Condensin-Condensin Interactions Facilitate Mitotic Chromosome Assembly in Xenopus Egg Extracts

Condensins are large protein complexes that play a central role in mitotic chromosome assembly in eukaryotes. Our previous mutational analyses of condensin I, combined with Xenopus egg cell-free extracts, provided evidence that dynamic condensin-condensin interactions, triggered by a contact between the CAP-D2 and SMC4 subunits, underlie proper chromosome assembly and shaping. To examine whether and how the hypothesized condensin-condensin interactions contribute to chromosome assembly, here we employed a rapamycin-inducible FKBP-FRB dimerization system to artificially tether CAP-D2 and SMC4 either between different complexes (inter-complex tethering) or within single complexes (intra-complex tethering). The ability of the resulting complexes to assemble mitotic chromosomes was then assessed in Xenopus egg extracts. We found that inter-complex tethering enhances condensin I loading and facilitates mitotic chromosome assembly, whereas intra-complex tethering restricts its function. Moreover, deficiencies in the D2-SMC4 contact caused by an SMC4 W-loop mutation were partially compensated by inter-complex tethering. Together, these findings provide direct evidence that condensin-condensin interactions facilitate mitotic chromosome assembly.