The meiosis-specific Cdc20 family-member Ama1 promotes binding of the Ssp2 activator to the Smk1 MAP kinase

The MAPK homolog, Smk1, promotes assembly of the glucan layer of the spore wall in S. cerevisiae

Smk1 is a MAPK homolog in the yeast Saccharomyces cerevisiae that controls the postmeiotic program of spore wall assembly. During this program, haploid cells are surrounded by a layer of mannan and then a layer of glucan. These inner layers of the spore wall resemble the vegetative cell wall. Next, the outer layers consisting of chitin/chitosan and then dityrosine are assembled. The outer layers are spore-specific and provide protection against environmental stressors. Smk1 is required for the proper assembly of spore walls. However, the protective properties of the outer layers have limited our understanding of how Smk1 controls this morphogenetic program. Mutants lacking the chitin deacetylases, Cda1 and Cda2, form spores that lack the outer layers of the spore wall. In this study, cda1,2∆ cells were used to demonstrate that Smk1 promotes deposition of the glucan layer of the spore wall through the partially redundant glucan synthases Gsc2 and Fks3. Although Gsc2 is localized to sites of spore wall assembly in the wild type, it is mislocalized in the mother cell cytoplasm in the smk1∆ mutant. These findings suggest that Smk1 controls assembly of the spore wall by regulating the localization of Gsc2 during sporogenesis.

Meiotic Cytokinesis in Saccharomyces cerevisiae: Spores That Just Need Closure

In the budding yeast Saccharomyces cerevisiae, sporulation occurs during starvation of a diploid cell and results in the formation of four haploid spores forming within the mother cell ascus. Meiosis divides the genetic material that is encapsulated by the prospore membrane that grows to surround the haploid nuclei; this membrane will eventually become the plasma membrane of the haploid spore. Cellularization of the spores occurs when the prospore membrane closes to capture the haploid nucleus along with some cytoplasmic material from the mother cell, and thus, closure of the prospore membrane is the meiotic cytokinetic event. This cytokinetic event involves the removal of the leading-edge protein complex, a complex of proteins that localizes to the leading edge of the growing prospore membrane. The development and closure of the prospore membrane must be coordinated with other meiotic exit events such as spindle disassembly. Timing of the closure of the prospore membrane depends on the meiotic exit pathway, which utilizes Cdc15, a Hippo-like kinase, and Sps1, an STE20 family GCKIII kinase, acting in parallel to the E3 ligase Ama1-APC/C. This review describes the sporulation process and focuses on the development of the prospore membrane and the regulation of prospore membrane closure.

Isc10, an inhibitor of the Smk1 MAPK, prevents activation-loop autophosphorylation and substrate phosphorylation through separate mechanisms

Many eukaryotic protein kinases are activated by the intramolecular autophosphorylation of activation loop residues. Smk1 is a meiosis-specific mitogen-activated protein kinase (MAPK) in yeast that autophosphorylates its activation loop tyrosine and thereby upregulates catalytic output. This reaction is controlled by an inhibitor, Isc10, that binds the MAPK during meiosis I and an activator, Ssp2, that binds Smk1/Isc10 during meiosis II. Upon completion of the meiotic divisions, Isc10 is degraded, and Smk1 undergoes autophosphorylation to generate the high activity form of the MAPK that controls spore formation. How Isc10 inhibits Smk1 is not clear. Here, we use a bacterial coexpression/reconstitution system to define a domain in the carboxy-terminal half of Isc10 that specifically inhibits Smk1 autophosphorylation. Nevertheless, Smk1 bound by this domain is able to phosphorylate other substrates, and it phosphorylates the amino-terminal half of Isc10 on serine 97. In turn, the phosphorylated motif in Isc10 inhibits the Smk1 active site. These data show that Isc10 inhibits autophosphorylation and the phosphorylation of substrates by separate mechanisms. Furthermore, we demonstrate Isc10 can inhibit the autophosphorylation of the mammalian intestinal cell kinase ICK1 (also known as CILK1), suggesting a conserved mechanism of action. These findings define a novel class of developmentally regulated molecules that prevent the self-activation of MAPKs and MAPK-like enzymes.

Distinct requirements for the COMPASS core subunits Set1, Swd1, and Swd3 during meiosis in the budding yeast Saccharomyces cerevisiae

Meiosis-specific chromatin structures, guided by histone modifications, are critical mediators of a meiotic transient transcription program and progression through prophase I. Histone H3K4 can be methylated up to three times by the Set1-containing COMPASS complex and each methylation mark corresponds to a different chromatin conformation. The level of H3K4 modification is directed by the activity of additional COMPASS components. In this study, we characterized the role of the COMPASS subunits during meiosis in Saccharomyces cerevisiae. In vegetative cells, previous studies revealed a role for subunits Swd2, Sdc1, and Bre2 for H3K4me2 while Spp1 supported trimethylation. However, we found that Bre2 and Sdc1 are required for H3K4me3 as yeast prepare to enter meiosis while Spp1 is not. Interestingly, we identified distinct meiotic functions for the core COMPASS complex members that required for all H3K4me, Set1, Swd1, and Swd3. While Set1 and Swd1 are required for progression through early meiosis, Swd3 is critical for late meiosis and spore morphogenesis. Furthermore, the meiotic requirement for Set1 is independent of H3K4 methylation, suggesting the presence of nonhistone substrates. Finally, checkpoint suppression analyses indicate that Set1 and Swd1 are required for both homologous recombination and chromosome segregation. These data suggest that COMPASS has important new roles for meiosis that are independent of its well-characterized functions during mitotic divisions.

Isc10, an Inhibitor That Links the Anaphase-Promoting Complex to a Meiosis-Specific Mitogen-Activated Protein Kinase

Smk1 is a meiosis-specific mitogen-activated protein kinase (MAPK) in yeast that controls spore differentiation. It is activated by a MAPK binding protein, Ssp2, upon completion of the meiotic divisions. The activation of Smk1 by Ssp2 is positively regulated by a meiosis-specific coactivator of the anaphase promoting complex (APC/C) E3 ubiquitin ligase, Ama1. Here, we identify Isc10 as an inhibitor that links APC/C^Ama1 to Smk1 activation. Isc10 and Smk1 form an inhibited complex during meiosis I (MI). Ssp2 is produced later in the program, and it forms a ternary complex with Isc10 and Smk1 during MII that is poised for activation. Upon completion of MII, Isc10 is ubiquitylated and degraded in an AMA1-dependent manner, thereby triggering the activation of Smk1 by Ssp2. Mutations that caused Ssp2 to be produced before MII, or isc10Δ mutations, modestly reduced the efficiency of spore differentiation whereas spores were nearly absent in the double mutant. These findings define a pathway that couples spore differentiation to the G₀-like phase of the cell cycle.

RNA Recognition-like Motifs Activate a Mitogen-Activated Protein Kinase

Smk1 is a mitogen-activated protein kinase (MAPK) family member in the yeast Saccharomyces cerevisiae that controls the postmeiotic program of spore formation. Ssp2 is a meiosis-specific protein that activates Smk1 and triggers the autophosphorylation of its activation loop. A fragment of Ssp2 that is sufficient to activate Smk1 contains two segments that resemble RNA recognition motifs (RRMs). Mutations in either of these motifs eliminated Ssp2's ability to activate Smk1. In contrast, deletions and insertions within the segment linking the RRM-like motifs only partially reduced the activity of Ssp2. Moreover, when the two RRM-like motifs were expressed as separate proteins in bacteria, they activated Smk1. We also find that both motifs can be cross-linked to Smk1 and that at least one of the motifs binds near the ATP-binding pocket of the MAPK. These findings demonstrate that motifs related to RRMs can directly activate protein kinases.

Yeast can accommodate phosphotyrosine: v-Src toxicity in yeast arises from a single disrupted pathway

Tyrosine phosphorylation is a key biochemical signal that controls growth and differentiation in multicellular organisms. Saccharomyces cerevisiae and nearly all other unicellular eukaryotes lack intact phosphotyrosine signaling pathways. However, many of these organisms have primitive phosphotyrosine-binding proteins and tyrosine phosphatases, leading to the assumption that the major barrier for emergence of phosphotyrosine signaling was the negative consequences of promiscuous tyrosine kinase activity. In this work, we reveal that the classic oncogene v-Src, which phosphorylates many dozens of proteins in yeast, is toxic because it disrupts a specific spore wall remodeling pathway. Using genetic selections, we find that expression of a specific cyclic peptide, or overexpression of SMK1, a MAP kinase that controls spore wall assembly, both lead to robust growth despite a continuous high level of phosphotyrosine in the yeast proteome. Thus, minimal genetic manipulations allow yeast to tolerate high levels of phosphotyrosine. These results indicate that the introduction of tyrosine kinases within single-celled organisms may not have been a major obstacle to the evolution of phosphotyrosine signaling.