A midzone-based ruler adjusts chromosome compaction to analphase spindle length

Abstract

Partitioning of chromatids during mitosis requires that chromosomecompaction and spindle length scale appropriately with each other. However,it is not clear whether chromosome condensation and spindle elongation arelinked. Here we have used chromosome fusions to examine the impact ofincreased chromosome length during yeast mitosis. We find that yeast cellscould cope with a >50% increase in the length of their longest chromosomearm by decreasing the physical length of the mitotic chromosome arm through1) reducing the number of copies of the repetitive rDNA array and 2) byincreasing the level of mitotic condensation. Consistently, cells carrying thefused chromosomes became more sensitive to loss of condensin- and itsregulator polo kinase/Cdc5. Length-dependent stimulation of condensationtook place during anaphase and depended on aurora/Ipl1 activity, itslocalization to the spindle midzone, and phosphorylation of histone H3 onSer10, a known Ipl1 substrate. The anaphase spindle therefore may functionas a ruler to adapt the condensation of chromosomes to spindle length.Consistent with this, chromosome condensation levels correlate with thelength of anaphase spindles.