Tiered assembly of the yeast Far3-7-8-9-10-11 complex at the endoplasmic reticulum

STRIPAK complex defects result in pseudosexual reproduction in Cryptococcus neoformans

STRIPAK is an evolutionarily conserved signaling complex that coordinates diverse cellular processes across fungi and animals. In the human fungal pathogen Cryptococcus neoformans , STRIPAK was recently shown to play critical roles in maintaining genome stability and controlling both sexual and asexual development. In Cryptococcus , sexual reproduction is closely linked to virulence, and our findings demonstrate that the STRIPAK complex plays key roles in both processes. Here, we further investigate the specific roles of the STRIPAK catalytic subunit Pph22 and its regulatory partner Far8 during sexual development. We show that while pph22 Δ mutants are defective in α- a sexual reproduction, exhibiting impaired meiotic progression and a failure to produce viable spores, the deletion of PPH22 resulted in exclusive pseudosexual reproduction, with progeny inheriting nuclear genomes solely from the wild-type parent. Overexpression of PPG1 , a related phosphatase, rescued growth and developmental defects in pph22 Δ mutants, and restored the preference for α- a sexual reproduction over pseudosexual reproduction during mating, suggesting functional redundancy within the STRIPAK signaling network. Furthermore, deletion of FAR8 , another component of the STRIPAK complex, also led to a high rate of pseudosexual reproduction during α- a sexual mating, reinforcing the role of STRIPAK in modulating reproductive modes in C. neoformans , possibly through regulating nuclear inheritance and meiotic progression. Together, these findings highlight the distinct contributions of STRIPAK to sexual reproduction in C. neoformans and suggest that disruptions of this complex affect genome integrity and inheritance mechanisms, with broader implications for fungal adaptation and pathogenesis.

STRIPAK, a fundamental signaling hub of eukaryotic development

SUMMARYThe striatin-interacting phosphatase and kinase (STRIPAK) complex is involved in the regulation of many developmental processes in eukaryotic microorganisms and all animals, including humans. STRIPAK is a component of protein phosphatase 2A (PP2A), a highly conserved serine-threonine phosphatase composed of catalytic subunits (PP2Ac), a scaffolding subunit (PP2AA) and various substrate-directing B regulatory subunits. In particular, the B''' regulatory subunit called striatin has evoked major interest over the last 20 years. Studies in fungal systems have contributed substantially to our current knowledge about STRIPAK composition, assembly, and cellular localization, as well as its regulatory role in autophagy and the morphology of fungal development. STRIPAK represents a signaling hub with many kinases and thus integrates upstream and downstream information from many conserved eukaryotic signaling pathways. A profound understanding of STRIPAK's regulatory role in fungi opens the gateway to understanding the multifarious functions carried out by STRIPAK in higher eukaryotes, including its contribution to malignant cell growth.

The Cryptococcus neoformans STRIPAK complex controls genome stability, sexual development, and virulence

The eukaryotic serine/threonine protein phosphatase PP2A is a heterotrimeric enzyme composed of a scaffold A subunit, a regulatory B subunit, and a catalytic C subunit. Of the four known B subunits, the B"' subunit (known as striatin) interacts with the multi-protein striatin-interacting phosphatase and kinase (STRIPAK) complex. Orthologs of STRIPAK components were identified in Cryptococcus neoformans, namely PP2AA/Tpd3, PP2AC/Pph22, PP2AB/Far8, STRIP/Far11, SLMAP/Far9, and Mob3. Structural modeling, protein domain analysis, and detected protein-protein interactions suggest C. neoformans STRIPAK is assembled similarly to the human and fungal orthologs. Here, STRIPAK components Pph22, Far8, and Mob3 were functionally characterized. Whole-genome sequencing revealed that mutations in STRIPAK complex subunits lead to increased segmental and chromosomal aneuploidy, suggesting STRIPAK functions in maintaining genome stability. We demonstrate that PPH22 is a haploinsufficient gene: heterozygous PPH22/pph22Δ mutant diploid strains exhibit defects in hyphal growth and sporulation and have a significant fitness disadvantage when grown in competition against a wild-type diploid. Deletion mutants pph22Δ, far8Δ, and mob3Δ exhibit defects in mating and sexual differentiation, including impaired hyphae, basidia, and basidiospore production. Loss of either PPH22 or FAR8 in a haploid background leads to growth defects at 30°C, severely reduced growth at elevated temperature, abnormal cell morphology, and impaired virulence. Additionally, pph22Δ strains frequently accumulate suppressor mutations that result in overexpression of another putative PP2A catalytic subunit, PPG1. The pph22Δ and far8Δ mutants are also unable to grow in the presence of the calcineurin inhibitors cyclosporine A or FK506, and thus these mutations are synthetically lethal with loss of calcineurin activity. Conversely, mob3Δ mutants display increased thermotolerance, capsule production, and melanization, and are hypervirulent in a murine infection model. Taken together, these findings reveal that the C. neoformans STRIPAK complex plays an important role in genome stability, vegetative growth, sexual development, and virulence in this prominent human fungal pathogen.

The Cryptococcus neoformans STRIPAK complex controls genome stability, sexual development, and virulence

The eukaryotic serine/threonine protein phosphatase PP2A is a heterotrimeric enzyme composed of a scaffold A subunit, a regulatory B subunit, and a catalytic C subunit. Of the four known B subunits, the B"' subunit (known as striatin) interacts with the multi-protein striatin-interacting phosphatase and kinase (STRIPAK) complex. Orthologs of STRIPAK components were identified in Cryptococcus neoformans, namely PP2AA/Tpd3, PP2AC/Pph22, PP2AB/Far8, STRIP/Far11, SLMAP/Far9, and Mob3. Structural modeling, protein domain analysis, and detected protein-protein interactions suggest C. neoformans STRIPAK is assembled similarly to the human and fungal orthologs. Here, STRIPAK components Pph22, Far8, and Mob3 were functionally characterized. Whole-genome sequencing revealed that mutations in STRIPAK complex subunits lead to increased segmental and chromosomal aneuploidy, suggesting STRIPAK functions in maintaining genome stability. We demonstrate that PPH22 is a haploinsufficient gene: heterozygous PPH22/pph22Δ mutant diploid strains exhibit defects in hyphal growth and sporulation and have a significant fitness disadvantage when grown in competition against a wild-type diploid. Deletion mutants pph22Δ, far8Δ, and mob3Δ exhibit defects in mating and sexual differentiation, including impaired hyphae, basidia, and basidiospore production. Loss of either PPH22 or FAR8 in a haploid background leads to growth defects at 30°C, severely reduced growth at elevated temperature, abnormal cell morphology, and impaired virulence. Additionally, pph22Δ strains frequently accumulate suppressor mutations that result in overexpression of another putative PP2A catalytic subunit, PPG1. The pph22Δ and far8Δ mutants are also unable to grow in the presence of the calcineurin inhibitors cyclosporine A or FK506, and thus these mutations are synthetically lethal with loss of calcineurin activity. Conversely, mob3Δ mutants display increased thermotolerance, capsule production, and melanization, and are hypervirulent in a murine infection model. Taken together, these findings reveal that the C. neoformans STRIPAK complex plays an important role in genome stability, vegetative growth, sexual development, and virulence in this prominent human fungal pathogen.

The PP2A-like phosphatase Ppg1 mediates assembly of the Far complex to balance gluconeogenic outputs and enables adaptation to glucose depletion

To sustain growth in changing nutrient conditions, cells reorganize outputs of metabolic networks and appropriately reallocate resources. Signaling by reversible protein phosphorylation can control such metabolic adaptations. In contrast to kinases, the functions of phosphatases that enable metabolic adaptation as glucose depletes are poorly studied. Using a Saccharomyces cerevisiae deletion screen, we identified the PP2A-like phosphatase Ppg1 as required for appropriate carbon allocations towards gluconeogenic outputs-trehalose, glycogen, UDP-glucose, UDP-GlcNAc-after glucose depletion. This Ppg1 function is mediated via regulation of the assembly of the Far complex-a multi-subunit complex that tethers to the ER and mitochondrial outer membranes forming localized signaling hubs. The Far complex assembly is Ppg1 catalytic activity-dependent. Ppg1 regulates the phosphorylation status of multiple ser/thr residues on Far11 to enable the proper assembly of the Far complex. The assembled Far complex is required to maintain gluconeogenic outputs after glucose depletion. Glucose in turn regulates Far complex amounts. This Ppg1-mediated Far complex assembly, and Ppg1-Far complex dependent control of gluconeogenic outputs enables adaptive growth under glucose depletion. Our study illustrates how protein dephosphorylation is required for the assembly of a multi-protein scaffold present in localized cytosolic pools, to thereby alter gluconeogenic flux and enable cells to metabolically adapt to nutrient fluctuations.

Analysis of microRNA expression profiles during the differentiation of chicken embryonic stem cells into male germ cells

The differentiation of embryonic stem cells (ESCs) into germ cells in vitro could have very promising applications for infertility treatment and could provide an excellent model for uncovering the molecular mechanisms of germline generation. This study aimed to investigate the differentially expressed miRNAs (DEMs) during the differentiation of chicken ESCs (cESCs) into male germ cells and to establish a profile of the DEMs. Cells before and after induction were subjected to miRNA sequencing (miRNA-seq). A total of 113 DEMs were obtained, including 61 upregulated and 52 downregulated DEMs. GO and KEGG enrichment analyses showed that the target genes were enriched mainly in the MAPK signaling pathway, HTLV infection signaling pathway, cell adhesion molecule (CAM)-related pathways, viral myocarditis, Wnt signaling pathway, ABC transporters, TGF-β signaling pathways, Notch signaling pathways and insulin signaling pathway. The target genes of the miRNAs were related to cell binding, cell parts and biological regulatory processes. Six DEMs, let-7k-5p, miR-132c-5p, miR-193a-5p, miR-202-5p, miR-383-5p and miR-6553-3p, were assessed by qRT-PCR, and the results were consistent with the results of miRNA-seq. Based on qRT-PCR and western blot verification, miR-383-5p and its putative target gene STRN3 were selected to construct an STRN3 3'-UTR dual-luciferase gene reporter vector and its mutant vector. The double luciferase reporter activity of the cotransfected STRN3-WT + miR-383-5p mimics group was significantly lower (by approximately 46%) than that of the other five groups (p < 0.01). There was no significant difference in luciferase activity among the other 5 groups. This study establishes a DEM profile during the process of cESC differentiation into male germ cells; illustrates the mechanisms by which miRNAs regulate target genes; provides a theoretical basis for further research on the mechanisms of the formation and regulation of male germ cells; and provides an important strategy for gene editing, animal genetic resource protection and transgenic animal production.

In the Rat Midbrain, SG2NA and DJ-1 have Common Interactome, Including Mitochondrial Electron Transporters that are Comodulated Under Oxidative Stress

Scaffold proteins Striatin and SG2NA assemble kinases and phosphatases into the signalling complexes called STRIPAK. Dysfunctional STRIPAKs cause cancer, cerebral cavernous malformations, etc. DJ-1, a sensor for oxidative stress, has long been associated with the Parkinson's disease, cancer, and immune disorders. SG2NA interacts with DJ-1 and Akt providing neuroprotection under oxidative stress. To dissect the role of SG2NA and DJ-1 in neuronal pathobiology, rat midbrain extracts were immunoprecipitated with SG2NA and sixty-three interacting proteins were identified. BN-PAGE followed by the LC-MS/MS showed 1030 comigrating proteins as the potential constituents of the multimeric complexes formed by SG2NA. Forty-three proteins were common between those identified by co-immunoprecipitation and the BN-PAGE. Co-immunoprecipitation with DJ-1 identified 179 interacting partners, of which forty-one also interact with SG2NA. Among those forty-one proteins immunoprecipitated with both SG2NA and DJ-1, thirty-nine comigrated with SG2NA in the BN-PAGE, and thus are bonafide constituents of the supramolecular assemblies comprising both DJ-1 and SG2NA. Among those thirty-nine proteins, seven are involved in mitochondrial oxidative phosphorylation. In rotenone-treated rats having Parkinson's like symptoms, the levels of both SG2NA and DJ-1 increased in the mitochondria; and the association of SG2NA with the electron transport complexes enhanced. In the hemi-Parkinson's model, where the rats were injected with 6-OHDA into the midbrain, the occupancy of SG2NA and DJ-1 in the mitochondrial complexes also increased. Our study thus reveals a new family of potential STRIPAK assemblies involving both SG2NA and DJ-1, with key roles in protecting midbrain from the oxidative stress.

FARL-11 (STRIP1/2) is required for sarcomere and sarcoplasmic reticulum organization in C. elegans

Protein phosphatase 2A (PP2A) functions in a variety of cellular contexts. PP2A can assemble into four different complexes based on the inclusion of different regulatory or targeting subunits. The B''' regulatory subunit "striatin" forms the STRIPAK complex consisting of striatin, a catalytic subunit (PP2AC), striatin-interacting protein 1 (STRIP1), and MOB family member 4 (MOB4). In yeast and Caenorhabditis elegans, STRIP1 is required for formation of the endoplasmic reticulum (ER). Because the sarcoplasmic reticulum (SR) is the highly organized muscle-specific version of ER, we sought to determine the function of the STRIPAK complex in muscle using C. elegans. CASH-1 (striatin) and FARL-11 (STRIP1/2) form a complex in vivo, and each protein is localized to SR. Missense mutations and single amino acid losses in farl-11 and cash-1 each result in similar sarcomere disorganization. A missense mutation in farl-11 shows no detectable FARL-11 protein by immunoblot, disruption of SR organization around M-lines, and altered levels of the SR Ca+2 release channel UNC-68.

The GET pathway serves to activate Atg32-mediated mitophagy by ER targeting of the Ppg1-Far complex

Mitophagy removes defective or superfluous mitochondria via selective autophagy. In yeast, the pro-mitophagic protein Atg32 localizes to the mitochondrial surface and interacts with the scaffold protein Atg11 to promote degradation of mitochondria. Although Atg32-Atg11 interactions are thought to be stabilized by Atg32 phosphorylation, how this posttranslational modification is regulated remains obscure. Here, we show that cells lacking the guided entry of the tail-anchored protein (GET) pathway exhibit reduced Atg32 phosphorylation and Atg32-Atg11 interactions, which can be rescued by additional loss of the ER-resident Ppg1-Far complex, a multi-subunit phosphatase negatively acting in mitophagy. In GET-deficient cells, Ppg1-Far is predominantly localized to mitochondria. An artificial ER anchoring of Ppg1-Far in GET-deficient cells significantly ameliorates defects in Atg32-Atg11 interactions and mitophagy. Moreover, disruption of GET and Msp1, an AAA-ATPase that extracts non-mitochondrial proteins localized to the mitochondrial surface, elicits synthetic defects in mitophagy. Collectively, we propose that the GET pathway mediates ER targeting of Ppg1-Far, thereby preventing dysregulated suppression of mitophagy activation.

FARL-11 (STRIP1/2) is Required for Sarcomere and Sarcoplasmic Reticulum Organization in C. elegans

Protein phosphatase 2A (PP2A) functions in a variety of cellular contexts. PP2A can assemble into four different complexes based on the inclusion of different regulatory or targeting subunits. The B''' regulatory subunit "striatin" forms the STRIPAK complex consisting of striatin, a catalytic subunit (PP2AC), striatin interacting protein 1 (STRIP1), and MOB family member 4 (MOB4). In yeast and C. elegans, STRIP1 is required for formation of the endoplasmic reticulum (ER). Since the sarcoplasmic reticulum (SR) is the highly organized muscle-specific version of ER, we sought to determine the function of the STRIPAK complex in muscle using C. elegans . CASH-1 (striatin) and FARL-11 (STRIP1/2) form a complex in vivo , and each protein is localized to SR. Missense mutations and single amino acid losses in farl-11 and cash-1 each result in similar sarcomere disorganization. A missense mutation in farl-11 shows no detectable FARL-11 protein by immunoblot, disruption of SR organization around M-lines, and altered levels of the SR Ca +2 release channel UNC-68.

Summary

Protein phosphatase 2A forms a STRIPAK complex when it includes the targeting B''' subunit "striatin" and STRIP1. STRIP1 is required for formation of ER. We show that in muscle STRIP1 is required for organization of SR and sarcomeres.

Post-transcriptional control of fungal cell wall synthesis

Pathogenic fungi hide from their hosts by camouflage, obscuring immunogenic cell wall components such as beta-glucan with innocuous coverings such as mannoproteins and alpha-glucan that are less readily recognised by the host. Attempts to understand how such processes are regulated have met with varying success. Typically studies focus on understanding the transcriptional response of fungi to either their reservoir environment or the host. However, such approaches do not fully address this research question, due to the layers of post-transcriptional and post-translational regulation that occur within a cell. Although in animals the impact of post-transcriptional and post-translational regulation has been well characterised, our knowledge of these processes in the fungal kingdom is more limited. Mutations in RNA-binding proteins, like Ssd1 and Candida albicans Slr1, affect cell wall composition and fungal virulence indicating that post-transcriptional regulation plays a key role in these processes. Here, we review the current state of knowledge of fungal post-transcriptional regulation, and link this to potential mechanisms of immune evasion by drawing on studies from model yeast and plant pathogenic fungi. We highlight several RNA-binding proteins that regulate cell wall synthesis and could be involved in local translation of cell wall components. Expanding our knowledge on post-transcriptional regulation in human fungal pathogens is essential to fully comprehend fungal virulence strategies and for the design of novel antifungal therapies.

The protein N-terminal acetyltransferase A complex contributes to yeast mitophagy via promoting expression and phosphorylation of Atg32

Mitophagy is an evolutionarily conserved catabolic process that selectively degrades damaged or superfluous mitochondria via autophagy. Although mitophagy is considered to be critical to maintain cellular homeostasis, detailed mechanisms of mitophagy remain largely unknown. In the budding yeast Saccharomyces cerevisiae, the protein N-terminal acetyltransferase A (NatA) complex is important for transcriptional induction of the pro-mitophagic factor Atg32 and efficient degradation of mitochondria under prolonged respiratory conditions. Overexpression of Atg32 only partially recovers mitophagy in cells lacking NatA, raising the possibility that NatA may contribute to mitophagy via additional mechanisms. Here we demonstrate that Atg32 phosphorylation, which is required for facilitating mitophagy, is altered in respiring NatA-deficient cells. Hyperphosphorylation of Atg32 partially rescues mitophagy in cells lacking NatA. Notably, mitophagy is mostly restored in NatA-null cells overexpressing hyperphosphorylated Atg32. Loss of NatA does not impair the interaction of phosphorylated Atg32 with Atg11, a scaffold protein critical for selective autophagy, suggesting that NatA-dependent Atg32 phosphorylation promotes mitophagy independently of Atg32-Atg11 interactions. We propose that NatA-mediated protein N-terminal acetylation acts in Atg32 expression and phosphorylation to drive mitophagy.

STRIPAK, a Key Regulator of Fungal Development, Operates as a Multifunctional Signaling Hub

The striatin-interacting phosphatases and kinases (STRIPAK) multi subunit complex is a highly conserved signaling complex that controls diverse developmental processes in higher and lower eukaryotes. In this perspective article, we summarize how STRIPAK controls diverse developmental processes in euascomycetes, such as fruiting body formation, cell fusion, sexual and vegetative development, pathogenicity, symbiosis, as well as secondary metabolism. Recent structural investigations revealed information about the assembly and stoichiometry of the complex enabling it to act as a signaling hub. Multiple organellar targeting of STRIPAK subunits suggests how this complex connects several signaling transduction pathways involved in diverse cellular developmental processes. Furthermore, recent phosphoproteomic analysis shows that STRIPAK controls the dephosphorylation of subunits from several signaling complexes. We also refer to recent findings in yeast, where the STRIPAK homologue connects conserved signaling pathways, and based on this we suggest how so far non-characterized proteins may functions as receptors connecting mitophagy with the STRIPAK signaling complex. Such lines of investigation should contribute to the overall mechanistic understanding of how STRIPAK controls development in euascomycetes and beyond.

Molecular mechanisms and physiological functions of mitophagy

Degradation of mitochondria via a selective form of autophagy, named mitophagy, is a fundamental mechanism conserved from yeast to humans that regulates mitochondrial quality and quantity control. Mitophagy is promoted via specific mitochondrial outer membrane receptors, or ubiquitin molecules conjugated to proteins on the mitochondrial surface leading to the formation of autophagosomes surrounding mitochondria. Mitophagy-mediated elimination of mitochondria plays an important role in many processes including early embryonic development, cell differentiation, inflammation, and apoptosis. Recent advances in analyzing mitophagy in vivo also reveal high rates of steady-state mitochondrial turnover in diverse cell types, highlighting the intracellular housekeeping role of mitophagy. Defects in mitophagy are associated with various pathological conditions such as neurodegeneration, heart failure, cancer, and aging, further underscoring the biological relevance. Here, we review our current molecular understanding of mitophagy, and its physiological implications, and discuss how multiple mitophagy pathways coordinately modulate mitochondrial fitness and populations.

Association and dissociation between the mitochondrial Far complex and Atg32 regulate mitophagy

Mitophagy plays an important role in mitochondrial homeostasis. In yeast, the phosphorylation of the mitophagy receptor Atg32 by casein kinase 2 is essential for mitophagy. This phosphorylation is counteracted by the yeast equivalent of the STRIPAK complex consisting of the PP2A-like protein phosphatase Ppg1 and Far3-7-8-9-10-11 (Far complex), but the underlying mechanism remains elusive. Here we show that two subpopulations of the Far complex reside in the mitochondria and endoplasmic reticulum, respectively, and play distinct roles; the former inhibits mitophagy via Atg32 dephosphorylation, and the latter regulates TORC2 signaling. Ppg1 and Far11 form a subcomplex, and Ppg1 activity is required for the assembling integrity of Ppg1-Far11-Far8. The Far complex preferentially interacts with phosphorylated Atg32, and this interaction is weakened by mitophagy induction. Furthermore, the artificial tethering of Far8 to Atg32 prevents mitophagy. Taken together, the Ppg1-mediated Far complex formation and its dissociation from Atg32 are crucial for mitophagy regulation.

STRIPAK, a highly conserved signaling complex, controls multiple eukaryotic cellular and developmental processes and is linked with human diseases

The striatin-interacting phosphatases and kinases (STRIPAK) complex is evolutionary highly conserved and has been structurally and functionally described in diverse lower and higher eukaryotes. In recent years, this complex has been biochemically characterized better and further analyses in different model systems have shown that it is also involved in numerous cellular and developmental processes in eukaryotic organisms. Further recent results have shown that the STRIPAK complex functions as a macromolecular assembly communicating through physical interaction with other conserved signaling protein complexes to constitute larger dynamic protein networks. Here, we will provide a comprehensive and up-to-date overview of the architecture, function and regulation of the STRIPAK complex and discuss key issues and future perspectives, linked with human diseases, which may form the basis of further research endeavors in this area. In particular, the investigation of bi-directional interactions between STRIPAK and other signaling pathways should elucidate upstream regulators and downstream targets as fundamental parts of a complex cellular network.

Strip and Cka negatively regulate JNK signalling during Drosophila spermatogenesis

One fundamental property of a stem cell niche is the exchange of molecular signals between its component cells. Niche models, such as the Drosophila melanogaster testis, have been instrumental in identifying and studying the conserved genetic factors that contribute to niche molecular signalling. Here, we identify jam packed (jam), an allele of Striatin interacting protein (Strip), which is a core member of the highly conserved Striatin-interacting phosphatase and kinase (STRIPAK) complex. In the developing Drosophila testis, Strip cell-autonomously regulates the differentiation and morphology of the somatic lineage, and non-cell-autonomously regulates the proliferation and differentiation of the germline lineage. Mechanistically, Strip acts in the somatic lineage with its STRIPAK partner, Connector of kinase to AP-1 (Cka), where they negatively regulate the Jun N-terminal kinase (JNK) signalling pathway. Our study reveals a novel role for Strip/Cka in JNK pathway regulation during spermatogenesis within the developing Drosophila testis.

A synthetic non-histone substrate to study substrate targeting by the Gcn5 HAT and sirtuin HDACs

Gcn5 and sirtuins are highly conserved histone acetyltransferase (HAT) and histone deacetylase (HDAC) enzymes that were first characterized as regulators of gene expression. Although histone tails are important substrates of these enzymes, they also target many nonhistone proteins that function in diverse biological processes. However, the mechanisms used by these enzymes to choose their nonhistone substrates are unknown. Previously, we used SILAC-based MS to identify novel nonhistone substrates of Gcn5 and sirtuins in yeast and found a shared target consensus sequence. Here, we use a synthetic biology approach to demonstrate that this consensus sequence can direct acetylation and deacetylation targeting by these enzymes in vivo Remarkably, fusion of the sequence to a nonsubstrate confers de novo acetylation that is regulated by both Gcn5 and sirtuins. We exploit this synthetic fusion substrate as a tool to define subunits of the Gcn5-containing SAGA and ADA complexes required for nonhistone protein acetylation. In particular, we find a key role for the Ada2 and Ada3 subunits in regulating acetylations on our fusion substrate. In contrast, other subunits tested were largely dispensable, including those required for SAGA stability. In an extended analysis, defects in proteome-wide acetylation observed in ada3Δ mutants mirror those in ada2Δ mutants. Altogether, our work argues that nonhistone protein acetylation by Gcn5 is determined in part by specific amino acids surrounding target lysines but that even optimal sequences require both Ada2 and Ada3 for robust acetylation. The synthetic fusion substrate we describe can serve as a tool to further dissect the regulation of both Gcn5 and sirtuin activities in vivo.

A novel STRIPAK complex component mediates hyphal fusion and fruiting-body development in filamentous fungi

The STRIPAK complex is involved in growth, cell fusion, development and signaling pathways, and thus malfunctions in the human STRIPAK complex often result in severe neuronal diseases and cancer. Despite the high degree of general conservation throughout the complex, several STRIPAK complex-associated small coiled-coil proteins of animals and yeasts are not conserved across species. As there are no data for filamentous ascomycetes, we addressed this through affinity purification with HA-tagged striatin ortholog PRO11 in Sordaria macrospora. Combining the method with liquid chromatography-mass spectrometry, we were able to co-purify STRIPAK complex interactor 1 (SCI1), the first STRIPAK-associated small coiled-coil protein in filamentous ascomycetes. Using yeast two-hybrid experiments, we identified SCI1 protein regions required for SCI1-PRO11 interaction, dimerization of SCI1 and interaction with other STRIPAK components. Further, both proteins PRO11 and SCI1 co-localize with the nuclear basket protein SmPOM152 at the nuclear envelope. Expression of the gene sci1 occurs during early developmental stages of S. macrospora, and the protein SCI1 in combination with PRO11 is required for cell fusion, vegetative growth and sexual development. The results of the present study will help to understand the underlying molecular mechanisms of STRIPAK signaling and function in cellular development and diseases in higher eukaryotes.

STRIP1, a core component of STRIPAK complexes, is essential for normal mesoderm migration in the mouse embryo

Striatin-interacting phosphatases and kinases (STRIPAK) complexes can regulate the cytoskeleton and cell migration in cell lines, but their roles in vivo in mammals are not known. Here, we show that mouse embryos that lack striatin-interacting protein 1 (STRIP1), a core component of STRIPAK complexes, arrest at midgestation with striking morphological defects. Strip1 mutants lack a trunk, and both paraxial and axial mesoderm fail to elongate along the anterior–posterior body axis. Mesodermal cells from Strip1 mutants have defects in actin organization, focal adhesions, and cell migration that can account for the failure of normal mesoderm migration. The findings demonstrate that STRIPAK is a critical regulator of mammalian cell migration and is likely to have important roles in tumor progression as well as development.

Regulated mesoderm migration is necessary for the proper morphogenesis and organ formation during embryonic development. Cell migration and its dependence on the cytoskeleton and signaling machines have been studied extensively in cultured cells; in contrast, remarkably little is known about the mechanisms that regulate mesoderm cell migration in vivo. Here, we report the identification and characterization of a mouse mutation in striatin-interacting protein 1 (Strip1) that disrupts migration of the mesoderm after the gastrulation epithelial-to-mesenchymal transition (EMT). STRIP1 is a core component of the biochemically defined mammalian striatin-interacting phosphatases and kinase (STRIPAK) complexes that appear to act through regulation of protein phosphatase 2A (PP2A), but their functions in mammals in vivo have not been examined. Strip1-null mutants arrest development at midgestation with profound disruptions in the organization of the mesoderm and its derivatives, including a complete failure of the anterior extension of axial mesoderm. Analysis of cultured mesoderm explants and mouse embryonic fibroblasts from null mutants shows that the mesoderm migration defect is correlated with decreased cell spreading, abnormal focal adhesions, changes in the organization of the actin cytoskeleton, and decreased velocity of cell migration. The results show that STRIPAK complexes are essential for cell migration and tissue morphogenesis in vivo.

FvSTR1, a striatin orthologue in Fusarium virguliforme, is required for asexual development and virulence

The soil-borne fungus Fusarium virguliforme causes sudden death syndrome (SDS), one of the most devastating diseases of soybean in North and South America. Despite the importance of SDS, a clear understanding of the fungal pathogenicity factors that affect the development of this disease is still lacking. We have identified FvSTR1, a F. virguliforme gene, which encodes a protein similar to a family of striatin proteins previously reported to regulate signalling pathways, cell differentiation, conidiation, sexual development, and virulence in filamentous fungi. Striatins are multi-domain proteins that serve as scaffolding units in the striatin-interacting phosphatase and kinase (STRIPAK) complex in fungi and animals. To address the function of a striatin homologue in F. virguliforme, FvSTR1 was disrupted and functionally characterized using a gene knock out strategy. The resulting Fvstr1 mutants were largely impaired in conidiation and pigmentation, and displayed defective conidia and conidiophore morphology compared to the wild-type and ectopic transformants. Greenhouse virulence assays revealed that the disruption of FvSTR1 resulted in complete loss of virulence in F. virguliforme. Microtome studies using fluorescence microscopy showed that the Fvstr1 mutants were defective in their ability to colonize the vascular system. The Fvstr1 mutants also showed a reduced transcript level of genes involved in asexual reproduction and in the production of secondary metabolites. These results suggest that FvSTR1 has a critical role in asexual development and virulence in F. virguliforme.

A role for post-transcriptional control of endoplasmic reticulum dynamics and function in C. elegans germline stem cell maintenance

Membrane-bound receptors, which are crucial for mediating several key developmental signals, are synthesized on endoplasmic reticulum (ER). The functional integrity of ER must therefore be important for the regulation of at least some developmental programs. However, the developmental control of ER function is not well understood. Here, we identify the C. elegans protein FARL-11, an ortholog of the mammalian STRIPAK complex component STRIP1/2 (FAM40A/B), as an ER protein. In the C. elegans embryo, we find that FARL-11 is essential for the cell cycle-dependent morphological changes of ER and for embryonic viability. In the germline, FARL-11 is required for normal ER morphology and for membrane localization of the GLP-1/Notch receptor involved in germline stem cell (GSC) maintenance. Furthermore, we provide evidence that PUF-8, a key translational regulator in the germline, promotes the translation of farl-11 mRNA. These findings reveal that ER form and function in the C. elegans germline are post-transcriptionally regulated and essential for the niche-GSC signaling mediated by GLP-1.

The composition and function of the striatin-interacting phosphatases and kinases (STRIPAK) complex in fungi

The striatin-interacting phosphatases and kinases (STRIPAK) complex is a highly conserved eukaryotic protein complex that was recently described for diverse animal and fungal species. Here, we summarize our current knowledge about the composition and function of the STRIPAK complex from the ascomycete Sordaria macrospora, which we discovered by investigating sexually sterile mutants (pro), having a defect in fruiting body development. Mass spectrometry and yeast two-hybrid analysis defined core subunits of the STRIPAK complex, which have structural homologs in animal and other fungal organisms. These subunits (and their mammalian homologs) are PRO11 (striatin), PRO22 (STRIP1/2), SmMOB3 (Mob3), PRO45 (SLMAP), and PP2AA, the structural, and PP2Ac, the catalytic subunits of protein phosphatase 2A (PP2A). Beside fruiting body formation, the STRIPAK complex controls vegetative growth and hyphal fusion in S. macrospora. Although the contribution of single subunits to diverse cellular and developmental processes is not yet fully understood, functional analysis has already shown that mammalian homologs are able to substitute the function of distinct fungal STRIPAK subunits. This underscores the view that fungal model organisms serve as useful tools to get a molecular insight into cellular and developmental processes of eukaryotes in general. Future work will unravel the precise localization of single subunits within the cell and decipher their STRIPAK-related and STRIPAK-independent functions. Finally, evidence is accumulating that there is a crosstalk between STRIPAK and various signaling pathways, suggesting that eukaryotic development is dependent on STRIPAK signaling.

Germinal Center Kinases SmKIN3 and SmKIN24 Are Associated with the Sordaria macrospora Striatin-Interacting Phosphatase and Kinase (STRIPAK) Complex

The striatin-interacting phosphatase and kinase (STRIPAK) complex is composed of striatin, protein phosphatase PP2A and protein kinases that regulate development in animals and fungi. In the filamentous ascomycete Sordaria macrospora, it is required for fruiting-body development and cell fusion. Here, we report on the presence and function of STRIPAK-associated kinases in ascomycetes. Using the mammalian germinal center kinases (GCKs) MST4, STK24, STK25 and MINK1 as query, we identified the two putative homologs SmKIN3 and SmKIN24 in S. macrospora. A BLASTP search revealed that both kinases are conserved among filamentous ascomycetes. The physical interaction of the striatin homolog PRO11 with SmKIN3 and SmKIN24 were verified by yeast two-hybrid (Y2H) interaction studies and for SmKIN3 by co-Immunoprecipitation (co-IP). In vivo localization found that both kinases were present at the septa and deletion of both Smkin3 and Smkin24 led to abnormal septum distribution. While deletion of Smkin3 caused larger distances between adjacent septa and increased aerial hyphae, deletion of Smkin24 led to closer spacing of septa and to sterility. Although phenotypically distinct, both kinases appear to function independently because the double-knockout strain ΔSmkin3/ΔSmkin24 displayed the combined phenotypes of each single-deletion strain.

A fungal sarcolemmal membrane-associated protein (SLMAP) homolog plays a fundamental role in development and localizes to the nuclear envelope, endoplasmic reticulum, and mitochondria

Sarcolemmal membrane-associated protein (SLMAP) is a tail-anchored protein involved in fundamental cellular processes, such as myoblast fusion, cell cycle progression, and chromosomal inheritance. Further, SLMAP misexpression is associated with endothelial dysfunctions in diabetes and cancer. SLMAP is part of the conserved striatin-interacting phosphatase and kinase (STRIPAK) complex required for specific signaling pathways in yeasts, filamentous fungi, insects, and mammals. In filamentous fungi, STRIPAK was initially discovered in Sordaria macrospora, a model system for fungal differentiation. Here, we functionally characterize the STRIPAK subunit PRO45, a homolog of human SLMAP. We show that PRO45 is required for sexual propagation and cell-to-cell fusion and that its forkhead-associated (FHA) domain is essential for these processes. Protein-protein interaction studies revealed that PRO45 binds to STRIPAK subunits PRO11 and SmMOB3, which are also required for sexual propagation. Superresolution structured-illumination microscopy (SIM) further established that PRO45 localizes to the nuclear envelope, endoplasmic reticulum, and mitochondria. SIM also showed that localization to the nuclear envelope requires STRIPAK subunits PRO11 and PRO22, whereas for mitochondria it does not. Taken together, our study provides important insights into fundamental roles of the fungal SLMAP homolog PRO45 and suggests STRIPAK-related and STRIPAK-unrelated functions.

Silencing of STRN4 suppresses the malignant characteristics of cancer cells

The striatin family of proteins, comprising STRN, STRN3 and STRN4, are multidomain-containing proteins that associate with additional proteins to form a large protein complex. We previously reported that STRN4 directly associated with protein kinases, such as MINK1, TNIK and MAP4K4, which are associated with tumor suppression or tumor progression. However, it remains unclear whether STRN4 is associated with tumor progression. In this report, we examined the role that STRN4 plays in cancer malignancy. We show that depletion of STRN4 suppresses proliferation, migration, invasion and the anchorage-independent growth of cancer cells. In addition, STRN4 knockdown increases the sensitivity of pancreatic cancer cells to gemcitabine. Finally, we show that STRN4 knockdown suppresses the proliferation and metastasis of cancer cells in mice. Our results demonstrate a possible role of STRN4 in tumor progression.

HAM-2 and HAM-3 are central for the assembly of the Neurospora STRIPAK complex at the nuclear envelope and regulate nuclear accumulation of the MAP kinase MAK-1 in a MAK-2-dependent manner

Intercellular communication and somatic cell fusion are important for fungal colony establishment, multicellular differentiation and have been associated with host colonization and virulence of pathogenic species. By a combination of genetic, biochemical and live cell imaging techniques, we characterized the Neurospora crassa STRIPAK complex that is essential for self-signalling and consists of the six proteins HAM-2/STRIP, HAM-3/striatin, HAM-4/SLMAP, MOB-3/phocein, PPG-1/PP2A-C and PP2A-A. We describe that the core STRIPAK components HAM-2 and HAM-3 are central for the assembly of the complex at the nuclear envelope, while the phosphatase PPG-1 only transiently associates with this central subcomplex. Our data connect the STRIPAK complex with two MAP kinase pathways: (i) nuclear accumulation of the cell wall integrity MAP kinase MAK-1 depends on the functional integrity of the STRIPAK complex at the nuclear envelope, and (ii) phosphorylation of MOB-3 by the MAP kinase MAK-2 impacts the nuclear accumulation of MAK-1. In summary, these data support a model, in which MAK-2-dependent phosphorylation of MOB-3 is part of a MAK-1 import mechanism. Although self-communication remained intact in the absence of nuclear MAK-1 accumulation, supporting the presence of multiple mechanisms that co-ordinate robust intercellular communication, proper fruiting body morphology was dependent on the MAK-2-phosphorylated N-terminus of MOB-3.