Stu2 promotes mitotic spindle elongation in anaphase

During anaphase, mitotic spindles elongate up to five times their metaphase length. This process, known as anaphase B, is essential for correct segreg ation of chromosomes. Here, we examine the control of spindle length during anaphase in the budding yeast Saccharomyces cerevisiae. We show that micro tubule stabilization during anaphase requires the microtubule-associated pr otein Stu2. We further show that the activity of Stu2 is opposed by the act ivity of the kinesin-related protein Kip3. Reexamination of the kinesin hom ology tree suggests that KIP3 is the S. cerevisiae orthologue of the microt ubule-destabilizing subfamily of kinesins (Kin I). We conclude that a balan ce of activity between evolutionally conserved microtubule-stabilizing and microtubule-destabilizing factors is essential for correct spindle elongati on during anaphase B.