Functionally distinct isoforms of Cik1 are differentially regulated by APC/C-mediated proteolysis

Mol Cell. 2009 Mar 13;33(5):581-90. doi: 10.1016/j.molcel.2009.01.032.


Cik1, in association with the kinesin Kar3, controls both the mitotic spindle and nuclear fusion during mating. Here, we show that there are two Cik1 isoforms, and that the mitotic form includes an N-terminal domain required for ubiquitination by the Anaphase-Promoting Complex/Cyclosome (APC/C). During vegetative growth, Cik1 is expressed during mitosis and regulates the mitotic spindle, allowing for accurate chromosome segregation. After mitosis, APC/C(Cdh1) targets Cik1 for ubiquitin-mediated proteolysis. Upon exposure to the mating pheromone alpha factor, a smaller APC/C-resistant Cik1 isoform is expressed from an alternate transcriptional start site. This shorter Cik1 isoform is stable and cannot be ubiquitinated by APC/C(Cdh1). Moreover, the two Cik1 isoforms are functionally distinct. Cells that express only the long isoform have defects in nuclear fusion, whereas cells expressing only the short isoform have an increased rate of chromosome loss. These results demonstrate a coupling of transcriptional regulation and APC/C-mediated proteolysis.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, U.S. Gov't, Non-P.H.S.

MeSH terms

  • Anaphase-Promoting Complex-Cyclosome
  • Cdh1 Proteins
  • Chromosome Segregation
  • Gene Expression Regulation, Fungal
  • Mating Factor
  • Membrane Fusion
  • Microtubule Proteins / genetics
  • Microtubule Proteins / metabolism*
  • Microtubule-Associated Proteins / metabolism
  • Mitosis* / genetics
  • Mutation
  • Peptide Hydrolases / genetics
  • Peptide Hydrolases / metabolism*
  • Peptides / metabolism
  • Promoter Regions, Genetic
  • Protein Isoforms
  • Protein Processing, Post-Translational*
  • Protein Stability
  • Saccharomyces cerevisiae / genetics
  • Saccharomyces cerevisiae / growth & development
  • Saccharomyces cerevisiae / metabolism*
  • Saccharomyces cerevisiae Proteins / genetics
  • Saccharomyces cerevisiae Proteins / metabolism*
  • Spindle Apparatus / metabolism*
  • Time Factors
  • Transcription, Genetic
  • Ubiquitin-Protein Ligase Complexes / genetics
  • Ubiquitin-Protein Ligase Complexes / metabolism*
  • Ubiquitination


  • CDH1 protein, S cerevisiae
  • CIK1 protein, S cerevisiae
  • Cdh1 Proteins
  • KAR3 protein, S cerevisiae
  • Microtubule Proteins
  • Microtubule-Associated Proteins
  • Peptides
  • Protein Isoforms
  • Saccharomyces cerevisiae Proteins
  • Mating Factor
  • Ubiquitin-Protein Ligase Complexes
  • Anaphase-Promoting Complex-Cyclosome
  • Peptide Hydrolases