Crowding of molecular motors determines microtubule depolymerization
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.