Condensin I stabilizes chromosomes mechanically through a dynamic interaction in live cells

BACKGROUND: Restructuring chromatin into morphologically distinct chromosomes is essential for cell division, but the molecular mechanisms underlying this process are poorly understood. Condensin complexes have been proposed as key factors, although controversial conclusions about their contribution to chromosome structure were reached by different experimental approaches in fixed cells or cell extracts. Their function under physiological conditions still needs to be defined. RESULTS: Here, we investigated the specific functions of condensin I and II in live cells by fluorescence microscopy and RNAi depletion. Photobleaching and quantitative time-lapse imaging showed that GFP-tagged condensin II bound stably to chromosomes throughout mitosis. By contrast, the canonical condensin I interacted dynamically with chromatin after completion of prophase compaction, reaching steady-state levels on chromosomes before congression. In condensin I-depleted cells, compaction was normal, but chromosomes were mechanically labile and unable to withstand spindle forces during alignment. However, normal levels of condensin II were not required for chromosome stability. CONCLUSIONS: We conclude that while condensin I seems dispensable for normal chromosome compaction, its dynamic binding after nuclear envelope breakdown locks already condensed chromatin in a rigid state required for mechanically stable spindle attachment.