Cell Cycle-dependent Expression and Nucleolar Localization of hCAP-H

Abstract

Condensin is a conserved 13S heteropentamer composed of two nonidentical structural maintenance of chromosome (SMC) family proteins, in Xenopus XCAP-C and XCAP-E, and three regulatory subunits, XCAP-D2, XCAP-G, and XCAP-H. Both biochemical and genetic analyses have demonstrated an essential role for the 13S condensin complex in mitotic chromosome condensation. Further, a potential requirement for condensin in completion of chromatid arm separation in early anaphase is demonstrated by the mutational phenotypes of the Drosophila homologues of XCAP-H, barren and XCAP-C, DmSMC4. In this study we have investigated the expression and subcellular distribution of hCAP-H, the human homolog of XCAP-H, in order to better understand its cellular functions. Transcription of hCAP-H was restricted to proliferating cells with highest expression during the G 2 phase of the cell cycle. In contrast, cellular hCAP-H protein levels were constant throughout the cell cycle. hCAP-H was found to be associated with mitotic chromosomes exhibiting a nonuniform but symmetric distribution along sister chromatids. The symmetry of hCAP-H association with sister chromatids suggests that there are sequence-dependent domains of condensin aggregation. During interphase hCAP-H, -C, and -E, have distinct punctate nucleolar localization, suggesting that condensin may associate with and modulate the conformation and function of rDNA. hCAP-H association with condensed chromatin was not observed in the early phase of chromosome condensation when histone H3 phosphorylation has already taken place. This finding is consistent with the hypothesis that histone H3 phosphorylation precedes condensin-mediated condensation.

Nucleolar association of pEg7 and XCAP-E, two members ofXenopus laeviscondensin complex in interphase cells

“…Most authors demonstrated diffuse distribution of SMC proteins throughout the nucleus (Saitoh et al, 1994;Hirano and Mitchison, 1994), whereas Journal of Cell Science 116 (9) others reported concentration of these proteins in discrete subnuclear domains of an unknown nature (Schmiesing et al, 1998). Recently, nucleolar localization of the human condensins hCAP-H has been reported (Cabello et al, 1999;Cabello et al, 2001). Similar results were obtained for S. cerevisiae, where GFP-tagged pSmc4 was shown to accumulate in the nucleolus prior to entry into mitosis (Freeman et al, 2000).…”

“…Taking into account the fact that intracellular levels of condensin subunits remain practically constant throughout the cell cycle [this study; (Schmeising et al, 1998;Cabello et al, 2001)], one should expect condensins to exert some yet unknown function(s) during interphase. It is a plausible hypothesis that condensins act as regulators of gene expression by controlling the accessibility of chromatin loci to transcription factors via local condensation/decondensation of interphase chromosomes.…”

Condensin Loaded onto the Replication Fork Barrier Site in the rRNA Gene Repeats during S Phase in a FOB1-Dependent Fashion To Prevent Contraction of a Long Repetitive Array in Saccharomyces cerevisiae

“…In performing both of these roles, the cells seem to acquire a higher potential for stably maintaining long repeat structures; this is because replication fork blocking activity and condensin are conserved in various organisms. From this point of view, it is very interesting that condensin in human cells is concentrated in the nucleolus during the interphase (8). Further, in bacteria, both SMC-like proteins (e.g., that encoded by mukB in Escherichia coli) and replication fork blocking systems (Ter site and Tus protein) are found, and chromosome segregation defects were observed in mutants of SMC-related genes (6,19,21,22,37,39,53).…”

Transcript profiling of Wilms tumors reveals connections to kidney morphogenesis and expression patterns associated with anaplasia

“…Depletion of centromeric MCAK leads to chromosome congression and segregation defect due to improper kinetochore attachments (Kim et al, 1997;Sugata et al, 2000;KlineSmith et al, 2004). hCAP-H plays a central role in mitotic chromosome condensation (Cabello et al, 2001;Kimura et al, 2001). SRP1, the yeast homologous of KPNA2, has been shown to be critical for normal migration, division and integrity of nuclei during mitosis (Kussel and Frasch, 1995).…”