Ubiquitination-dependent control of sexual differentiation in fission yeast

A bifunctional snoRNA with separable activities in guiding rRNA 2'-O-methylation and scaffolding gametogenesis effectors

Small nucleolar RNAs are non-coding transcripts that guide chemical modifications of RNA substrates and modulate gene expression at the epigenetic and post-transcriptional levels. However, the extent of their regulatory potential and the underlying molecular mechanisms remain poorly understood. Here, we identify a conserved, previously unannotated intronic C/D-box snoRNA, termed snR107, hosted in the fission yeast long non-coding RNA mamRNA and carrying two independent cellular functions. On the one hand, snR107 guides site-specific 25S rRNA 2'-O-methylation and promotes pre-rRNA processing and 60S subunit biogenesis. On the other hand, snR107 associates with the gametogenic RNA-binding proteins Mmi1 and Mei2, mediating their reciprocal inhibition and restricting meiotic gene expression during sexual differentiation. Both functions require distinct cis-motifs within snR107, including a conserved 2'-O-methylation guiding sequence. Together, our results position snR107 as a dual regulator of rRNA modification and gametogenesis effectors, expanding our vision on the non-canonical functions exerted by snoRNAs in cell fate decisions.

Regulation of sexual differentiation initiation in Schizosaccharomyces pombe

The fission yeast Schizosaccharomyces pombe is an excellent model organism to explore cellular events owing to rich tools in genetics, molecular biology, cellular biology, and biochemistry. Schizosaccharomyces pombe proliferates continuously when nutrients are abundant but arrests in G1 phase upon depletion of nutrients such as nitrogen and glucose. When cells of opposite mating types are present, cells conjugate, fuse, undergo meiosis, and finally form 4 spores. This sexual differentiation process in S. pombe has been studied extensively. To execute sexual differentiation, the glucose-sensing cAMP-PKA (cyclic adenosine monophosphate-protein kinase A) pathway, nitrogen-sensing TOR (target of rapamycin) pathway, and SAPK (stress-activating protein kinase) pathway are crucial, and the MAPK (mitogen-activating protein kinase) cascade is essential for pheromone sensing. These signals regulate ste11 at the transcriptional and translational levels, and Ste11 is modified in multiple ways. This review summarizes the initiation of sexual differentiation in S. pombe based on results I have helped to obtain, including the work of many excellent researchers.

Thirty Years with ERH: An mRNA Splicing and Mitosis Factor Only or Rather a Novel Genome Integrity Protector?

ERH is a 100 to about 110 aa nuclear protein with unique primary and three-dimensional structures that are very conserved from simple eukaryotes to humans, albeit some species have lost its gene, with most higher fungi being a noteworthy example. Initially, studies on Drosophila melanogaster implied its function in pyrimidine metabolism. Subsequently, research on Xenopus laevis suggested that it acts as a transcriptional repressor. Finally, studies in humans pointed to a role in pre-mRNA splicing and in mitosis but further research, also in Caenorhabditis elegans and Schizosaccharomyces pombe, demonstrated its much broader activity, namely involvement in the biogenesis of mRNA, and miRNA, piRNA and some other ncRNAs, and in repressive heterochromatin formation. ERH interacts with numerous, mostly taxon-specific proteins, like Mmi1 and Mei2 in S. pombe, PID-3/PICS-1, TOST-1 and PID-1 in C. elegans, and DGCR8, CIZ1, PDIP46/SKAR and SAFB1/2 in humans. There are, however, some common themes in this wide range of processes and partners, such as: (a) ERH homodimerizes to form a scaffold for several complexes involved in the metabolism of nucleic acids, (b) all these RNAs are RNA polymerase II transcripts, (c) pre-mRNAs, whose splicing depends on ERH, are enriched in transcripts of DNA damage response and DNA metabolism genes, and (d) heterochromatin is formed to silence unwanted transcription, e.g., from repetitive elements. Thus, it seems that ERH has been adopted for various pathways that serve to maintain genome integrity.

A dual, catalytic role for the fission yeast Ccr4-Not complex in gene silencing and heterochromatin spreading

Heterochromatic gene silencing relies on combinatorial control by specific histone modifications, the occurrence of transcription, and/or RNA degradation. Once nucleated, heterochromatin propagates within defined chromosomal regions and is maintained throughout cell divisions to warrant proper genome expression and integrity. In the fission yeast Schizosaccharomyces pombe, the Ccr4-Not complex partakes in gene silencing, but its relative contribution to distinct heterochromatin domains and its role in nucleation versus spreading have remained elusive. Here, we unveil major functions for Ccr4-Not in silencing and heterochromatin spreading at the mating type locus and subtelomeres. Mutations of the catalytic subunits Caf1 or Mot2, involved in RNA deadenylation and protein ubiquitinylation, respectively, result in impaired propagation of H3K9me3 and massive accumulation of nucleation-distal heterochromatic transcripts. Both silencing and spreading defects are suppressed upon disruption of the heterochromatin antagonizing factor Epe1. Overall, our results position the Ccr4-Not complex as a critical, dual regulator of heterochromatic gene silencing and spreading.

Long Non-Coding RNAs in the Control of Gametogenesis: Lessons from Fission Yeast

Long non-coding RNAs (lncRNAs) contribute to cell fate decisions by modulating genome expression and stability. In the fission yeast Schizosaccharomyces pombe, the transition from mitosis to meiosis results in a marked remodeling of gene expression profiles, which ultimately ensures gamete production and inheritance of genetic information to the offspring. This key developmental process involves a set of dedicated lncRNAs that shape cell cycle-dependent transcriptomes through a variety of mechanisms, including epigenetic modifications and the modulation of transcription, post-transcriptional and post-translational regulations, and that contribute to meiosis-specific chromosomal events. In this review, we summarize the biology of these lncRNAs, from their identification to mechanism of action, and discuss their regulatory role in the control of gametogenesis.

The CNOT4 Subunit of the CCR4-NOT Complex is Involved in mRNA Degradation, Efficient DNA Damage Repair, and XY Chromosome Crossover during Male Germ Cell Meiosis

The CCR4-NOT complex is a major mRNA deadenylase in eukaryotes, comprising the catalytic subunits CNOT6/6L and CNOT7/8, as well as CNOT4, a regulatory subunit with previously undetermined functions. These subunits have been hypothesized to play synergistic biochemical functions during development. Cnot7 knockout male mice have been reported to be infertile. In this study, viable Cnot6/6l double knockout mice are constructed, and the males are fertile. These results indicate that CNOT7 has CNOT6/6L-independent functions in vivo. It is also demonstrated that CNOT4 is required for post-implantation embryo development and meiosis progression during spermatogenesis. Conditional knockout of Cnot4 in male germ cells leads to defective DNA damage repair and homologous crossover between X and Y chromosomes. CNOT4 functions as a previously unrecognized mRNA adaptor of CCR4-NOT by targeting mRNAs to CNOT7 for deadenylation of poly(A) tails, thereby mediating the degradation of a subset of transcripts from the zygotene to pachytene stage. The mRNA removal promoted by the CNOT4-regulated CCR4-NOT complex during the zygotene-to-pachytene transition is crucial for the appropriate expression of genes involved in the subsequent events of spermatogenesis, normal DNA double-strand break repair during meiosis, efficient crossover between X and Y chromosomes, and ultimately, male fertility.

A scaffold lncRNA shapes the mitosis to meiosis switch

Long non-coding RNAs (lncRNAs) contribute to the regulation of gene expression in response to intra- or extracellular signals but the underlying molecular mechanisms remain largely unexplored. Here, we identify an uncharacterized lncRNA as a central player in shaping the meiotic gene expression program in fission yeast. We report that this regulatory RNA, termed mamRNA, scaffolds the antagonistic RNA-binding proteins Mmi1 and Mei2 to ensure their reciprocal inhibition and fine tune meiotic mRNA degradation during mitotic growth. Mechanistically, mamRNA allows Mmi1 to target Mei2 for ubiquitin-mediated downregulation, and conversely enables accumulating Mei2 to impede Mmi1 activity, thereby reinforcing the mitosis to meiosis switch. These regulations also occur within a unique Mmi1-containing nuclear body, positioning mamRNA as a spatially-confined sensor of Mei2 levels. Our results thus provide a mechanistic basis for the mutual control of gametogenesis effectors and further expand our vision of the regulatory potential of lncRNAs.

The Regulatory Properties of the Ccr4-Not Complex

The mammalian Ccr4–Not complex, carbon catabolite repression 4 (Ccr4)-negative on TATA-less (Not), is a large, highly conserved, multifunctional assembly of proteins that acts at different cellular levels to regulate gene expression. In the nucleus, it is involved in the regulation of the cell cycle, chromatin modification, activation and inhibition of transcription initiation, control of transcription elongation, RNA export, nuclear RNA surveillance, and DNA damage repair. In the cytoplasm, the Ccr4–Not complex plays a central role in mRNA decay and affects protein quality control. Most of our original knowledge of the Ccr4–Not complex is derived, primarily, from studies in yeast. More recent studies have shown that the mammalian complex has a comparable structure and similar properties. In this review, we summarize the evidence for the multiple roles of both the yeast and mammalian Ccr4–Not complexes, highlighting their similarities.

Comprehensive Proteomic Analysis of Lysine Ubiquitination in Seedling Leaves of Nicotiana tabacum

Lysine ubiquitination, a widely studied posttranslational modification, plays vital roles in various biological processes in eukaryotic cells. Although several studies have examined the plant ubiquitylome, no such research has been performed in tobacco, a model plant for molecular biology. Here, we comprehensively analyzed lysine ubiquitination in tobacco (Nicotiana tabacum) using LC-MS/MS along with highly sensitive immune-affinity purification. In total, 964 lysine-ubiquitinated (K^ub) sites were identified in 572 proteins. Extensive bioinformatics studies revealed the distribution of these proteins in various cellular locations, including the cytoplasm, chloroplast, nucleus, and plasma membrane. Notably, 25% of the K^ub proteins were located in the chloroplast of which 21 were enzymatically involved in important pathways, that is, photosynthesis and carbon fixation. Western blot analysis indicated that TMV infection can cause changes in ubiquitination levels. This is the first comprehensive proteomic analysis of lysine ubiquitination in tobacco, illustrating the vital role of ubiquitination in various physiological and biochemical processes and representing a valuable addition to the existing landscape of lysine ubiquitination.

Formation of S. pombe Erh1 homodimer mediates gametogenic gene silencing and meiosis progression

Timely and accurate expression of the genetic information relies on the integration of environmental cues and the activation of regulatory networks involving transcriptional and post-transcriptional mechanisms. In fission yeast, meiosis-specific transcripts are selectively targeted for degradation during mitosis by the EMC complex, composed of Erh1, the ortholog of human ERH, and the YTH family RNA-binding protein Mmi1. Here, we present the crystal structure of Erh1 and show that it assembles as a homodimer. Mutations of amino acid residues to disrupt Erh1 homodimer formation result in loss-of-function phenotypes, similar to erh1∆ cells: expression of meiotic genes is derepressed in mitotic cells and meiosis progression is severely compromised. Interestingly, formation of Erh1 homodimer is dispensable for interaction with Mmi1, suggesting that only fully assembled EMC complexes consisting of two Mmi1 molecules bridged by an Erh1 dimer are functionally competent. We also show that Erh1 does not contribute to Mmi1-dependent down-regulation of the meiosis regulator Mei2, supporting the notion that Mmi1 performs additional functions beyond EMC. Overall, our results provide a structural basis for the assembly of the EMC complex and highlight its biological relevance in gametogenic gene silencing and meiosis progression.

meiRNA, A Polyvalent Player in Fission Yeast Meiosis

A growing number of recent studies have revealed that non-coding RNAs play a wide variety of roles beyond expectation. A lot of non-coding RNAs have been shown to function by forming intracellular structures either in the nucleus or the cytoplasm. In the fission yeast Schizosaccharomyces pombe, a non-coding RNA termed meiRNA has been shown to play multiple vital roles in the course of meiosis. meiRNA is tethered to its genetic locus after transcription and forms a peculiar intranuclear dot structure. It ensures stable expression of meiotic genes in cooperation with an RNA-binding protein Mei2. Chromosome-associated meiRNA also facilitates recognition of homologous chromosome loci and induces robust pairing. In this review, the quarter-century history of meiRNA, from its identification to functional characterization, will be outlined.

A conserved CAF40-binding motif in metazoan NOT4 mediates association with the CCR4-NOT complex

The multisubunit CCR4-NOT mRNA deadenylase complex plays important roles in the posttranscriptional regulation of gene expression. The NOT4 E3 ubiquitin ligase is a stable component of the CCR4-NOT complex in yeast but does not copurify with the human or Drosophila melanogaster complex. Here we show that the C-terminal regions of human and D. melanogaster NOT4 contain a conserved sequence motif that directly binds the CAF40 subunit of the CCR4-NOT complex (CAF40-binding motif [CBM]). In addition, nonconserved sequences flanking the CBM also contact other subunits of the complex. Crystal structures of the CBM-CAF40 complex reveal a mutually exclusive binding surface for NOT4 and Roquin or Bag of marbles mRNA regulatory proteins. Furthermore, CAF40 depletion or structure-guided mutagenesis to disrupt the NOT4-CAF40 interaction impairs the ability of NOT4 to elicit decay of tethered reporter mRNAs in cells. Together with additional sequence analyses, our results reveal the molecular basis for the association of metazoan NOT4 with the CCR4-NOT complex and show that it deviates substantially from yeast. They mark the NOT4 ubiquitin ligase as an ancient but nonconstitutive cofactor of the CCR4-NOT deadenylase with potential recruitment and/or effector functions.

m⁶A mRNA Destiny: Chained to the rhYTHm by the YTH-Containing Proteins

The control of gene expression is a multi-layered process occurring at the level of DNA, RNA, and proteins. With the emergence of highly sensitive techniques, new aspects of RNA regulation have been uncovered leading to the emerging field of epitranscriptomics dealing with RNA modifications. Among those post-transcriptional modifications, N6-methyladenosine (m⁶A) is the most prevalent in messenger RNAs (mRNAs). This mark can either prevent or stimulate the formation of RNA-protein complexes, thereby influencing mRNA-related mechanisms and cellular processes. This review focuses on proteins containing a YTH domain (for YT521-B Homology), a small building block, that selectively detects the m⁶A nucleotide embedded within a consensus motif. Thereby, it contributes to the recruitment of various effectors involved in the control of mRNA fates through adjacent regions present in the different YTH-containing proteins.

mmi1 and rep2 mRNAs are novel RNA targets of the Mei2 RNA-binding protein during early meiosis in Schizosaccharomyces pombe

The RNA-binding protein Mei2 is crucial for meiosis in Schizosaccharomyces pombe. In mei2 mutants, pre-meiotic S-phase is blocked, along with meiosis. Mei2 binds a long non-coding RNA (lncRNA) called meiRNA, which is a 'sponge RNA' for the meiotic inhibitor protein Mmi1. The interaction between Mei2, meiRNA and Mmi1 protein is essential for meiosis. But mei2 mutants have stronger and different phenotypes than meiRNA mutants, since mei2Δ arrests before pre-meiotic S, while the meiRNA mutant arrests after pre-meiotic S but before meiosis. This suggests Mei2 may bind additional RNAs. To identify novel RNA targets of Mei2, which might explain how Mei2 regulates pre-meiotic S, we used RNA immunoprecipitation and cross-linking immunoprecipitation. In addition to meiRNA, we found the mRNAs for mmi1 (which encodes Mmi1) and for the S-phase transcription factor rep2 There were also three other RNAs of uncertain relevance. We suggest that at meiotic initiation, Mei2 may sequester rep2 mRNA to help allow pre-meiotic S, and then may bind both meiRNA and mmi1 mRNA to inactivate Mmi1 at two levels, the protein level (as previously known), and also the mRNA level, allowing meiosis. We call Mei2-meiRNA a 'double sponge' (i.e. binding both an mRNA and its encoded protein).