215 at different concentrations to
XMAP215-depleted extracts. Similarly, spindle length increased
with increasing wildtype XMAP215 concentrations. However, at
some point adding more than XMAP215 did not result in longer
spindles but spindle length plateaued, consistent with existing
evidence for an upper limit to spindle length
30
. When we corre-
lated spindle length with the maximum microtubule growth pro-
motion, spindle length was directly proportional to the maximal
growth promoted by XMAP215 (Figure 3g), exactly as predicted
by our model.
Interestingly, we observed that spindle shape remains cons-
tant while spindle length changes significantly with varying
XMAP215 activities
17
. This is surprising and suggests that shape
is determined by distinct mechanisms that are independent from
those determining length. From a developmental or scaling point
of view, the separation of length and shape is appealing as it
allows fine-tuning of spindle length without changing its overall
morphology. We thus propose that force balances set the shape
of the metaphase spindle. Nevertheless, it remains to be shown
experimentally which forces shape the metaphase spindle and
to characterize the molecular factors that underlie these forces.
To conclude, our molecular understanding of XMAP215 has allo-
wed us to perform a “synthetic biology” experiment, in which we
can limit the length of a spindle solely by using an appropriate,
engineered mutant. Combining experiments and theory has allo-
wed us to bridge scales from the level of single microtubule dy-
namics to the overall organization of the spindle by using mass
balance. Many manipulations of microtubule dynamics alter the
size and shape of the spindle in a variety of systems
31-36
. Our mo-
del provides a conceptual framework for understanding why per-
turbations in microtubule dynamics often result in spindle length
changes. Any modulation that solely affects a single parameter
of spindle organization will change the mass balance and thus
result in changes in spindle length.
Material Properties and Organelle Scaling.
Our experiments suggest that
Xenopus
spindles join a growing
number of cellular structures that can be treated as active li-
quids. One example are RNA/protein complexes of P granules
in
C. elegans
that were shown to exhibit liquid-like behaviours,
which suggest a simple physical picture for P granule localizati-
on
37
. Likewise, nucleoli in amphibian oocyte behave like liquid-
like droplets
38
. Only recently, the centrosome was suggested
to be an autocatalytic drop, which explains the formation of
centrosomes only around centrioles and their scaling with cell
size (Zwicker
et al.
, unpublished and
39
). One characteristic of all
these highly dynamic structures is that they are non-membrane
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