Structural basis of HEAT‐kleisin interactions in the human condensin I subcomplex

Condensin I is a multi‐protein complex that plays an essential role in mitotic chromosome assembly and segregation in eukaryotes. It is composed of five subunits: two SMC (SMC2 and SMC4), a kleisin (CAP‐H), and two HEAT‐repeat (CAP‐D2 and CAP‐G) subunits. Although balancing acts of the two HEAT‐repeat subunits have been demonstrated to enable this complex to support the dynamic assembly of chromosomal axes in vertebrate cells, its underlying mechanisms remain poorly understood. Here, we report the crystal structure of a human condensin I subcomplex comprising hCAP‐G and hCAP‐H. hCAP‐H binds to the concave surfaces of a harp‐shaped HEAT‐repeat domain of hCAP‐G. Physical interaction between hCAP‐G and hCAP‐H is indeed essential for mitotic chromosome assembly recapitulated in Xenopus egg cell‐free extracts. Furthermore, this study reveals that the human CAP‐G‐H subcomplex has the ability to interact with not only double‐stranded DNA, but also single‐stranded DNA, suggesting functional divergence of the vertebrate condensin I complex in proper mitotic chromosome assembly. Synopsis Condensin I has a central role in mitotic chromosome assembly and segregation. The crystal structure of a human condensin I subcomplex reveals that the interaction between hCAP‐G and hCAP‐H is essential for mitotic chromosome assembly and DNA binding. The crystal structure of the hCAP‐G‐H condensin I subcomplex shows an “open” conformation. The interaction between hCAP‐G and hCAP‐H is required for proper assembly of mitotic chromosomes. hCAP‐G‐H interacts not only with double‐stranded DNA, but also single‐stranded DNA. Graphical Abstract Condensin I has a central role in mitotic chromosome assembly and segregation. The crystal structure of a human condensin I subcomplex reveals that the interaction between hCAP‐G and hCAP‐H is essential for mitotic chromosome assembly and DNA binding.