SUMMARY The Anaphase Promoting Complex/Cyclosome (APC/C) is an ubiquitin ligase and core component of the cell cycle oscillator. During G1-phase APC/C binds to its substrate receptor Cdh1 and APC/CCdh1 plays an important role in restricting S-phase entry and maintaining genome integrity. We describe a reciprocal feedback circuit between APC/C and a second ubiquitin ligase, the SCF (Skp1-Cul1-F box). We show that Cyclin F, a cell cycle regulated substrate receptor (F-box protein) for the SCF, is targeted for degradation by APC/C. Furthermore, we establish that Cdh1 is itself a substrate of SCFCyclin F. Cyclin F loss impairs Cdh1 degradation and delays S-phase entry, and this delay is reversed by simultaneous removal of Cdh1. These data indicate that the coordinated, temporal ordering of Cyclin F and Cdh1 degradation, organized in a double-negative feedback loop, represents a fundamental aspect of cell cycle control. This mutual antagonism could be a feature of other oscillating systems.

SUMMARYThe transcription factor FOXM1 contributes to cell cycle progression and is significantly upregulated in basal-like breast cancer (BLBC). Despite its importance in normal and cancer cell cycles, we lack a complete understanding of mechanisms that regulate FOXM1. We identified USP21 in an RNAi-based screen for deubiquitinases that control FOXM1 abundance. USP21 increases the stability of FOXM1, and USP21 binds and deubiquitinates FOXM1 in vivo and in vitro, indicating a direct enzyme-substrate relationship. Depleting USP21 downregulates the FOXM1 transcriptional network and causes a signifi-cant delay in cell cycle progression. Significantly, USP21 depletion sensitized BLBC cell lines and mouse xenograft tumors to paclitaxel, an anti-mitotic, frontline therapy in BLBC treatment. USP21 is the most frequently amplified deubiquitinase in BLBC patient tumors, and its amplification co-occurs with the upregulation of FOXM1 protein. Altogether, these data suggest a role for USP21 in the proliferation and potentially treatment of FOXM1-high, USP21-high BLBC.

SUMMARY The oncogenic AKT kinase is a key regulator of apoptosis, cell growth and cell cycle progression. Despite its important role in proliferation, it remains largely unknown how AKT is mechanistically linked to cell cycle. We show here that Cyclin F, a substrate receptor F-box protein for the SCF family of E3 ubiquitin ligases, is a bona fide AKT substrate. Cyclin F expression oscillates throughout the cell cycle, a rare feature among the 69 human F-box proteins, and all of its known substrates are involved in proliferation. AKT phosphorylation of Cyclin F enhances its stability and promotes assembly into productive E3 ligase complexes. Importantly, expression of mutant versions of Cyclin F that cannot be phosphorylated by AKT impair cell cycle entry. Our data suggest that Cyclin F transmits mitogen signaling through AKT to the core cell cycle machinery. This discovery has potential implications for proliferative control in malignancies where AKT is activated.

The oncogenic transcription factor FoxM1 plays a vital role in cell cycle progression, is activated in numerous human malignancies, and is linked to chromosome instability. We characterize here a cullin 4-based E3 ubiquitin ligase and its substrate receptor, VprBP/DCAF1 (CRL4 VprBP ), which we show regulate FoxM1 ubiquitylation and degradation. Paradoxically, we also found that the substrate receptor VprBP is a potent FoxM1 activator. VprBP depletion reduces expression of FoxM1 target genes and impairs mitotic entry, whereas ectopic VprBP expression strongly activates a FoxM1 transcriptional reporter. VprBP binding to CRL4 is reduced during mitosis, and our data suggest that VprBP activation of FoxM1 is ligase independent. This implies a nonproteolytic activation mechanism that is reminiscent of, yet distinct from, the ubiquitin-dependent transactivation of the oncoprotein Myc by other E3s. Significantly, VprBP protein levels were upregulated in high-grade serous ovarian patient tumors, where the FoxM1 signature is amplified. These data suggest that FoxM1 abundance and activity are controlled by VprBP and highlight the functional repurposing of E3 ligase substrate receptors independent of the ubiquitin system. KEYWORDS cell cycle, cullin ring ligase, FoxM1, transcriptional regulation, ubiquitination C hanges in gene expression combined with targeted protein degradation dynamically shape the protein landscape. Gene expression is coordinated by transcription factors that specify genes for activation and cofactors that modulate transcription factor activity or alter the local chromatin environment. Posttranslational modifications (PTMs) play a crucial role in transcriptional dynamics. Phosphorylation, acetylation, methylation, and ubiquitylation of histone proteins are well studied and contribute significantly to gene expression dynamics (1). Similarly, posttranslational modification of transcription factors plays an important role in regulating genome output.FoxM1 is an oncogenic, cell cycle-regulated transcription factor that was discovered as both a marker and a key mediator of cell proliferation (2-4). Subsequent work clarified the importance of FoxM1 in proliferation through its role in cell cycle progression (reviewed in reference 5). FoxM1 controls the mitotic transcriptional program, and its depletion significantly impairs normal mitotic entry and progression (6-9). In addition, FoxM1 and its transcriptional network have been associated with numerous cancers (5, 10). Notably, FoxM1 is the key regulator of a proliferative gene expression signature found in high-grade serous ovarian cancer (HGSOC), basal-like breast cancers,

The mitotic spindle is composed of dynamic microtubules and associated proteins that together direct chromosome movement during mitosis. The spindle plays a vital role in accurate chromosome segregation fidelity and is a therapeutic target in cancer. Nevertheless, the molecular mechanisms by which many spindle-associated proteins function remains unknown. The cleolar andpindle-ssociated rotein NUSAP1 is a microtubule-binding protein implicated in spindle stability and chromosome segregation. We show here that NUSAP1 localizes to dynamic spindle microtubules in a unique chromosome-centric pattern, in the vicinity of overlapping microtubules, during metaphase and anaphase of mitosis. Mass spectrometry-based analysis of endogenous NUSAP1 interacting proteins uncovered a cell cycle-regulated interaction between the RanBP2-RanGAP1-UBC9 SUMO E3 ligase complex and NUSAP1. Like NUSAP1 depletion, RanBP2 depletion impaired the response of cells to the microtubule poison Taxol. NUSAP1 contains a conserved SAP domain (SAF-A/B, Acinus, and PIAS). SAP domains are common among many other SUMO E3s, and are implicated in substrate recognition and ligase activity. We speculate that NUSAP1 contributes to accurate chromosome segregation by acting as a co-factor for RanBP2-RanGAP1-UBC9 during cell division.