title: Timely activation of budding yeast APCCdh1 involves degradation of its inhibitor, Acm1, by an unconventional proteolytic mechanism.
abstract: Regulated proteolysis mediated by the ubiquitin proteasome system is a fundamental and essential feature of the eukaryotic cell division cycle. Most proteins with cell cycle-regulated stability are targeted for degradation by one of two related ubiquitin ligases, the Skp1-cullin-F box protein (SCF) complex or the anaphase-promoting complex (APC). Here we describe an unconventional cell cycle-regulated proteolytic mechanism that acts on the Acm1 protein, an inhibitor of the APC activator Cdh1 in budding yeast. Although Acm1 can be recognized as a substrate by the Cdc20-activated APC (APCCdc20) in anaphase, APCCdc20 is neither necessary nor sufficient for complete Acm1 degradation at the end of mitosis. An APC-independent, but 26S proteasome-dependent, mechanism is sufficient for complete Acm1 clearance from late mitotic and G1 cells. Surprisingly, this mechanism appears distinct from the canonical ubiquitin targeting pathway, exhibiting several features of ubiquitin-independent proteasomal degradation. For example, Acm1 degradation in G1 requires neither lysine residues in Acm1 nor assembly of polyubiquitin chains. Acm1 was stabilized though by conditional inactivation of the ubiquitin activating enzyme Uba1, implying some requirement for the ubiquitin pathway, either direct or indirect. We identified an amino terminal predicted disordered region in Acm1 that contributes to its proteolysis in G1. Although ubiquitin-independent proteasome substrates have been described, Acm1 appears unique in that its sensitivity to this mechanism is strictly cell cycle-regulated via cyclin-dependent kinase (Cdk) phosphorylation. As a result, Acm1 expression is limited to the cell cycle window in which Cdk is active. We provide evidence that failure to eliminate Acm1 impairs activation of APCCdh1 at mitotic exit, justifying its strict regulation by cell cycle-dependent transcription and proteolytic mechanisms. Importantly, our results reveal that strict cell-cycle expression profiles can be established independent of proteolysis mediated by the APC and SCF enzymes.
authors: Department of Biochemistry, Purdue University, West Lafayette, Indiana, United States of America.
authors: Michael
authors: M
authors: Melesse
authors: Department of Biochemistry, Purdue University, West Lafayette, Indiana, United States of America.
authors: Eunyoung
authors: E
authors: Choi
authors: Department of Biochemistry, Purdue University, West Lafayette, Indiana, United States of America.
authors: Hana
authors: H
authors: Hall
authors: Department of Biochemistry, Purdue University, West Lafayette, Indiana, United States of America.
authors: Michael J
authors: MJ
authors: Walsh
authors: Department of Biochemistry, Purdue University, West Lafayette, Indiana, United States of America.
authors: M Ariel
authors: MA
authors: Geer
authors: Department of Biochemistry, Purdue University, West Lafayette, Indiana, United States of America; Center for Cancer Research, Purdue University, West Lafayette, Indiana, United States of America.
authors: Mark C
authors: MC
authors: Hall
mesh_terms: D000705:Anaphase; D064173:Anaphase-Promoting Complex-Cyclosome / Q000378:metabolism*; D064200:Cdh1 Proteins / Q000378:metabolism*; D018797:Cell Cycle Proteins / Q000737:chemistry / Q000378:metabolism*; D018844:Cyclin-Dependent Kinases / Q000378:metabolism; D007976:Leupeptins / Q000494:pharmacology; D008938:Mitosis; D010766:Phosphorylation; D046988:Proteasome Endopeptidase Complex / Q000378:metabolism; D059748:Proteolysis / Q000187:drug effects; D044843:SKP Cullin F-Box Protein Ligases / Q000378:metabolism; D029701:Saccharomyces cerevisiae Proteins / Q000737:chemistry / Q000378:metabolism*; D004718:Saccharomycetales / Q000378:metabolism*; D044767:Ubiquitin-Protein Ligases / Q000378:metabolism
chemical_list: C516306:Acm1 protein, S cerevisiae; C577583:CDH1 protein, S cerevisiae; D064200:Cdh1 Proteins; D018797:Cell Cycle Proteins; D007976:Leupeptins; D029701:Saccharomyces cerevisiae Proteins; D064173:Anaphase-Promoting Complex-Cyclosome; D044843:SKP Cullin F-Box Protein Ligases; D044767:Ubiquitin-Protein Ligases; D018844:Cyclin-Dependent Kinases; D046988:Proteasome Endopeptidase Complex; C061553:ATP dependent 26S protease; C072553:benzyloxycarbonylleucyl-leucyl-leucine aldehyde