The Smc5/6 complex contributes to mitoticchromosome assembly in human cell lines
Genomic instability is considered a hallmark of cancer. Structural maintenance of chromosomes (SMC) proteins constitute the core of critical complexes involved in accurate chromosome structure and organization, two fundamental requirements for faithful chromosome segregation and maintenance of genomic information. The Smc5/6 complex is essential; is conserved from yeast to humans; and participates in homologous recombination repair, ribosomal DNA stability, and telomere maintenance, although how it regulates these processes remains an open question. Here we analyzed the cell cycle localization of the human Smc5/6 complex and its contribution to chromosome organization during cell cycle progression. Time-course analysis of Smc5 and Smc6 by chromatin fractionation, immunofluorescence, and live-cell imaging indicated that these proteins were associated with chromatin during interphase and were subsequently removed from the nucleus once chromosome condensation took place. Smc5 and Smc6 depletion resulted in aberrant chromosome structures during mitosis, indicating a defect in mitotic chromosome organization. Lagging chromosomes, anaphase bridges, and micronuclei were also frequently observed, indicating that defective mitotic chromosome organization substantially disturbs chromosome segregation. Immunofluorescence microscopy as well as genomic studies of these depleted cells indicated the abnormal distribution of topoisomerase IIa and condensin subunit Smc2, confirming the implication of Smc5/6 in high-order chromatin organization during mitosis.