Interaction of Gcn4 with target gene chromatin is modulated by proteasome function

The Sts1 nuclear import adapter uses a non-canonical bipartite nuclear localization signal and is directly degraded by the proteasome

The proteasome is an essential regulator of protein homeostasis. In yeast and many mammalian cells, proteasomes strongly concentrate in the nucleus. Sts1 from the yeast Saccharomyces cerevisiae is an essential protein linked to proteasome nuclear localization. Here, we show that Sts1 contains a non-canonical bipartite nuclear localization signal (NLS) important for both nuclear localization of Sts1 itself and the proteasome. Sts1 binds the karyopherin-α import receptor (Srp1) stoichiometrically, and this requires the NLS. The NLS is essential for viability, and over-expressed Sts1 with an inactive NLS interferes with 26S proteasome import. The Sts1-Srp1 complex binds preferentially to fully assembled 26S proteasomes in vitro Sts1 is itself a rapidly degraded 26S proteasome substrate; notably, this degradation is ubiquitin independent in cells and in vitro and is inhibited by Srp1 binding. Mutants of Sts1 are stabilized, suggesting that its degradation is tightly linked to its role in localizing proteasomes to the nucleus. We propose that Sts1 normally promotes nuclear import of fully assembled proteasomes and is directly degraded by proteasomes without prior ubiquitylation following karyopherin-α release in the nucleus.

Ubiquitin-proteasome system modulates zygotic genome activation in early mouse embryos and influences full-term development

Maternal RNA/protein degradation and zygotic genome activation (ZGA), occurring during maternal-to-zygotic transition (MZT), are the first essential events for the development of pre-implantation embryos. Previously, we have shown the importance of the ubiquitin-proteasome system (UPS) for initiation of minor ZGA at the 1-cell stage of mouse embryos. However, little is known about the mechanism of involvement of the UPS-degraded maternal proteins in ZGA. In this study, we investigated the effect of inhibiting maternal protein degradation by the reversible proteasome inhibitor, MG132, on post-implantation development and ZGA regulation during early cleavage stages. Our study revealed that zygotic transcription by RNA polymerase II (Pol II) at the 1-cell stage was delayed and the full-term development was affected by transient proteasome inhibition during 1 to 9 h post-insemination (hpi). Furthermore, we found that the transient inhibition of proteasome activity at the 2-cell stage delayed the onset of transcription of some major ZGA genes. These results support the model hypothesizing the requirement of sequential degradation of maternal proteins by UPS for the proper onset of ZGA and normal progression of MZT in early mouse embryos.

MYC Modulation around the CDK2/p27/SKP2 Axis

MYC is a pleiotropic transcription factor that controls a number of fundamental cellular processes required for the proliferation and survival of normal and malignant cells, including the cell cycle. MYC interacts with several central cell cycle regulators that control the balance between cell cycle progression and temporary or permanent cell cycle arrest (cellular senescence). Among these are the cyclin E/A/cyclin-dependent kinase 2 (CDK2) complexes, the CDK inhibitor p27^KIP1 (p27) and the E3 ubiquitin ligase component S-phase kinase-associated protein 2 (SKP2), which control each other by forming a triangular network. MYC is engaged in bidirectional crosstalk with each of these players; while MYC regulates their expression and/or activity, these factors in turn modulate MYC through protein interactions and post-translational modifications including phosphorylation and ubiquitylation, impacting on MYC's transcriptional output on genes involved in cell cycle progression and senescence. Here we elaborate on these network interactions with MYC and their impact on transcription, cell cycle, replication and stress signaling, and on the role of other players interconnected to this network, such as CDK1, the retinoblastoma protein (pRB), protein phosphatase 2A (PP2A), the F-box proteins FBXW7 and FBXO28, the RAS oncoprotein and the ubiquitin/proteasome system. Finally, we describe how the MYC/CDK2/p27/SKP2 axis impacts on tumor development and discuss possible ways to interfere therapeutically with this system to improve cancer treatment.

Identification of Genes in Saccharomyces cerevisiae that Are Haploinsufficient for Overcoming Amino Acid Starvation

The yeast Saccharomyces cerevisiae responds to amino acid deprivation by activating a pathway conserved in eukaryotes to overcome the starvation stress. We have screened the entire yeast heterozygous deletion collection to identify strains haploinsufficient for growth in the presence of sulfometuron methyl, which causes starvation for isoleucine and valine. We have discovered that cells devoid of MET15 are sensitive to sulfometuron methyl, and loss of heterozygosity at the MET15 locus can complicate screening the heterozygous deletion collection. We identified 138 cases of loss of heterozygosity in this screen. After eliminating the issues of the MET15 loss of heterozygosity, strains isolated from the collection were retested on sulfometuron methyl. To determine the general effect of the mutations for a starvation response, SMM-sensitive strains were tested for the ability to grow in the presence of canavanine, which induces arginine starvation, and strains that were MET15 were also tested for growth in the presence of ethionine, which causes methionine starvation. Many of the genes identified in our study were not previously identified as starvation-responsive genes, including a number of essential genes that are not easily screened in a systematic way. The genes identified span a broad range of biological functions, including many involved in some level of gene expression. Several unnamed proteins have also been identified, giving a clue as to possible functions of the encoded proteins.