Chromosome Condensation Factor Brn1p Is Required for Chromatid Separation in Mitosis

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Abstract
This work describes BRN1, the budding yeast homologue of Drosophila Barren andXenopus condensin subunit XCAP-H. TheDrosophila protein is required for proper chromosome segregation in mitosis, and Xenopus protein functions in mitotic chromosome condensation. Mutant brn1 cells show a defect in mitotic chromosome condensation and sister chromatid separation and segregation in anaphase. Chromatid cohesion before anaphase is properly maintained in the mutants. Somebrn1 mutant cells apparently arrest in S-phase, pointing to a possible function for Brn1p at this stage of the cell cycle. Brn1p is a nuclear protein with a nonuniform distribution pattern, and its level is up-regulated at mitosis. Temperature-sensitive mutations ofBRN1 can be suppressed by overexpression of a novel geneYCG1, which is homologous to anotherXenopus condensin subunit, XCAP-G. Overexpression ofSMC2, a gene necessary for chromosome condensation, and a homologue of the XCAP-E condensin, does not suppress brn1, pointing to functional specialization of components of the condensin complex.