Condensin I and II behaviour in interphase nuclei and cells undergoing premature chromosome condensation.

Condensin is an integral component of the mitotic chromosome condensation machinery, which ensures orderly segregation of chromosomes during cell division. In metazoans, condensin exists as two complexes, condensin I and II. It is not yet clear what roles these complexes may play outside mitosis, and so we have examined their behaviour both in normal interphase and in premature chromosome condensation (PCC). We find that a small fraction of condensin I is retained in interphase nuclei, and our data suggests that this interphase nuclear condensin I is active in both gene regulation and chromosome condensation. Furthermore, live cell imaging demonstrates condensin II dramatically increases on G1 nuclei following completion of mitosis. Our PCC studies show condensins I and II and topoisomerase II localise to the chromosome axis in G1-PCC and G2/M-PCC, while KIF4 binding is altered. Individually, condensins I and II are dispensable for PCC. However, when both are knocked out, G1-PCC chromatids are less well structured. Our results define new roles for the condensins during interphase and provide new information about the mechanism of PCC.