Smc Family Proteins And Associated Factors In Mitotic Ch

Child Health And Human Development

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Abstract

The eukaryotic ATP-ases of SMC family (structural maintenance of chromosomes) form several essential eukaryotic protein complexes that determine the higher-order chromosome structure and dynamics in eukaryotic cells. One of these complexes, termed condensin, is in the current focus of studies by the Unit of Chromosome Structure and Function.[unreadable] Condensin complex constitutes the main molecular machinery of chromosome condensation, a process indispensable for proper separation of sister chromatids and their segregation during anaphase. In budding yeast and higher eukaryotes condensin is composed of five essential subunits: Smc2, Smc4, Ycs5/Ycg1, Ycs4 and Brn1. At present, the molecular mechanism of condensin activity in vivo is unknown. In order to understand the essence of condensin activity in chromatin the studies in the Unit were focused on several particular readouts of this activity, more recently on (1) the specificity of condensin targeting to the nucleolar chromatin and (2) the role of active condensin for functionality of other chromosomal site (recently identified in the Unit using a whole-genome analysis).[unreadable] (1) Previous studies in the Unit identified nucleolar chromatin (the rDNA genomic locus, or nucleolar organizer) as the major binding site for condensin in S. cerevisiae. The roles of genetic and epigenetic factors in recognition of this specific chromosomal domain by condensin were extensively investigated. Four condensin binding sites within the 9-kb rDNA unit were found to be controlled by two pathways, both involved in the complex process of maintaining the nucleolar organizer. First, analysis of condensin bindig to these sites in the course of cell cycle and by means of modulating the nucleolar transcription levels has established that condensin occupancy of the rDNA is negatively regulated by Pol I transcription. The sites that are most affected are located within the transcribed portion of rDNA, which encodes the 35S RNA precursor. Second, the condensin sites in the intergene region of rDNA are sensitive to proficiency of the replication fork block (RFB) function of the Fob1 protein. Surprisingly, the FOB1 gene deletion also resulted in activation of Pol I transcription and reduction of condensin binding to the 35S gene. Thus these two pathways are semi-independent in controlling condensin binding to rDNA. These results allowed to propose a model suggesting that the non-transcribed rDNA repeats, which are constitutively present in this tandemly repeated locus and comprise about a half of all rDNA, are used for mitotic segregation of the nucleolus in its active/transcribed form, which is characteristic for yeast cells. Moreover, its is likely that this requirement for segregation of active nucleolus embodies the special role played by condensin in the mitotic nucleolar segregation in budding yeast. This model was validated by direct time-lapse microscopic observations of nucleolar segregation in live budding yeast cells. In the strain with only twenty rDNA repeats, where nearly all copies are transcribed throughout the cell cycle, segregation of the nucleolus (Sik1-RFP marker) was significantly delayed, as compared to the rest of the genome (Scc3-GFP marker). [unreadable] (2) As we showed recently, condensin has several critical sites located outside of the nucleolar organizer. These sites correlate with landmarks of of chromosome organization: DNA replication termination zones, subtelomeric regions and centromeres. Studies on these sites with well-defined functions in the nucleus allow to understand the role of condensin activity in the functioning of these chromosomal domains. We characterized the scope of condensin involvement in kinetochore function and found that condensin mutants elicit cell cycle arrest by activation of the spindle checkpoint pathway, mediated by centromere defects. The deficiency of the specialized centromere histone Cse4p (yeast CENP-A) may underlie these defects. This finding opens an intriguing possibility that condensin has a cis-function at the centromere, which is important for kinetochore assembly and/or function.