Bypassing anaphase by fission yeast cut9 mutation: requirement of cut9+ to initiate anaphase.

A novel anaphase block phenotype was found in fission yeast temperature-sensitive cut9 mutants. Cells enter mitosis with chromosome condensation and short spindle formation, then block anaphase, but continue to progress into postanaphase events such as degradation of the spindle, reformation of the postanaphase cytoplasmic microtubule arrays, septation, and cytokinesis. The cut9 mutants are defective in the onset of anaphase and possibly in the restraint of postanaphase events until the completion of anaphase. The cut9+ gene encodes a 78-kD protein containing the 10 34-amino acid repeats, tetratricopeptide repeats (TPR), and similar to budding yeast Cdc16. It is essential for viability, and the mutation sites reside in the TPR. The three genes, namely, nuc2+, scn1+, and scn2+, genetically interact with cut9+. The nuc2+ and cut9+ genes share an essential function to initiate anaphase. The cold-sensitive scn1 and scn2 mutations, defective in late anaphase, can suppress the ts phenotype of cut9.