Schizosaccharomyces pombe RanGAP Homolog, SpRna1, Is Required for Centromeric Silencing and Chromosome SegregationD

We isolated 11 independent temperature-sensitive (ts) mutants of Schizosaccharomyces pombe RanGAP, SpRna1 that have several amino acid changes in the conserved domains of RanGAP. Resulting Sprna1 ts showed a strong defect in mitotic chromosome segregation, but did not in nucleocytoplasmic transport and microtubule formation. In addition to Sprna1 + and Spksp1 + , the clr4 + (histone H3-K9 methyltransferase), the S. pombe gene, SPAC25A8.01c, designated snf2SR + (a member of the chromatin remodeling factors, Snf2 family with DNA-dependent ATPase activity), but not the spi1 + ( S. pombe Ran homolog), rescued a lethality of Sprna1 ts . Both Clr4 and Snf2 were reported to be involved in heterochromatin formation essential for building the centromeres. Consistently, Sprna1 ts was defective in gene-silencing at the centromeres. But a silencing at the telomere, another heterochromatic region, was normal in all of Sprna1 ts strains, indicating SpRna1 in general did not function for a heterochromatin formation. snf2SR + rescued a centromeric silencing defect and Δ clr4 + was synthetic lethal with Sprna1 ts . Taken together, SpRna1 was suggested to function for constructing the centromeres, by cooperating with Clr4 and Snf2SR. Loss of SpRna1 activity, therefore, caused chromosome missegregation.