  Assembly and disassembly dynamics of microtubules (MTs) is tightly controlled
by MT associated proteins. Here, we investigate how plus-end-directed
depolymerases of the kinesin-8 family regulate MT depolymerization dynamics.
Employing an individual-based model, we reproduce experimental findings.
Moreover, crowding is identified as the key regulatory mechanism of
depolymerization dynamics. Our analysis gives two qualitatively distinct
regimes. For motor densities above a particular threshold, a macroscopic
traffic jam emerges at the plus-end and the MT dynamics become independent of
the motor concentration. Below this threshold, microscopic traffic jams at the
tip arise which cancel out the effect of the depolymerization kinetics such
that the depolymerization speed is solely determined by the motor density.
Because this density changes over the MT length, length-dependent regulation is
possible. Remarkably, motor cooperativity does not affect the depolymerization
speed but only the end-residence time of depolymerases.
