Microtubule-Driven Multimerization Recruits ase1p onto Overlapping Microtubules

Microtubule (MT) crosslinking proteins of the ase1p/PRC1/Map65 family play a major role in the construction of MT networks such as the mitotic spindle. Most homologs in this family have been shown to localize with a remarkable specificity to sets of MTs that overlap with an antiparallel relative orientation [1-4]. Regulatory proteins bind to ase1p/PRC1/Map65 and appear to use the localization to set up precise spatial signals [5-10]. Here, we present evidence for a mechanism of localized protein multimerization underlying the specific targeting of ase1p, the fision yeast homolog. In controlled in vitro experiments, dimers of ase1-GFP diffused along the surface of single MTs and, at concentrations above a certain threshold, assembled into static multimeric structures. We observed that this threshold was significantly lower on overlapping MTs. We also observed diffusion and multimerization of ase1-GFP on MTs inside living cells, suggesting that a multimerization-driven localization mechanism is relevant in vivo. The domains responsible for MT binding and multimerization were identified via a series of ase1p truncations. Our findings show that cells use a finely tuned cooperative localization mechanism that exploits differences in the geometry and concentration of ase1p binding sites along single and overlapping MTs

Dynamic reorganization of Eg5 in the mammalian spindle throughout mitosis requires dynein and TPX2

The kinesin Eg5 moves toward minus ends of astral microtubules in early mitosis, switching to plus-end motion in anaphase. Dynein is required for minus-end motion; depletion of TPX2 results in a switch to plus-end motion. On midzone microtubules, Eg5 moves in both directions. Our results explain the redistribution of Eg5 throughout mitosis