CUT1 IS LOADED ONTO THE SPINDLE BY BINDING TO CUT2 AND PROMOTES ANAPHASE SPINDLE MOVEMENT UPON CUT2 PROTEOLYSIS

Background: The Cut1 and Cut2 proteins of the fission yeast Schizosacc haromyces pombe form a complex and are required for the separation of sister chromatids during anaphase. Polyubiquitinated Cut2 degrades at the onset of anaphase and this degradation, like that of mitotic cycli n, is dependent on the anaphase-promoting complex/cyclosome. Expressio n of Cut2 that cannot be degraded blocks sister chromatid separation a nd anaphase spindle elongation. Here, we have investigated the role of the Cut1-Cut2 interaction in sister chromatid separation. Results: Th e carboxyl terminus of Cut2 interacts with the amino terminus of Cut1, and temperature-sensitive cut2 mutants expressed Cut2 proteins that co ntain substitutions in the carboxyl terminus and fail to interact with Cut1, resulting in aberrant anaphase, Localization of Cut1 alters dra matically during the cell cycle. Cut1 is retained in the cytoplasm dur ing interphase and moves to the mitotic spindle pole bodies and the sp indle upon entry into prophase, when spindles are formed. The associat ion between Cut2 and Cut1 is needed for the localization of Cut1 to th e spindles, as Cut1 remains unbound to the spindle if complex formatio n is impaired. Cut2 degrades during anaphase, but Cut1 remains bound t o the anaphase spindle. This association with the anaphase spindle req uires the conserved carboxyl terminus of Cut1. Conclusions: Complex fo rmation between Cut1 and Cut2 is needed for the onset of normal anapha se. Cut2 is required for loading Cut1 onto the spindle at prophase and Cut2 proteolysis is needed for the active participation of Cut1 in si ster chromatid separation. (C) Current Biology Ltd ISSN 0960-9822.