Ubiquitylation regulates interactions of astral microtubules with the cleavage apparatus.
BACKGROUND
Correct positioning of the mitotic spindle relative to the cleavage apparatus is crucial for successful mitosis. In budding yeast, the Adenomatous Polyposis Coli-related protein Kar9, yeast EB1, and Myo2, a type V myosin, mediate positioning of the mitotic spindle close to the septin-anchored cleavage apparatus at the bud neck.


RESULTS
We find that Kar9 is ubiquitylated and degraded by the proteasome. Ubiquitylation requires the ubiquitin-conjugating enzymes Ubc1 and Ubc4 and phosphorylation of Kar9 by yeast Cdk1. Importantly, Kar9 ubiquitylation and degradation depend on an intact cleavage apparatus. Kar9 is stabilized in septin mutant cells or cells lacking the bud neck formin Bnr1, but not in the bud formin Bni1 or the actomyosin ring. Transport of Kar9 to the bud neck by Myo2 is also required for Kar9 degradation. Abrogation of Kar9 phosphorylation and ubiquitylation increases interactions of astral microtubules (aMTs) with the bud neck and causes spindle mispositioning. Photoconversion experiments showed that Kar9 association with aMTs is stable.


CONCLUSIONS
We propose that ubiquitylation controls interactions of aMTs with the cleavage apparatus through localized disassembly of Kar9 complexes.
Kammerer
Daniel
D
Heidelberg University Biochemistry Center (BZH), Heidelberg, Germany.
Stevermann
Lea
L
Liakopoulos
Dimitris
D
D048749:Cytokinesis / Q000502:physiology*; D008856:Microscopy, Fluorescence; D008869:Microtubule-Associated Proteins / Q000378:metabolism; D008870:Microtubules / Q000378:metabolism / Q000502:physiology*; D008938:Mitosis / Q000502:physiology*; D008954:Models, Biological; D018995:Myosin Heavy Chains / Q000378:metabolism; D024701:Myosin Type V / Q000378:metabolism; D009687:Nuclear Proteins / Q000378:metabolism*; D010766:Phosphorylation; D012441:Saccharomyces cerevisiae; D029701:Saccharomyces cerevisiae Proteins / Q000378:metabolism*; D008941:Spindle Apparatus / Q000502:physiology*; D054875:Ubiquitination
C405804:EB1 microtubule binding proteins; C110443:KAR9 protein, S cerevisiae; C077512:MYO2 protein, S cerevisiae; D008869:Microtubule-Associated Proteins; D009687:Nuclear Proteins; D029701:Saccharomyces cerevisiae Proteins; D024701:Myosin Type V; D018995:Myosin Heavy Chains