  The availability of protein is an important factor for the determination of
the size of the mitotic spindle. Involved in spindle-size regulation is
kinesin-8, a molecular motor and microtubule (MT) depolymerase, which is known
to tightly control MT length. Here, we propose and analyze a theoretical model
in which kinesin-induced MT depolymerization competes with spontaneous
polymerization while supplies of both tubulin and kinesin are limited. In
contrast to previous studies where resources were unconstrained, we find that,
for a wide range of concentrations, MT length regulation is bistable. We test
our predictions by conducting in vitro experiments, and find that the bistable
behavior manifests in a bimodal MT length distribution.
