Cell cycle-dependent and independent mating blocks ensure fungal zygote survival and ploidy maintenance.

To ensure genome stability, sexually reproducing organisms require that mating brings together exactly 2 haploid gametes and that meiosis occurs only in diploid zygotes. In the fission yeast Schizosaccharomyces pombe, fertilization triggers the Mei3-Pat1-Mei2 signaling cascade, which represses subsequent mating and initiates meiosis. Here, we establish a degron system to specifically degrade proteins postfusion and demonstrate that mating blocks not only safeguard zygote ploidy but also prevent lysis caused by aberrant fusion attempts. Using long-term imaging and flow-cytometry approaches, we identify previously unrecognized and independent roles for Mei3 and Mei2 in zygotes. We show that Mei3 promotes premeiotic S-phase independently of Mei2 and that cell cycle progression is both necessary and sufficient to reduce zygotic mating behaviors. Mei2 not only imposes the meiotic program and promotes the meiotic cycle, but also blocks mating behaviors independently of Mei3 and cell cycle progression. Thus, we find that fungi preserve zygote ploidy and survival by at least 2 mechanisms where the zygotic fate imposed by Mei2 and the cell cycle reentry triggered by Mei3 synergize to prevent zygotic mating.

Gamete fusion triggers bipartite transcription factor assembly to block re-fertilization.

The ploidy cycle, which is integral to sexual reproduction, requires meiosis to halve chromosome numbers as well as mechanisms that ensure zygotes are formed by exactly two partners1-4. During sexual reproduction of the fungal model organism Schizosaccharomyces pombe, haploid P and M cells fuse to form a diploid zygote that immediately enters meiosis5. Here we reveal that rapid post-fusion reconstitution of a bipartite transcription factor blocks re-fertilization. We first identify mutants that undergo transient cell fusion involving cytosol exchange but not karyogamy, and show that this drives distinct cell fates in the two gametes. The P partner undergoes lethal haploid meiosis, whereas the M cell persists in mating. The zygotic transcription that drives meiosis is rapidly initiated first from the P parental genome, even in wild-type cells. This asymmetric gene expression depends on a bipartite complex formed post-fusion between the cytosolic M-cell-specific peptide Mi and the nuclear P-cell-specific homeobox protein Pi6,7, which captures Mi in the P nucleus. Zygotic transcription is thus poised to initiate in the P nucleus as fast as Mi reaches it after fusion, a design that we reconstruct using two synthetic interactors localized to the nucleus and the cytosol of two respective partner cells. Notably, delaying zygotic transcription-by postponing Mi expression or deleting its transcriptional target in the P genome-leads to zygotes fusing with additional gametes, thus forming polyploids and eventually aneuploid progeny. The signalling cascade to block re-fertilization shares components with, but bifurcates from, meiotic induction8-10. Thus, a cytoplasmic connection upon gamete fusion leads to asymmetric reconstitution of a bipartite transcription factor to rapidly block re-fertilization and induce meiosis, ensuring genome maintenance during sexual reproduction.

MINDY-1 Is a Member of an Evolutionarily Conserved and Structurally Distinct New Family of Deubiquitinating Enzymes.

Deubiquitinating enzymes (DUBs) remove ubiquitin (Ub) from Ub-conjugated substrates to regulate the functional outcome of ubiquitylation. Here we report the discovery of a new family of DUBs, which we have named MINDY (motif interacting with Ub-containing novel DUB family). Found in all eukaryotes, MINDY-family DUBs are highly selective at cleaving K48-linked polyUb, a signal that targets proteins for degradation. We identify the catalytic activity to be encoded within a previously unannotated domain, the crystal structure of which reveals a distinct protein fold with no homology to any of the known DUBs. The crystal structure of MINDY-1 (also known as FAM63A) in complex with propargylated Ub reveals conformational changes that realign the active site for catalysis. MINDY-1 prefers cleaving long polyUb chains and works by trimming chains from the distal end. Collectively, our results reveal a new family of DUBs that may have specialized roles in regulating proteostasis.

A toolbox of stable integration vectors in the fission yeast Schizosaccharomyces pombe.

Schizosaccharomyces pombe is a widely used model organism to study many aspects of eukaryotic cell physiology. Its popularity as an experimental system partially stems from the ease of genetic manipulations, where the innate homology-targeted repair is exploited to precisely edit the genome. While vectors to incorporate exogenous sequences into the chromosomes are available, most are poorly characterized. Here, we show that commonly used fission yeast vectors, which upon integration produce repetitive genomic regions, give rise to unstable genomic loci. We overcome this problem by designing a new series of stable integration vectors (SIVs) that target four different prototrophy genes. SIVs produce non-repetitive, stable genomic loci and integrate predominantly as single copy. Additionally, we develop a set of complementary auxotrophic alleles that preclude false-positive integration events. We expand the vector series to include antibiotic resistance markers, promoters, fluorescent tags and terminators, and build a highly modular toolbox to introduce heterologous sequences. Finally, as proof of concept, we generate a large set of ready-to-use, fluorescent probes to mark organelles and cellular processes with a wide range of applications in fission yeast research.This article has an associated First Person interview with the first author of the paper.

The Mitotic Exit Network and Cdc14 phosphatase initiate cytokinesis by counteracting CDK phosphorylations and blocking polarised growth.

Polarisation of the actin cytoskeleton must cease during cytokinesis, to support efficient assembly and contraction of the actomyosin ring at the site of cell division, but the underlying mechanisms are still understood poorly in most species. In budding yeast, the Mitotic Exit Network (MEN) releases Cdc14 phosphatase from the nucleolus during anaphase, leading to the inactivation of mitotic forms of cyclin-dependent kinase (CDK) and the onset of septation, before G1-CDK can be reactivated and drive re-polarisation of the actin cytoskeleton to a new bud. Here, we show that premature inactivation of mitotic CDK, before release of Cdc14, allows G1-CDK to divert the actin cytoskeleton away from the actomyosin ring to a new site of polarised growth, thereby delaying progression through cytokinesis. Our data indicate that cells normally avoid this problem via the MEN-dependent release of Cdc14, which counteracts all classes of CDK-mediated phosphorylations during cytokinesis and blocks polarised growth. The dephosphorylation of CDK targets is therefore central to the mechanism by which the MEN and Cdc14 initiate cytokinesis and block polarised growth during late mitosis.

Tethering of SCF(Dia2) to the Replisome Promotes Efficient Ubiquitylation and Disassembly of the CMG Helicase.

Disassembly of the Cdc45-MCM-GINS (CMG) DNA helicase, which unwinds the parental DNA duplex at eukaryotic replication forks, is the key regulated step during replication termination but is poorly understood. In budding yeast, the F-box protein Dia2 drives ubiquitylation of the CMG helicase at the end of replication, leading to a disassembly pathway that requires the Cdc48 segregase. The substrate-binding domain of Dia2 comprises leucine-rich repeats, but Dia2 also has a TPR domain at its amino terminus that interacts with the Ctf4 and Mrc1 subunits of the replisome progression complex, which assembles around the CMG helicase at replication forks. Previous studies suggested two disparate roles for the TPR domain of Dia2, either mediating replisome-specific degradation of Mrc1 and Ctf4 or else tethering SCF(Dia2) (SCF [Skp1/cullin/F-box protein]) to the replisome to increase its local concentration at replication forks. Here, we show that SCF(Dia2) does not mediate replisome-specific degradation of Mrc1 and Ctf4, either during normal S phase or in response to replication stress. Instead, the tethering of SCF(Dia2) to the replisome progression complex increases the efficiency of ubiquitylation of the Mcm7 subunit of CMG, both in vitro and in vivo. Correspondingly, loss of tethering reduces the efficiency of CMG disassembly in vivo and is synthetic lethal in combination with a disassembly-defective allele of CDC48. Residual ubiquitylation of Mcm7 in dia2-ΔTPR cells is still CMG specific, highlighting the complex regulation of the final stages of chromosome replication, about which much still remains to be learned.

Hof1 and Rvs167 have redundant roles in actomyosin ring function during cytokinesis in budding yeast.

The Hof1 protein (Homologue of Fifteen) regulates formation of the primary septum during cytokinesis in the budding yeast Saccharomyces cerevisiae, whereas the orthologous Cdc15 protein in fission yeast regulates the actomyosin ring by using its F-BAR domain to recruit actin nucleators to the cleavage site. Here we show that budding yeast Hof1 also contributes to actin ring assembly in parallel with the Rvs167 protein. Simultaneous deletion of the HOF1 and RVS167 genes is lethal, and cells fail to assemble the actomyosin ring as they progress through mitosis. Although Hof1 and Rvs167 are not orthologues, they both share an analogous structure, with an F-BAR or BAR domain at the amino terminus, capable of inducing membrane curvature, and SH3 domains at the carboxyl terminus that bind to specific proline-rich targets. The SH3 domain of Rvs167 becomes essential for assembly of the actomyosin ring in cells lacking Hof1, suggesting that it helps to recruit a regulator of the actin cytoskeleton. This new function of Rvs167 appears to be independent of its known role as a regulator of the Arp2/3 actin nucleator, as actin ring assembly is not abolished by the simultaneous inactivation of Hof1 and Arp2/3. Instead we find that recruitment to the bud-neck of the Iqg1 actin regulator is defective in cells lacking Hof1 and Rvs167, though future studies will be needed to determine if this reflects a direct interaction between these factors. The redundant role of Hof1 in actin ring assembly suggests that the mechanism of actin ring assembly has been conserved to a greater extent across evolution than anticipated previously.