EB1 (green), at microtubule ends, helps make spindles.

Increased microtubule dynamics during mitosis allows for the construction of a spindle, but may make the cytoskeleton vulnerable to the type of defects noted by Rogers et al. on page 873. They find that depletion of a fly microtubule- associated protein (MAP) called EB1 leads to problems in spindle construction, positioning, and functioning, probably because of a failure to counteract microtubule-destabilizing factors.

The product of the budding yeast homologue of EB1, called Bim1p, has been identified as a plus-end microtubule-binding protein necessary for positioning the spindle and promoting microtubule dynamics. Rogers et al. looked first in fly tissue culture cells that were grown so they flattened to double their normal diameter. This allowed the authors to visualize individual microtubules in vivo and confirm that microtubule dynamics are suppressed upon loss of EB1. This probably reflects the presence of an endogenous factor (normally counteracted by EB1) that suppresses dynamics, as microtubules left to their own devices are inherently dynamic.

The loss of EB1 does not, however, have a marked effect on the overall extent of interphase microtubule arrays. Mitosis is another matter. Astral and interpolar microtubules (both of which lack the protective cap present on kinetochore microtubules) are greatly reduced in numbers. The authors suggest that a microtubule-destabilizing factor up- regulated in mitosis may be responsible. Whatever the cause, the upshot is a spindle that wanders far from the center of the cell, as it is no longer braced or tugged into position by astral microtubules that contact the cortex. The loss of interpolar microtubules results in partially collapsed spindles that segregate chromosomes more slowly in anaphase.

Extending the research beyond EB1 will be easier with the cell culture system. Rogers et al. are interested in characterizing several factors, including those that load EB1 at microtubule ends and those that counteract it either by suppressing microtubule dynamics or promoting microtubule destruction. ▪