TORC2 signaling is antagonized by protein phosphatase 2A and the Far complex in Saccharomyces cerevisiae

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.

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.

Striatin family proteins: The neglected scaffolds

The Striatin family of proteins constitutes Striatin, SG2NA, and Zinedin. Members of this family of proteins act as a signaling scaffold due to the presence of multiple protein-protein interaction domains. At least two members of this family, namely Zinedin and SG2NA, have a proven role in cancer cell proliferation. SG2NA, the second member of this family, undergoes alternative splicing and gives rise to several isoforms which are differentially regulated in a tissue-dependent manner. SG2NA evolved earlier than the other two members of the family, and SG2NA undergoes not only alternative splicing but also other posttranscriptional gene regulation. Striatin also undergoes alternative splicing, and as a result, it gives rise to multiple isoforms. It has been shown that this family of proteins plays a significant role in estrogen signaling, neuroprotection, cancer as well as in cell cycle regulation. Members of the striatin family form a complex network of signaling hubs with different kinases and phosphatases, and other signaling proteins named STRIPAK. Here, in the present manuscript, we thoroughly reviewed the findings on striatin family members to elaborate on the overall structural and functional idea of this family of proteins. We also commented on the involvement of these proteins in STRIPAK complexes and their functional relevance.

The STRIPAK component SipC is involved in morphology and cell-fate determination in the nematode-trapping fungus Duddingtonia flagrans

The striatin-interacting phosphatase and kinase (STRIPAK) complex is a highly conserved eukaryotic signaling hub involved in the regulation of many cellular processes. In filamentous fungi, STRIPAK controls multicellular development, hyphal fusion, septation, and pathogenicity. In this study, we analyzed the role of the STRIPAK complex in the nematode-trapping fungus Duddingtonia flagrans which forms three-dimensional, adhesive trapping networks to capture Caenorhabditis elegans. Trap networks consist of several hyphal loops which are morphologically and functionally different from vegetative hyphae. We show that lack of the STRIPAK component SipC (STRIP1/2/HAM-2/PRO22) results in incomplete loop formation and column-like trap structures with elongated compartments. The misshapen or incomplete traps lost their trap identity and continued growth as vegetative hyphae. The same effect was observed in the presence of the actin cytoskeleton drug cytochalasin A. These results could suggest a link between actin and STRIPAK complex functions.

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.

Characterization of Strip1 Expression in Mouse Cochlear Hair Cells

Striatin-interacting protein 1 (Strip1) is a core component of the striatin interacting phosphatase and kinase (STRIPAK) complex, which is involved in embryogenesis and development, circadian rhythms, type 2 diabetes, and cancer progression. However, the expression and role of Strip1 in the mammalian cochlea remains unclear. Here we studied the expression and function of Strip1 in the mouse cochlea by using Strip1 knockout mice. We first found that the mRNA and protein expression of Strip1 increases as mice age starting from postnatal day (P) 3 and reaches its highest expression level at P30 and that the expression of Strip1 can be detected by immunofluorescent staining starting from P14 only in cochlear HCs, and not in supporting cells (SCs). Next, we crossed Strip1 heterozygous knockout (Strip +/-) mice to obtain Strip1 homozygous knockout (Strip1-/-) mice for studying the role of Strip1 in cochlear HCs. However, no Strip1-/- mice were obtained and the ratio of Strip +/- to Strip1+/+ mice per litter was about 2:1, which suggested that homozygous Strip1 knockout is embryonic lethal. We measured hearing function and counted the HC number in P30 and P60 Strip +/- mice and found that they had normal hearing ability and HC numbers compared to Strip1+/+ mice. Our study suggested that Strip1 probably play important roles in HC development and maturation, which needs further study in the future.

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.

The STRIPAK Complex Regulates Response to Chemotherapy Through p21 and p27

The STRIPAK complex has been linked to a variety of biological processes taking place during embryogenesis and development, but its role in cancer has only just started to be defined. Here, we expand on previous work indicating a role for the scaffolding protein STRIP1 in cancer cell migration and metastasis. We show that cell cycle arrest and decreased proliferation are seen upon loss of STRIP1 in MDA-MB-231 cells due to the induction of cyclin dependent kinase inhibitors, including p21 and p27. We demonstrate that p21 and p27 induction is observed in a subpopulation of cells having low DNA damage response and that the p21^high/γH2AX^low ratio within single cells can be rescued by depleting MST3&4 kinases. While the loss of STRIP1 decreases cell proliferation and tumor growth, cells treated with low dosage of chemotherapeutics in vitro paradoxically escape therapy-induced senescence and begin to proliferate after recovery. This corroborates with already known research on the dual role of p21 and indicates that STRIP1 also plays a contradictory role in breast cancer, suppressing tumor growth, but once treated with chemotherapeutics, allowing for possible recurrence and decreased patient survival.

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.

Ser/Thr protein phosphatases in fungi: structure, regulation and function

Reversible phospho-dephosphorylation of proteins is a major mechanism for the control of cellular functions. By large, Ser and Thr are the most frequently residues phosphorylated in eukar-yotes. Removal of phosphate from these amino acids is catalyzed by a large family of well-conserved enzymes, collectively called Ser/Thr protein phosphatases. The activity of these enzymes has an enormous impact on cellular functioning. In this work we pre-sent the members of this family in S. cerevisiae and other fungal species, and review the most recent findings concerning their regu-lation and the roles they play in the most diverse aspects of cell biology.

Assembly of a heptameric STRIPAK complex is required for coordination of light-dependent multicellular fungal development with secondary metabolism in Aspergillus nidulans

Eukaryotic striatin forms striatin-interacting phosphatase and kinase (STRIPAK) complexes that control many cellular processes including development, cellular transport, signal transduction, stem cell differentiation and cardiac functions. However, detailed knowledge of complex assembly and its roles in stress responses are currently poorly understood. Here, we discovered six striatin (StrA) interacting proteins (Sips), which form a heptameric complex in the filamentous fungus Aspergillus nidulans. The complex consists of the striatin scaffold StrA, the Mob3-type kinase coactivator SipA, the SIKE-like protein SipB, the STRIP1/2 homolog SipC, the SLMAP-related protein SipD and the catalytic and regulatory phosphatase 2A subunits SipE (PpgA), and SipF, respectively. Single and double deletions of the complex components result in loss of multicellular light-dependent fungal development, secondary metabolite production (e.g. mycotoxin Sterigmatocystin) and reduced stress responses. sipA (Mob3) deletion is epistatic to strA deletion by supressing all the defects caused by the lack of striatin. The STRIPAK complex, which is established during vegetative growth and maintained during the early hours of light and dark development, is mainly formed on the nuclear envelope in the presence of the scaffold StrA. The loss of the scaffold revealed three STRIPAK subcomplexes: (I) SipA only interacts with StrA, (II) SipB-SipD is found as a heterodimer, (III) SipC, SipE and SipF exist as a heterotrimeric complex. The STRIPAK complex is required for proper expression of the heterotrimeric VeA-VelB-LaeA complex which coordinates fungal development and secondary metabolism. Furthermore, the STRIPAK complex modulates two important MAPK pathways by promoting phosphorylation of MpkB and restricting nuclear shuttling of MpkC in the absence of stress conditions. SipB in A. nidulans is similar to human suppressor of IKK-ε(SIKE) protein which supresses antiviral responses in mammals, while velvet family proteins show strong similarity to mammalian proinflammatory NF-KB proteins. The presence of these proteins in A. nidulans further strengthens the hypothesis that mammals and fungi use similar proteins for their immune response and secondary metabolite production, respectively.

The multisubunit STRIPAK complex has been studied from yeast to human and plays a range of roles from cell-cycle arrest, fruit body formation to neuronal functions. Molecular assembly of the STRIPAK complex and its roles in stress responses are not well-documented. Fungi, with an estimated 1.5 million members are friends and foes of mankind, acting as pathogens, natural product and enzyme producers. In filamentous fungus Aspergillus nidulans, we found a heptameric STRIPAK core complex made from three subcomplexes, which sits on the nuclear envelope and coordinates signal influx for light-dependent fungal development, secondary metabolism and stress responses. STRIPAK complex controls activities of two major Mitogen Activated Protein Kinase (MAPK) signaling pathways through either promoting their phosphorylation or limiting their nuclear localization under resting conditions. These findings establish a basis for how fungi govern signal influx by using multimeric scaffold protein complexes on the nuclear envelope to control different downstream pathways.

Fsr1, a striatin homologue, forms an endomembrane-associated complex that regulates virulence in the maize pathogen Fusarium verticillioides

Fsr1, a homologue of mammalian striatin, containing multiple protein-binding domains and a coiled-coil (CC) domain, is critical for Fusarium verticillioides virulence. In mammals, striatin interacts with multiple proteins to form a STRIPAK (striatin-interacting phosphatase and kinase) complex that regulates a variety of developmental processes and cellular mechanisms. In this study, we identified the homologue of a key mammalian STRIPAK component STRIP1/2 (striatin-interacting proteins 1 and 2) in F. verticillioides, FvStp1, which interacts with Fsr1 in vivo. Gene deletion analysis indicates that FvStp1 is critical for F. verticillioides stalk rot virulence. In addition, we identified three proteins, designated FvCyp1, FvScp1 and FvSel1, which interact with the Fsr1 CC domain via a yeast two-hybrid screen. Importantly, FvCyp1, FvScp1 and FvSel1 co-localize to endomembrane structures, each having a preferred localization in the cell, and they are all required for F. verticillioides stalk rot virulence. Moreover, these proteins are necessary for the correct localization of Fsr1 to the endoplasmic reticulum (ER) and nuclear envelope. Thus, we identified several novel components in the STRIPAK complex that regulates F. verticillioides virulence, and propose that the correct organization and localization of Fsr1 are critical for STRIPAK complex function.

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.

The TORC2-Dependent Signaling Network in the Yeast Saccharomyces cerevisiae

To grow, eukaryotic cells must expand by inserting glycerolipids, sphingolipids, sterols, and proteins into their plasma membrane, and maintain the proper levels and bilayer distribution. A fungal cell must coordinate growth with enlargement of its cell wall. In Saccharomyces cerevisiae, a plasma membrane-localized protein kinase complex, Target of Rapamicin (TOR) complex-2 (TORC2) (mammalian ortholog is mTORC2), serves as a sensor and master regulator of these plasma membrane- and cell wall-associated events by directly phosphorylating and thereby stimulating the activity of two types of effector protein kinases: Ypk1 (mammalian ortholog is SGK1), along with a paralog (Ypk2); and, Pkc1 (mammalian ortholog is PKN2/PRK2). Ypk1 is a central regulator of pathways and processes required for plasma membrane lipid and protein homeostasis, and requires phosphorylation on its T-loop by eisosome-associated protein kinase Pkh1 (mammalian ortholog is PDK1) and a paralog (Pkh2). For cell survival under various stresses, Ypk1 function requires TORC2-mediated phosphorylation at multiple sites near its C terminus. Pkc1 controls diverse processes, especially cell wall synthesis and integrity. Pkc1 is also regulated by Pkh1- and TORC2-dependent phosphorylation, but, in addition, by interaction with Rho1-GTP and lipids phosphatidylserine (PtdSer) and diacylglycerol (DAG). We also describe here what is currently known about the downstream substrates modulated by Ypk1-mediated and Pkc1-mediated phosphorylation.

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.

Catalytic Subunit 1 of Protein Phosphatase 2A Is a Subunit of the STRIPAK Complex and Governs Fungal Sexual Development

The generation of complex three-dimensional structures is a key developmental step for most eukaryotic organisms. The details of the molecular machinery controlling this step remain to be determined. An excellent model system to study this general process is the generation of three-dimensional fruiting bodies in filamentous fungi like Sordaria macrospora Fruiting body development is controlled by subunits of the highly conserved striatin-interacting phosphatase and kinase (STRIPAK) complex, which has been described in organisms ranging from yeasts to humans. The highly conserved heterotrimeric protein phosphatase PP2A is a subunit of STRIPAK. Here, catalytic subunit 1 of PP2A was functionally characterized. The Δpp2Ac1 strain is sterile, unable to undergo hyphal fusion, and devoid of ascogonial septation. Further, PP2Ac1, together with STRIPAK subunit PRO22, governs vegetative and stress-related growth. We revealed in vitro catalytic activity of wild-type PP2Ac1, and our in vivo analysis showed that inactive PP2Ac1 blocks the complementation of the sterile deletion strain. Tandem affinity purification, followed by mass spectrometry and yeast two-hybrid analysis, verified that PP2Ac1 is a subunit of STRIPAK. Further, these data indicate links between the STRIPAK complex and other developmental signaling pathways, implying the presence of a large interconnected signaling network that controls eukaryotic developmental processes. The insights gained in our study can be transferred to higher eukaryotes and will be important for understanding eukaryotic cellular development in general.

IMPORTANCE

The striatin-interacting phosphatase and kinase (STRIPAK) complex is highly conserved from yeasts to humans and is an important regulator of numerous eukaryotic developmental processes, such as cellular signaling and cell development. Although functional insights into the STRIPAK complex are accumulating, the detailed molecular mechanisms of single subunits are only partially understood. The first fungal STRIPAK was described in Sordaria macrospora, which is a well-established model organism used to study the formation of fungal fruiting bodies, three-dimensional organ-like structures. We analyzed STRIPAK subunit PP2Ac1, catalytic subunit 1 of protein phosphatase PP2A, to study the importance of the catalytic activity of this protein during sexual development. The results of our yeast two-hybrid analysis and tandem affinity purification, followed by mass spectrometry, indicate that PP2Ac1 activity connects STRIPAK with other signaling pathways and thus forms a large interconnected signaling network.

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.

Molecular Basis of the Rapamycin Insensitivity of Target Of Rapamycin Complex 2

Target of Rapamycin (TOR) plays central roles in the regulation of eukaryote growth as the hub of two essential multiprotein complexes: TORC1, which is rapamycin-sensitive, and the lesser characterized TORC2, which is not. TORC2 is a key regulator of lipid biosynthesis and Akt-mediated survival signaling. In spite of its importance, its structure and the molecular basis of its rapamycin insensitivity are unknown. Using crosslinking-mass spectrometry and electron microscopy, we determined the architecture of TORC2. TORC2 displays a rhomboid shape with pseudo-2-fold symmetry and a prominent central cavity. Our data indicate that the C-terminal part of Avo3, a subunit unique to TORC2, is close to the FKBP12-rapamycin-binding domain of Tor2. Removal of this sequence generated a FKBP12-rapamycin-sensitive TORC2 variant, which provides a powerful tool for deciphering TORC2 function in vivo. Using this variant, we demonstrate a role for TORC2 in G2/M cell-cycle progression.

Daytime CLOCK Dephosphorylation Is Controlled by STRIPAK Complexes in Drosophila

In the Drosophila circadian oscillator, the CLOCK/CYCLE complex activates transcription of period (per) and timeless (tim) in the evening. PER and TIM proteins then repress CLOCK (CLK) activity during the night. The pace of the oscillator depends upon post-translational regulation that affects both positive and negative components of the transcriptional loop. CLK protein is highly phosphorylated and inactive in the morning, whereas hypophosphorylated active forms are present in the evening. How this critical dephosphorylation step is mediated is unclear. We show here that two components of the STRIPAK complex, the CKA regulatory subunit of the PP2A phosphatase and its interacting protein STRIP, promote CLK dephosphorylation during the daytime. In contrast, the WDB regulatory PP2A subunit stabilizes CLK without affecting its phosphorylation state. Inhibition of the PP2A catalytic subunit and CKA downregulation affect daytime CLK similarly, suggesting that STRIPAK complexes are the main PP2A players in producing transcriptionally active hypophosphorylated CLK.

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.

Ribosomal protein S6, a target of rapamycin, is involved in the regulation of rRNA genes by possible epigenetic changes in Arabidopsis

The target of rapamycin (TOR) kinase pathway regulates various biological processes, including translation, synthesis of ribosomal proteins, and transcription of rRNA. The ribosomal protein S6 (RPS6) is one of the well known downstream components of the TOR pathway. Ribosomal proteins have been known to have diverse functions in regulating cellular metabolism as well as protein synthesis. So far, however, little is known about other possible role(s) of RPS6 in plants, besides being a component of the 40 S ribosomal subunit and acting as a target of TOR. Here, we report that RPS6 may have a novel function via interaction with histone deacetylase 2B (AtHD2B) that belongs to the plant-specific histone deacetylase HD2 family. RPS6 and AtHD2B were localized to the nucleolus. Co-expression of RPS6 and AtHD2B caused a change in the location of both RPS6 and AtHD2B to one or several nucleolar spots. ChIP analysis suggests that RPS6 directly interacts with the rRNA gene promoter. Protoplasts overexpressing both AtHD2B and RPS6 exhibited down-regulation of pre-18 S rRNA synthesis with a concomitant decrease in transcription of some of the ribosomal proteins, suggesting their direct role in ribosome biogenesis and plant development. This is consistent with the mutation in rps6b that results in reduction in 18 S rRNA transcription and decreased root growth. We propose that the interaction between RPS6 and AtHD2B brings about a change in the chromatin structure of rDNA and thus plays an important role in linking TOR signaling to rDNA transcription and ribosome biogenesis in plants.

STRIPAK complexes: structure, biological function, and involvement in human diseases

The mammalian striatin family consists of three proteins, striatin, S/G2 nuclear autoantigen, and zinedin. Striatin family members have no intrinsic catalytic activity, but rather function as scaffolding proteins. Remarkably, they organize multiple diverse, large signaling complexes that participate in a variety of cellular processes. Moreover, they appear to be regulatory/targeting subunits for the major eukaryotic serine/threonine protein phosphatase 2A. In addition, striatin family members associate with germinal center kinase III kinases as well as other novel components, earning these assemblies the name striatin-interacting phosphatase and kinase (STRIPAK) complexes. Recently, there has been a great increase in functional and mechanistic studies aimed at identifying and understanding the roles of STRIPAK and STRIPAK-like complexes in cellular processes of multiple organisms. These studies have identified novel STRIPAK and STRIPAK-like complexes and have explored their roles in specific signaling pathways. Together, the results of these studies have sparked increased interest in striatin family complexes because they have revealed roles in signaling, cell cycle control, apoptosis, vesicular trafficking, Golgi assembly, cell polarity, cell migration, neural and vascular development, and cardiac function. Moreover, STRIPAK complexes have been connected to clinical conditions, including cardiac disease, diabetes, autism, and cerebral cavernous malformation. In this review, we discuss the expression, localization, and protein domain structure of striatin family members. Then we consider the diverse complexes these proteins and their homologs form in various organisms, emphasizing what is known regarding function and regulation. Finally, we explore possible roles of striatin family complexes in disease, especially cerebral cavernous malformation.

The filamentous fungus Sordaria macrospora as a genetic model to study fruiting body development

Filamentous fungi are excellent experimental systems due to their short life cycles as well as easy and safe manipulation in the laboratory. They form three-dimensional structures with numerous different cell types and have a long tradition as genetic model organisms used to unravel basic mechanisms underlying eukaryotic cell differentiation. The filamentous ascomycete Sordaria macrospora is a model system for sexual fruiting body (perithecia) formation. S. macrospora is homothallic, i.e., self-fertile, easily genetically tractable, and well suited for large-scale genomics, transcriptomics, and proteomics studies. Specific features of its life cycle and the availability of a developmental mutant library make it an excellent system for studying cellular differentiation at the molecular level. In this review, we focus on recent developments in identifying gene and protein regulatory networks governing perithecia formation. A number of tools have been developed to genetically analyze developmental mutants and dissect transcriptional profiles at different developmental stages. Protein interaction studies allowed us to identify a highly conserved eukaryotic multisubunit protein complex, the striatin-interacting phosphatase and kinase complex and its role in sexual development. We have further identified a number of proteins involved in chromatin remodeling and transcriptional regulation of fruiting body development. Furthermore, we review the involvement of metabolic processes from both primary and secondary metabolism, and the role of nutrient recycling by autophagy in perithecia formation. Our research has uncovered numerous players regulating multicellular development in S. macrospora. Future research will focus on mechanistically understanding how these players are orchestrated in this fungal model system.

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.

Repression of class I transcription by cadmium is mediated by the protein phosphatase 2A

Toxic metals are part of our environment, and undue exposure to them leads to a variety of pathologies. In response, most organisms adapt their metabolism and have evolved systems to limit this toxicity and to acquire tolerance. Ribosome biosynthesis being central for protein synthesis, we analyzed in yeast the effects of a moderate concentration of cadmium (Cd(2+)) on Pol I transcription that represents >60% of the transcriptional activity of the cells. We show that Cd(2+) rapidly and drastically shuts down the expression of the 35S rRNA. Repression does not result from a poisoning of any of the components of the class I transcriptional machinery by Cd(2+), but rather involves a protein phosphatase 2A (PP2A)-dependent cellular signaling pathway that targets the formation/dissociation of the Pol I-Rrn3 complex. We also show that Pol I transcription is repressed by other toxic metals, such as Ag(+) and Hg(2+), which likewise perturb the Pol I-Rrn3 complex, but through PP2A-independent mechanisms. Taken together, our results point to a central role for the Pol I-Rrn3 complex as molecular switch for regulating Pol I transcription in response to toxic metals.

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

Target of rapamycin signaling is a conserved, essential pathway integrating nutritional cues with cell growth and proliferation. The target of rapamycin kinase exists in two distinct complexes, TORC1 and TORC2. It has been reported that protein phosphatase 2A (PP2A) and the Far3-7-8-9-10-11 complex (Far complex) negatively regulate TORC2 signaling in yeast. The Far complex, originally identified as factors required for pheromone-induced cell cycle arrest, and PP2A form the yeast counterpart of the STRIPAK complex, which was first isolated in mammals. The cellular localization of the Far complex has yet to be fully characterized. Here, we show that the Far complex localizes to the endoplasmic reticulum (ER) by analyzing functional GFP-tagged Far proteins in vivo. We found that Far9 and Far10, two homologous proteins each with a tail-anchor domain, localize to the ER in mutant cells lacking the other Far complex components. Far3, Far7, and Far8 form a subcomplex, which is recruited to the ER by Far9/10. The Far3-7-8- complex in turn recruits Far11 to the ER. Finally, we show that the tail-anchor domain of Far9 is required for its optimal function in TORC2 signaling. Our study reveals tiered assembly of the yeast Far complex at the ER and a function for Far complex's ER localization in TORC2 signaling.

Rts1-protein phosphatase 2A antagonizes Ptr3-mediated activation of the signaling protease Ssy5 by casein kinase I

The Ssy1-Ptr3-Ssy5 sensor of external amino acids couples Ssy1 receptor–initiated signals to casein kinase–dependent activation of the protease Ssy5. Here Ssy5 activity is shown to be tuned by interactions with Rts1-protein phosphatase 2A and the specific adapter protein Ptr3, which activates Ssy5 by mediating its proximity to casein kinase.

Ligand-induced conformational changes of plasma membrane receptors initiate signals that enable cells to respond to discrete extracellular cues. In response to extracellular amino acids, the yeast Ssy1-Ptr3-Ssy5 sensor triggers the endoproteolytic processing of transcription factors Stp1 and Stp2 to induce amino acid uptake. Activation of the processing protease Ssy5 depends on the signal-induced phosphorylation of its prodomain by casein kinase I (Yck1/2). Phosphorylation is required for subsequent Skp1/Cullin/Grr1 E3 ubiquitin ligase–dependent polyubiquitylation and proteasomal degradation of the inhibitory prodomain. Here we show that Rts1, a regulatory subunit of the general protein phosphatase 2A, and Ptr3 have opposing roles in controlling Ssy5 prodomain phosphorylation. Rts1 constitutively directs protein phosphatase 2A activity toward the prodomain, effectively setting a signaling threshold required to mute Ssy5 activation in the absence of amino acid induction. Ptr3 functions as an adaptor that transduces conformational signals initiated by the Ssy1 receptor to dynamically induce prodomain phosphorylation by mediating the proximity of the Ssy5 prodomain and Yck1/2. Our results demonstrate how pathway-specific and general signaling components function synergistically to convert an extracellular stimulus into a highly specific, tuned, and switch-like transcriptional response that is critical for cells to adapt to changes in nutrient availability.

mTOR in aging, metabolism, and cancer

The target of rapamycin (TOR) is a highly conserved serine/threonine kinase that is part of two structurally and functionally distinct complexes, TORC1 and TORC2. In multicellular organisms, TOR regulates cell growth and metabolism in response to nutrients, growth factors and cellular energy. Deregulation of TOR signaling alters whole body metabolism and causes age-related disease. This review describes the most recent advances in TOR signaling with a particular focus on mammalian TOR (mTOR) in metabolic tissues vis-a-vis aging, obesity, type 2 diabetes, and cancer.

mTOR in aging, metabolism, and cancer

The target of rapamycin (TOR) is a highly conserved serine/threonine kinase that is part of two structurally and functionally distinct complexes, TORC1 and TORC2. In multicellular organisms, TOR regulates cell growth and metabolism in response to nutrients, growth factors and cellular energy. Deregulation of TOR signaling alters whole body metabolism and causes age-related disease. This review describes the most recent advances in TOR signaling with a particular focus on mammalian TOR (mTOR) in metabolic tissues vis-a-vis aging, obesity, type 2 diabetes, and cancer.

A homologue of the human STRIPAK complex controls sexual development in fungi

Sexual development in fungi is a complex process involving the generation of new cell types and tissues - an essential step for all eukaryotic life. The characterization of sterile mutants in the ascomycete Sordaria macrospora has led to a number of proteins involved in sexual development, but a link between these proteins is still missing. Using a combined tandem-affinity purification/mass spectrometry approach, we showed in vivo association of developmental protein PRO22 with PRO11, homologue of mammalian striatin, and SmPP2AA, scaffolding subunit of protein phosphatase 2A. Further experiments extended the protein network to the putative kinase activator SmMOB3, known to be involved in sexual development. Extensive yeast two-hybrid studies allowed us to pinpoint functional domains involved in protein-protein interaction. We show for the first time that a number of already known factors together with new components associate in vivo to form a highly conserved multi-subunit complex. Strikingly, a similar complex has been described in humans, but the function of this so-called striatin interacting phosphatase and kinase (STRIPAK) complex is largely unknown. In S. macrospora, truncation of PRO11 and PRO22 leads to distinct defects in sexual development and cell fusion, indicating a role for the fungal STRIPAK complex in both processes.