Conserved Wat1/Pop3 WD-repeat protein of fission yeast secures genome stability through microtubule integrity and may be involved in mRNA maturation

Accurate chromosome segregation is dependent upon the integrity of mitotic spindles, which pull each pair of sister chromatids towards opposite poles. In this study, we have characterised fission yeast pop3-5235, a diploidisi ng mutant that is impaired in genome stability. Pop3 is the same as Wat1, a conserved protein containing 7 WD repeats. Pop3/Wat1 has also been isolate d from a two-hybrid screen as a binding partner to Prp2, the large subunit of the essential splicing factor U2AF. In wat1 mutants, the cellular amount of alpha -tubulin is decreased to very low levels, which results in compro mised microtubules and spindles, consequently leading to unequal chromosome separation. Further analysis shows that, in spite of the binding between W at1 and Prp2, Wat1 may not be involved directly in splicing reactions per s e. Instead, we find that Wat1 is required for the maintenance of alpha -tub ulin mRNA levels; moreover, transcript levels of genes other than the alpha -tubulin gene are also equally decreased in this mutant. Wild-type Wat1, b ut not the mutant protein, forms a large complex in the cell with several o ther proteins, suggesting that Wat1 functions as a structural linker in the complex. The results suggest that Wat1 plays a role in mRNA maturation as a coupling protein between splicing and synthesis and/or stabilisation.