![]() In addition to their mitotic functions, human condensins also play important roles in the prestressed condensed state of the nucleus, homologous recombination repair and gene expression during interphase. Different nuclear distribution, localization on chromosomesof human condensin I and human condenisn II during cell cycle are shown as Fig. Altogether, the differences in the timing of binding to chromosome and mutant phenotypes of dysfunction strongly indicated that human condensin I and II have fundamentally distinct functions during mitosis. Consistently, human condensin II can initiate sister chromatid resolution during S phase. Recently, it is identified telophase as a critical transition between condensin- and cohesin-driven chromosome folding. During telophase, human condensins are involved in the mitotic chromosome conformation transformation into the interphase state as well. Likewise, when human condensins are knocked down or dysfunction in human cells, chromatin bridges between daughter cells in anaphase and multiple nuclei in single cells are observed. During anaphase, when human condensins are depleted, chromosomes are formed with improperly structured kinetochores and chromosome bridges appear in the cell. It has been demonstrated that human condensin I but not condensin II can associate with KIF4A to confer rigidity to centromeres. This idea is somewhat inconsistent with a former study. Depletion of condensin II impacts chromosome mechanics more than depletion of condensin I and stiffness of the metaphase chromosome is more dependent on condensin II than on condensin I. Moreover, human condensins show a discontinuous pattern along mitotic chromosomes and play a major role in controlling the elastic stiffness of metaphase chromosomes. It is indicated that human condensin II but not condensin I is more indispensible for the salt-dependent, reversible reorganization of condensin II-based axes in chromosome shaping. Localization of human condensin II is centrally confined, but condensin I reaches ∼50% of the chromatid diameter from its center. Similarly, human condensin I complexes dynamically bind to chromosomes in two steps during prometaphase and early anaphase whereas human condensin II complexes are stably bound to chromosomes throughout mitosis. Once nuclear envelope breakdown, human condensin I rapidly associated with mitotic chromosomes then remained constant from prometaphase to late metaphase and chromatin bound human condensin I increased again just from anaphase onset until late anaphase when it dissociated from chromosomes. Human condensin I shows a two-step dynamic binding. During mitosis, initially condensin II participates in chromosome condensation within the nucleus in early prophase, whereas condensin I can interact with chromosomes only after the nuclear envelope breaks down. In detail, during interphase condensin I is present in the cytoplasm, whereas condensin II is enriched within the nucleus. Although both human condensins have similar components and alphabetic structure, they show different nuclear distribution, localization on chromosomes and play distinct roles in chromosome dynamics during mitosis. The canonical roles of human condensins are participated in chromosome dynamics, including chromosome condensation and segregation during mitosis. They are hCAP-H(NCAPH), hCAP-D2(NCAP-D2) and hCAP-G(NCAPG) for condensin I and hCAP-H2(NCAP-H2), hCAP-D3(NCAPD3) and hCAP-G2(NCAP-G2) for condensin II. Both human condensins are pentameric complexes composed of shared core SMC2/SMC4 (structural maintenance of chromosomes, SMC) heterodimer(also known as hCAP-E/hCAP-C)and three accessory non-SMC subunits, including a kleisin subunit and two HEAT-repeat proteins. The two kinds of condensins are also exist in human cells. Until now, most multicellular eukaryotes reported have two kinds of condensins, termed as condensin I and condensin II. Condensins were firstly identified for their fundamental roles in establishment and maintenance of mitotic chromosome condensation in cell-free system from Xenopus laevis eggs.
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