Deregulation of the G1/S-phase transition is the proximal cause of mortality in old yeast mother cells.

Budding yeast cells produce a finite number of daughter cells before they die. Why old yeast cells stop dividing and die is unclear. We found that age-induced accumulation of the G1/S-phase inhibitor Whi5 and defects in G1/S cyclin transcription cause cell cycle delays and genomic instability that result in cell death. We further identified extrachromosomal rDNA (ribosomal DNA) circles (ERCs) to cause the G1/S cyclin expression defect in old cells. Spontaneous segregation of Whi5 and ERCs into daughter cells rejuvenates old mothers, but daughters that inherit these aging factors die rapidly. Our results identify deregulation of the G1/S-phase transition as the proximal cause of age-induced proliferation decline and cell death in budding yeast.

PRMT6 functionally associates with PRMT5 to promote colorectal cancer progression through epigenetically repressing the expression of CDKN2B and CCNG1.

BACKGROUND: Protein arginine methyltransferase 6 (PRMT6) is a type I arginine methyltransferase that asymmetrically dimethylates histone H3 arginine 2 (H3R2me2a). However, the biological roles and underlying molecular mechanisms of PRMT6 in colorectal cancer (CRC) remain unclear. METHODS: PRMT6 expression in CRC tissue was examined using immunohistochemistry. The effect of PRMT6 on CRC cells was investigated in vitro and in vivo. Mass spectrometry, co-immunoprecipitation and GST pulldown assays were performed to identify interaction partners of PRMT6. RNA-seq, chromatin immunoprecipitation, Western blot and qRT-PCR assays were used to investigate the mechanism of PRMT6 in gene regulation. RESULTS: PRMT6 is significantly upregulated in CRC tissues and facilitates cell proliferation of CRC cells in vitro and in vivo. Through RNA-seq analysis, CDKN2B (p15INK4b) and CCNG1 were identified as new transcriptional targets of PRMT6. PRMT6-dependent H3R2me2a mark was predominantly deposited at the promoters of CDKN2B and CCNG1 in CRC cells. Furthermore, PRMT5 was firstly characterized as an interaction partner of PRMT6. Notably, H3R2me2a coincides with PRMT5-mediated H4R3me2s and H3R8me2s marks at the promoters of CDKN2B and CCNG1 genes, thus leading to transcriptional repression of these genes. CONCLUSIONS: PRMT6 functionally associates with PRMT5 to promote CRC progression through epigenetically repressing the expression of CDKN2B and CCNG1. These insights raise the possibility that combinational intervention of PRMT6 and PRMT5 may be a promising strategy for CRC therapy.

The stimulative function of long noncoding RNA CDKN2B-AS1 in osteosarcoma by targeting the microRNA-122/CCNG1 axis.

Osteosarcoma (OS), a prevalent aggressive malignancy in the bone, has limited therapeutic targets and diagnostic biomarkers. In the current investigation, RT-qPCR showed that CDKN2B-AS1 was enhanced in OS samples and cells. This research was set to examine the modulation of CDKN2B-AS1 in OS. The expression of CDKN2B-AS1 and downstream molecules was analyzed by RT-qPCR method. CCK8, EdU staining along with Transwell assays were applied to evaluate cell proliferation and invasion. Those in vitro investigations specified that silencing of CDKN2B-AS1 with shRNAs obviously impeded the proliferation and invasion of MG63 cells. To authenticate the relationships between CDKN2B-AS1 and microRNA-122-5p (miR-122-5p) or cyclin G1 (CCNG1) and miR-122-5p, we next employed luciferase reporter assay. We displayed that CDKN2B-AS1 repressed miR-122-5p to restore CCNG1 expression. All in all, our findings substantiated the indispensable function of CDKN2B-AS1 in OS progression and the possible molecular mechanism.

Cyclin G2 upregulation impairs migration, invasion, and network formation through RNF123/Dvl2/JNK signaling in the trophoblast cell line HTR8/SVneo, a possible role in preeclampsia.

Disruption of extravillous trophoblast (EVT) migration and invasion is considered to be responsible for pathological placentation in preeclampsia (PE). Cyclin G2 (CCNG2) is an atypical cyclin that inhibits cell cycle progression. However, its biological function and underlying molecular mechanism in PE are poorly understood. In this study, clinical data demonstrated that CCNG2 was significantly upregulated in PE placenta and associated with invasive EVT dysfunction. Additionally, Ccng2 knockout led to an attenuation of PE-like symptoms in the PE mouse model produced via treatment with NG-nitro-L-arginine methyl ester (L-NAME). In vitro, CCNG2 inhibited the migration, invasion, and endothelial-like network formation of human trophoblast cell line HTR8/SVneo. Mechanically, CCNG2 suppressed JNK-dependent Wnt/PCP signaling and its downstream indicators including epithelial-to-mesenchymal transition (EMT) markers and matrix metalloproteinases (MMPs) via promoting the polyubiquitination degradation of dishevelled 2 (Dvl2) protein in HTR8/SVneo cells. We also discovered that the E3 ligase Ring finger protein 123 (RNF123), as a novel CCNG2 target among HTR8/SVneo cells, interacted with Dvl2 and participated in CCNG2-induced polyubiquitination degradation of Dvl2. Moreover, we verified that the treatment of HTR8/SVneo cells with RNF123-specific siRNA improved polyubiquitination-induced degradation of Dvl2 and the activity of Wnt/PCP-JNK signaling mediated by CCNG2. Taken together, our results reveal that the CCNG2/RNF123/Dvl2/JNK axis may be involved in the pathogenesis and progression of PE through trophoblastic cell function modulation, thus probably providing us with new therapeutic strategies for PE treatment.

Locally acting transcription factors regulate p53-dependent cis-regulatory element activity.

The master tumor suppressor p53 controls transcription of a wide-ranging gene network involved in apoptosis, cell cycle arrest, DNA damage repair, and senescence. Recent studies revealed pervasive binding of p53 to cis-regulatory elements (CREs), which are non-coding segments of DNA that spatially and temporally control transcription through the combinatorial binding of local transcription factors. Although the role of p53 as a strong trans-activator of gene expression is well known, the co-regulatory factors and local sequences acting at p53-bound CREs are comparatively understudied. We designed and executed a massively parallel reporter assay (MPRA) to investigate the effect of transcription factor binding motifs and local sequence context on p53-bound CRE activity. Our data indicate that p53-bound CREs are both positively and negatively affected by alterations in local sequence context and changes to co-regulatory TF motifs. Our data suggest p53 has the flexibility to cooperate with a variety of transcription factors in order to regulate CRE activity. By utilizing different sets of co-factors across CREs, we hypothesize that global p53 activity is guarded against loss of any one regulatory partner, allowing for dynamic and redundant control of p53-mediated transcription.

Centromeric signaling proteins boost G1 cyclin degradation and modulate cell size in budding yeast.

Cell size scales with ploidy in a great range of eukaryotes, but the underlying mechanisms remain unknown. Using various orthogonal single-cell approaches, we show that cell size increases linearly with centromere (CEN) copy number in budding yeast. This effect is due to a G1 delay mediated by increased degradation of Cln3, the most upstream G1 cyclin acting at Start, and specific centromeric signaling proteins, namely Mad3 and Bub3. Mad3 binds both Cln3 and Cdc4, the adaptor component of the Skp1/Cul1/F-box (SCF) complex that targets Cln3 for degradation, these interactions being essential for the CEN-dosage dependent effects on cell size. Our results reveal a pathway that modulates cell size as a function of CEN number, and we speculate that, in cooperation with other CEN-independent mechanisms, it could assist the cell to attain efficient mass/ploidy ratios.

MicroRNA-874-mediated inhibition of the major G1/S phase cyclin, CCNE1, is lost in osteosarcomas.

The tumor microenvironment is characterized by nutrient-deprived conditions in which the cancer cells have to adapt for survival. Serum starvation resembles the growth factor deprivation characteristic of the poorly vascularized tumor microenvironment and has aided in the discovery of key growth regulatory genes and microRNAs (miRNAs) that have a role in the oncogenic transformation. We report here that miR-874 down-regulates the major G1/S phase cyclin, cyclin E1 (CCNE1), during serum starvation. Because the adaptation of cancer cells to the tumor microenvironment is vital for subsequent oncogenesis, we tested for miR-874 and CCNE1 interdependence in osteosarcoma cells. We observed that miR-874 inhibits CCNE1 expression in primary osteoblasts, but in aggressive osteosarcomas, miR-874 is down-regulated, leading to elevated CCNE1 expression and appearance of cancer-associated phenotypes. We established that loss of miR-874-mediated control of cyclin E1 is a general feature of osteosarcomas. The down-regulation of CCNE1 by miR-874 is independent of E2F transcription factors. Restoration of miR-874 expression impeded S phase progression, suppressing aggressive growth phenotypes, such as cell invasion, migration, and xenograft tumors, in nude mice. In summary, we report that miR-874 inhibits CCNE1 expression during growth factor deprivation and that miR-874 down-regulation in osteosarcomas leads to CCNE1 up-regulation and more aggressive growth phenotypes.

MicroRNA-23b and microRNA-27b plus flutamide treatment enhances apoptosis rate and decreases CCNG1 expression in a castration-resistant prostate cancer cell line.

The acquisition of a castration-resistant prostate cancer phenotype by prostate cancer cells is the alteration that has the worst prognosis for patients. The aim of this study was to evaluate the role of the microRNAs-23b/-27b as well as the possible CCNG1 target gene in tissue samples from patients with localized prostate cancer that progressed to castration-resistant prostate cancer and in a castration-resistant prostate cancer cell line (PC-3). The microRNAs and target gene expression levels of the surgical specimens were analyzed by quantitative real-time polymerase chain reaction. The prostate cancer cell line, PC-3, was transfected with pre-miR-23b, pre-miR-27b, and their respective controls using Lipofectamine RNAiMAX and exposed or not to flutamide. After transfections, expression levels of both the microRNAs and the gene, CCNG1, were analyzed by quantitative real-time polymerase chain reaction. The apoptosis and cell cycle assays were performed on the mini MUSE cytometer. MicroRNAs-23b/-27b were underexpressed in surgical specimens of prostate cancer; however, their target gene, CCNG1, was overexpressed in 69% of the cases. After transfection with the microRNAs-23b/-27b and flutamide, we observed a reduction in gene expression compared with cells that were treated only with microRNAs or only with flutamide. In the apoptosis assay, we demonstrated cell sensitization following transfection with microRNAs-23b/-27b and potentiation when co-administered with flutamide. The number of cells in apoptosis was almost three times higher with the simultaneous treatments (miR + flutamide) compared with the control (p < 0.05). In the cell cycle assay, only flutamide treatment showed better results; a higher number of cells were found in the G0-G1 phase, and a lower percentage of cells completed the final phase of the cycle (p < 0.05). We conclude that microRNAs-23b/-27b are downexpressed in prostate cancer, and their target gene, CCNG1, is overexpressed. We postulated that microRNAs-23b/-27b sensitize the PC-3 cell line and that after the addition of flutamide in the apoptosis assay, we would observe synergism in the treatments between miR and flutamide. In the cell cycle assay, the use of flutamide was sufficient to decrease the number of cells in mitosis. Therefore, we postulate that microRNAs, along with other drugs, may become very useful therapeutic tools in the treatment of castration-resistant prostate cancer.

MiR-122 modification enhances the therapeutic efficacy of adipose tissue-derived mesenchymal stem cells against liver fibrosis.

Mesenchymal stem cell (MSC) transplantation alone may be insufficient for treatment of liver fibrosis because of complicated histopathological changes in the liver. Given that miR-122 plays an essential role in liver fibrosis by negatively regulating the proliferation and transactivation of hepatic stellate cells (HSCs), this study investigated whether miR-122 modification can improve the therapeutic efficacy of adipose tissue-derived MSCs in treating liver fibrosis. MiR-122-modified AMSCs (AMSC-122) were constructed through lentivirus-mediated transfer of pre-miR-122. MiR-122-modified AMSCs expressed high level of miR-122, while they retained their phenotype and differentiation potential as naïve AMSCs. AMSC-122 more effectively suppressed the proliferation of and collagen maturation in HSCs than scramble miRNA-modified AMSCs. In addition, AMSC-derived exosomes mediated the miR-122 communication between AMSCs and HSCs, further affecting the expression levels of miR-122 target genes, such as insulin-like growth factor receptor 1 (IGF1R), Cyclin G(1) (CCNG1) and prolyl-4-hydroxylase α1 (P4HA1), which are involved in proliferation of and collagen maturation in HSCs. Moreover, miR-122 modification enhanced the therapeutic efficacy of AMSCs in the treatment of carbon tetrachloride (CCl4 )-induced liver fibrosis by suppressing the activation of HSCs and alleviating collagen deposition. Results demonstrate that miR-122 modification improves the therapeutic efficacy of AMSCs through exosome-mediated miR-122 communication; thus, miR-122 modification is a new potential strategy for treatment of liver fibrosis.

CyclinG1 Amplification Enhances Aurora Kinase Inhibitor-Induced Polyploid Resistance and Inhibition of Bcl-2 Pathway Reverses the Resistance.

BACKGROUND/AIMS: CyclinG1 (CycG1) is frequently overexpressed in solid tumors and overexpression of CycG1 promotes cell survival upon paclitaxel exposure by inducing polyploidy. Whether and how CycG1 regulates polyploidization caused by small molecular targeted inhibitors remains unclear. METHODS: Immunohistochemistry and immunoblotting were utilized to examine protein expression. Cell proliferation was measured by ATPlite assay, and cell cycle distribution and apoptosis were measured by flow cytometry and/or DNA fragmentation assays. RESULTS: Overexpression of CycG1 in breast cancer cells caused apoptosis-resistant polyploidy upon treatment with Aurora kinase inhibitor, ZM447439 (ZM). Addition of ABT-263, a small-molecule BH3 mimetic, to ZM, produced a synergistic loss of cell viability with greater sustained tumor growth inhibition in breast cancer cell lines. Decrease of Mcl-1 and increase of NOXA caused by ZM treatment, were responsible for the synergy. Furthermore, CycG1 was highly expressed in Triple-Negative-Breast-Cancer patients treated with paclitaxel and was paralleled by decreased cell survival. CONCLUSION: CycG1 is a crucial factor in ZM-induced polyploidy resistance, and ABT-263/ZM combination hold therapeutic utility in the CycG1-amplified subset of breast cancer and CycG1, thus, is a promising target in breast cancer.

MiR-19a, miR-122 and miR-223 are differentially regulated by hepatitis B virus X protein and involve in cell proliferation in hepatoma cells.

BACKGROUND: Hepatitis B virus (HBV) X protein (HBx) is a type of oncogenic protein involved in the progression of hepatocellular carcinoma (HCC) via interacting with host genes. Dysregulation of microRNAs (miRNAs) has been observed in HCC. This study aimed to investigate the role of HBx protein in the regulation of miR-19a, miR-122 and miR-223, and examine if these miRNAs involve in progression of malignant hepatocytes. METHODS: Quantitative real time PCR (qRT-PCR) was used to measure the expression of miR-19a, miR-122 and miR-223 in patient samples and in HepG2 cells transfected with HBx or 1.3 fold HBV genome and also in HepG2.2.15 cells, which stably produces HBV. Their target mRNAs and proteins-PTEN, cyclin G1 and c-myc were measured by qRT-PCR and western blot, respectively. The effect of miR-19a, miR-122 and miR-223, and their respective target genes, on cell proliferation was analyzed using 5-ethynyl-2-deoxyuridine incorporation and MTT assay. RESULTS: MiR-19a showed an up-regulation in HBV-positive HCC patients compared to healthy controls and HBV-negative HCC patients, while miR-122 and miR-223 showed a down-regulation compared to healthy controls, and miR-122 in HBV-positive HCC patients was also down-regulated when compared to HBV-negative HCC patients. MiR-19a was found to be up-regulated in HepG2 cells transfected with HBx or 1.3 fold HBV genome, but down-regulated in HepG2.2.15 cells. MiR-122 and miR-223 were down-regulated in HBx or 1.3 fold HBV transfected HepG2 cells as well as in HepG2.2.15 cell. Their target mRNAs and corresponding proteins-PTEN was down-regulated, while cyclin G1 and c-myc were found to be up-regulated. Modulated expression of miR-19a, miR-122 and miR-223 enhanced cell proliferation of HBx-transfected HepG2 cells, and rescue experiment further showed that their target genes-PTEN, cyclin G1and c-myc involved in cell proliferation of HBx-transfected HepG2 cells. CONCLUSIONS: The expression of miR-19a, miR-122 and miR-223 were differentially regulated by HBx protein, the differential expression of miR-19a, miR-122 and miR-223 plays an important role in cell proliferation of HCC. This study provides new insight into understanding how HBx protein interacts with miRNAs and subsequently regulates host function.

Huxley's revenge: cell-cycle entry, positive feedback, and the G1 cyclins.

In a recent issue of Nature, Skotheim et al. (2008) show that a transcriptional positive feedback loop plays a key role in the commitment to enter the yeast cell cycle.

Acetyl-CoA induces transcription of the key G1 cyclin CLN3 to promote entry into the cell division cycle in Saccharomyces cerevisiae.

In budding yeast cells, nutrient repletion induces rapid exit from quiescence and entry into a round of growth and division. The G1 cyclin CLN3 is one of the earliest genes activated in response to nutrient repletion. Subsequent to its activation, hundreds of cell-cycle genes can then be expressed, including the cyclins CLN1/2 and CLB5/6. Although much is known regarding how CLN3 functions to activate downstream targets, the mechanism through which nutrients activate CLN3 transcription in the first place remains poorly understood. Here we show that a central metabolite of glucose catabolism, acetyl-CoA, induces CLN3 transcription by promoting the acetylation of histones present in its regulatory region. Increased rates of acetyl-CoA synthesis enable the Gcn5p-containing Spt-Ada-Gcn5-acetyltransferase transcriptional coactivator complex to catalyze histone acetylation at the CLN3 locus alongside ribosomal and other growth genes to promote entry into the cell division cycle.

Molecular mechanisms associated with breast cancer based on integrated gene expression profiling by bioinformatics analysis.

In this study, we aimed to gain more insights into the underlying molecular mechanisms responsible for breast cancer (BC) progression. Three gene expression profiles of human BC were integrated and used to screen the differentially expressed genes (DEGs) between healthy breast samples and BC samples. Protein-protein interaction (PPI) network of DEGs was constructed by mapping DEGs into the Search Tool for the Retrieval of Interacting Genes (STRING) database; then the subnetworks of PPI were constructed with plug-in, MCODE and DEGs in Subnetwork 1 were analysed based on Kyoto Encyclopaedia of Genes and Genomes (KEGG) pathway database ( http://www.genome.jp/kegg /). In addition, co-expression network of DEGs was established using the Cytoscape. Totalally 931 DEGs were selected, including 340 up-regulated genes and 591 down-regulated genes. KEGG pathway analysis for DEGs in Subnetwork 1 showed that the pathogenesis of BC was associated with cell cycle, oocyte meiosis, progesterone-mediated oocyte maturation and p53 signalling pathways. Meanwhile, the most significant-related DEGs were found by co-expression network analysis of DEGs. In conclusion, CCNG1 might be involved in the progression of BC via inhibiting cell proliferation, and ADAMTS1 might play a crucial role in BC development through the regulation of angiogenesis.

Methylation-associated Has-miR-9 deregulation in paclitaxel- resistant epithelial ovarian carcinoma.

BACKGROUND: Drug resistance is still one of the key causes of death in epithelial ovarian carcinoma (EOC) patients, however there are very few strategies to reverse chemoresistance. Here we try to clarify whether and how miR-9 takes part in the regulation of paclitaxel sensitivity. METHODS: miR-9 expressions in EOC cells and tissues were detected by Realtime PCR. The target of miR-9 was validated through dual luciferase reporter assay and Western Blot. Methylation study, RNAi technique and cytotoxicity assay were used to determine the intrinsic mechanism of miR-9 in paclitaxel sensitivity regulation. RESULTS: miR-9 is down-regulated in paclitaxel resistant EOC. The patients with lower miR-9, Grade 3, Stage III -IV and suboptimal surgery present shorter survival time. miR-9 and suboptimal surgery are independent prognostic factors of EOC. Modulating miR-9 expression could change paclitaxel sensitivity of EOC cells. CCNG1, validated as a direct target of miR-9, mediates paclitaxel resistance. miR-9-1 and 3 gene hypermethylation would decrease miR-9 expression, while demethylation of miR-9 gene could restore miR-9 expression and improve paclitaxel sensitivity in chemoresistance EOC cells. Furthermore, methylation-associated miR-9 deregulation in EOC cells could be induced by paclitaxel exposure. CONCLUSIONS: Methylation-associated miR-9 down-regulation is probably one of the key mechanisms for paclitaxel resistance in EOC cells, via targeting CCNG1. Our findings may also provide a new potential therapeutic target to reverse paclitaxel resistance in EOC patients.

Oleanolic acid suppresses the proliferation of lung carcinoma cells by miR-122/Cyclin G1/MEF2D axis.

Oleanolic acid (OA) is a natural compound from plants with anti-tumor activities. However, the mechanism of the inhibitory effect of OA on cell cycle progression has not been completely explored. We employed several lung carcinoma cell lines to investigate the cell cycle-related molecular pathway affected by OA. The data revealed that OA suppressed the proliferation of lung cancer cells in both dose- and time-dependent manners, along with an increase in miR-122 abundance. The suppression of miR-122 abolished the effect of OA on lung cancer cells. CCNG1 and MEF2D, two putative miR-122 targets, were found to be downregulated by OA treatment. Restoring their expression counteracted the effect of OA on lung carcinoma cells. OA was further shown to induce the expression of miR-122-regulating transcriptional factors in lung cancer cells. Collectively, OA induced cell cycle arrest in lung cancer cells through miR-122/Cyclin G1/MEF2D pathway. This finding may contribute to the understanding of the molecular mechanism of OA's anti-tumor activity.

MiR-1271 Inhibits Ovarian Cancer Growth by Targeting Cyclin G1.

BACKGROUND: Ovarian cancer is the most lethal gynecological malignant cancer in the female genital system. The dysfunction of miRNA contributes to ovarian cancer development. MATERIAL AND METHODS: The miR-1271 level in ovarian cancer tissues and cells was assayed by qRT-PCR. The miR-1271 expression in cells was overexpressed by miRNA-mimic transfection and reduced by miRNA-antisense-oligonucleotide (ASO) transfection. Cell proliferation was analyzed by an MTT assay. The targeted genes were predicted by a bioinformatics algorithm and confirmed by a dual luciferase reporter assay. The protein level was assayed by Western blotting. RESULTS: The ovarian cancer tissue and cell lines showed low levels of miR-1271. Low levels of miR-1271 in ovarian cancer tissues were correlated with a low rate of patient survival, and the overexpression of miR-1271 inhibited the proliferation of ovarian cancer cells. The 3' UTR of cyclin G1 (CCNG1) was targeted by miR-1271. CONCLUSIONS: Low levels of miR-1271 in ovarian cancer tissues promoted cancer cell growth. MiR-1271 may be a new predictor of prognosis in ovarian cancer. MiR-1271 exerted its role by targeting CCNG1.

[Molecular cloning and tissue expression of the CCNG1 gene in sheep].

The CCNG1 gene encodes cyclin G1, which is an important cell cycle regulator and has been reported to be involved in reproductive biological processes, such as oocyte maturation and granule cell proliferation in mammals. But the study of CCNG1 in sheep has been rarely reported. To examine the effects of CCNG1 on estrous control and seasonal breeding in sheep, we first cloned and characterized the expression level of the sheep CCNG1 gene. Then by Real-time PCR, we detected and analyzed the expressions of CCNG1 gene at mRNA levels in the hypothalamus-pituitary-ovary (HPO) axis in different stages of an estrous cycle in Duo Lang sheep (non-seasonal breeding) and Merino sheep (seasonal breeding). The results showed that the open reading frame of the sheep CCNG1 gene is 885 bp in length and encodes 294 amino acids. Bioinformatic analysis indicated that the secondary structure of the sheep CCNG1 protein contained multiple phosphorylation sites and some Protein Kinase C phosphorylation sites. CCNG1 mRNA was identified in all tissues tested, with the levels in ovary and kidney higher than others. The expression profiles of CCNG1 in the HPO axis in different stages of an estrous cycle were similar in different sheep breeds: the expression levels of CCNG1 in the ovary, uterus, pineal gland and pituitary gland all peaked in the estrus phase. But there were significant differences for expression change extent of CCNG1 in ovaries in the oestrus and metestrus phase between different sheep breeds. The results suggested that CCNG1 probably participated in the regulation of estrous behavior and seasonal reproduction through controling the growth and development of follicles in sheep.

Defective in mitotic arrest 1 (Dma1) ubiquitin ligase controls G1 cyclin degradation.

Progression through the G(1) phase of the cell cycle is controlled by diverse cyclin-dependent kinases (CDKs) that might be associated to numerous cyclin isoforms. Given such complexity, regulation of cyclin degradation should be crucial for coordinating progression through the cell cycle. In Saccharomyces cerevisiae, SCF is the only E3 ligase known to date to be involved in G(1) cyclin degradation. Here, we report the design of a genetic screening that uncovered Dma1 as another E3 ligase that targets G(1) cyclins in yeast. We show that the cyclin Pcl1 is ubiquitinated in vitro and in vivo by Dma1, and accordingly, is stabilized in dma1 mutants. We demonstrate that Pcl1 must be phosphorylated by its own CDK to efficiently interact with Dma1 and undergo degradation. A nonphosphorylatable version of Pcl1 accumulates throughout the cell cycle, demonstrating the physiological relevance of the proposed mechanism. Finally, we present evidence that the levels of Pcl1 and Cln2 are independently controlled in response to nutrient availability. This new previously unknown mechanism for G(1) cyclin degradation that we report here could help elucidate the specific roles of the redundant CDK-cyclin complexes in G(1).