Cut8, essential for anaphase, controls localization of 26S proteasome, facilitating destruction of cyclin and Cut2

Background: Anaphase-promoting complex (APC)/cyclosome and 26S proteasome a re respectively required for polyubiquitination and degradation of mitotic cyclin and anaphase inhibitor Cut2 (Pds1/securin). In fission yeast, mutant cells defective in cyclosome and proteasome fail to complete mitosis and h ave hypercondensed chromosomes and a short spindle. A similar phenotype is seen in a temperature-sensitive strain cut8-563 at 36 degreesC, but the mol ecular basis for Cut8 function is little understood. Results: At high temperature, the level of Cut8 greatly increases and it be comes essential to the progression of anaphase. In cut8 mutants, chromosome mis-segregation and aberrant spindle dynamics occur, but cytokinesis takes place with normal timing, leading to the cut phenotype. This is due to the fact that destruction of mitotic cyclin and Cut? in the nucleus is dramati cally delayed, though polyubiquitination of Cdc13 occurs in cut8 mutant. Cu t8 is localized chiefly to the nucleus and nuclear periphery, a distributio n highly similar to that of 26S proteasome. In cut8 mutant, however, 26S pr oteasome becomes mostly cytoplasmic, showing that Cut8 is needed for its pr oper localization. Conclusion: Cut8 is a novel evolutionarily conserved heat-inducible regulat or. It facilitates anaphase-promoting proteolysis by recruiting 26S proteas ome to a functionally efficient nuclear location.