Cell cycle-dependent expression and nucleolar localization of hCAP-H

Condensin is a conserved 13S heteropentamer composed of two nonidentical st ructural maintenance of chromosome (SMC) family proteins, in Xenopus XCAP-C and XCAP-E, and three regulatory subunits, XCAP-D2, XCAP-G, and XCAP-H. Bo th biochemical and genetic analyses have demonstrated an essential role for the 13S condensin complex in mitotic chromosome condensation. Further, a p otential requirement for condensin in completion of chromatid arm separatio n in early anaphase is demonstrated by the mutational phenotypes of the Dro sophila homologues of XCAP-H, barren and XCAP-C, DmSMC4. In this study we h ave investigated the expression and subcellular distribution of hCAP-H, the human homolog of XCAP-H, in order to better understand its cellular functi ons. Transcription of hCAP-H was restricted to proliferating cells with hig hest expression during the G(2) phase of the cell cycle. In contrast, cellu lar hCAP-H protein levels were constant throughout the cell cycle. hCAP-H w as 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-depende nt domains of condensin aggregation. During interphase hCAP-H, -C, and -E, have distinct punctate nucleolar localization, suggesting that condensin ma y associate with and modulate the conformation and function of rDNA. hCAP-H association with condensed chromatin was not observed in the early phase o f chromosome condensation when histone H3 phosphorylation has already taken place. This finding is consistent with the hypothesis that histone H3 phos phorylation precedes condensin-mediated condensation.