Mechanisms Connecting the Conserved Protein Kinases Ssp1, Kin1, and Pom1 in Fission Yeast Cell Polarity and Division

Quantitative determination of the spatial distribution of components in single cells with CellDetail

The distribution of biomolecules within cells changes upon aging and diseases. To quantitatively determine the spatial distribution of components inside cells, we built the user-friendly open-source 3D-cell-image analysis platform Cell Detection and Analysis of Intensity Lounge (CellDetail). The algorithm within CellDetail is based on the concept of the dipole moment. CellDetail provides quantitative values for the distribution of the polarity proteins Cdc42 and Tubulin in young and aged hematopoietic stem cells (HSCs). Septin proteins form networks within cells that are critical for cell compartmentalization. We uncover a reduced level of organization of the Septin network within aged HSCs and within senescent human fibroblasts. Changes in the Septin network structure might therefore be a common feature of aging. The level of organization of the network of Septin proteins in aged HSCs can be restored to a youthful level by pharmacological attenuation of the activity of the small RhoGTPase Cdc42.

Fission yeast Bgs1 glucan synthase participates in the control of growth polarity and membrane traffic

Rod-shaped fission yeast grows through cell wall expansion at poles and septum, synthesized by essential glucan synthases. Bgs1 synthesizes the linear β(1,3)glucan of primary septum at cytokinesis. Linear β(1,3)glucan is also present in the wall poles, suggesting additional Bgs1 roles in growth polarity. Our study reveals an essential collaboration between Bgs1 and Tea1-Tea4, but not other polarity factors, in controlling growth polarity. Simultaneous absence of Bgs1 function and Tea1-Tea4 causes complete loss of growth polarity, spread of other glucan synthases, and spherical cell formation, indicating this defect is specifically due to linear β(1,3)glucan absence. Furthermore, linear β(1,3)glucan absence induces actin patches delocalization and sterols spread, which are ultimately responsible for the growth polarity loss without Tea1-Tea4. This suggests strong similarities in Bgs1 functions controlling actin structures during cytokinesis and polarized growth. Collectively, our findings unveil that cell wall β(1,3)glucan regulates polarized growth, like the equivalent extracellular matrix in neuronal cells.

Multi-omics analysis revealed that the protein kinase MoKin1 affected the cellular response to endoplasmic reticulum stress in the rice blast fungus, Magnaporthe oryzae

BACKGROUND

Previous studies have shown that protein kinase MoKin1 played an important role in the growth, conidiation, germination and pathogenicity in rice blast fungus, Magnaporthe oryzae. ΔMokin1 mutant showed significant phenotypic defects and significantly reduced pathogenicity. However, the internal mechanism of how MoKin1 affected the development of physiology and biochemistry remained unclear in M. oryzae.

RESULT

This study adopted a multi-omics approach to comprehensively analyze MoKin1 function, and the results showed that MoKin1 affected the cellular response to endoplasmic reticulum stress (ER stress). Proteomic analysis revealed that the downregulated proteins in ΔMokin1 mutant were enriched mainly in the response to ER stress triggered by the unfolded protein. Loss of MoKin1 prevented the ER stress signal from reaching the nucleus. Therefore, the phosphorylation of various proteins regulating the transcription of ER stress-related genes and mRNA translation was significantly downregulated. The insensitivity to ER stress led to metabolic disorders, resulting in a significant shortage of carbohydrates and a low energy supply, which also resulted in severe phenotypic defects in ΔMokin1 mutant. Analysis of MoKin1-interacting proteins indicated that MoKin1 really took participate in the response to ER stress.

CONCLUSION

Our results showed the important role of protein kinase MoKin1 in regulating cellular response to ER stress, providing a new research direction to reveal the mechanism of MoKin1 affecting pathogenic formation, and to provide theoretical support for the new biological target sites searching and bio-pesticides developing.

The serine/threonine protein kinase MpSTE1 directly governs hyphal branching in Monascus spp

Monascus spp. are commercially important fungi due to their ability to produce beneficial secondary metabolites such as the cholesterol-lowering agent lovastatin and natural food colorants azaphilone pigments. Although hyphal branching intensively influenced the production of these secondary metabolites, the pivotal regulators of hyphal development in Monascus spp. remain unclear. To identify these important regulators, we developed an artificial intelligence (AI)-assisted image analysis tool for quantification of hyphae-branching and constructed a random T-DNA insertion library. High-throughput screening revealed that a STE kinase, MpSTE1, was considered as a key regulator of hyphal branching based on the hyphal phenotype. To further validate the role of MpSTE1, we generated an mpSTE1 gene knockout mutant, a complemented mutant, and an overexpression mutant (OE::mpSTE1). Microscopic observations revealed that overexpression of mpSTE1 led to a 63% increase in branch number while deletion of mpSTE1 reduced the hyphal branching by 68% compared to the wild-type strain. In flask cultures, the strain OE::mpSTE1 showed accelerated growth and glucose consumption. More importantly, the strain OE::mpSTE1 produced 9.2 mg/L lovastatin and 17.0 mg/L azaphilone pigments, respectively, 47.0% and 30.1% higher than those of the wild-type strain. Phosphoproteomic analysis revealed that MpSTE1 directly phosphorylated 7 downstream signal proteins involved in cell division, cytoskeletal organization, and signal transduction. To our best knowledge, MpSTE1 is reported as the first characterized regulator for tightly regulating the hyphal branching in Monascus spp. These findings significantly expanded current understanding of the signaling pathway governing the hyphal branching and development in Monascus spp. Furthermore, MpSTE1 and its analogs were demonstrated as promising targets for improving production of valuable secondary metabolites. KEY POINTS: • MpSTE1 is the first characterized regulator for tightly regulating hyphal branching • Overexpression of mpSTE1 significantly improves secondary metabolite production • A high-throughput image analysis tool was developed for counting hyphal branching.

Polarity kinases that phosphorylate F-BAR protein Cdc15 have unique localization patterns during cytokinesis and contributions to preventing tip septation in Schizosaccharomyces pombe

The Schizosaccharomyces pombe F-BAR protein, Cdc15, facilitates the linkage between the cytokinetic ring and the plasma membrane. Cdc15 is phosphorylated on many sites by four polarity kinases and this antagonizes membrane interaction. Dephosphorylation of Cdc15 during mitosis induces its phase separation, allowing oligomerization, membrane association, and protein partner binding. Here, using live cell imaging we examined whether spatial separation of Cdc15 from its four identified kinases potentially explains their diverse effects on tip septation and the mitotic Cdc15 phosphorylation state. We identified a correlation between kinase localization and their ability to antagonize Cdc15 cytokinetic ring and membrane localization.

Multiple polarity kinases inhibit phase separation of F-BAR protein Cdc15 and antagonize cytokinetic ring assembly in fission yeast

The F-BAR protein Cdc15 is essential for cytokinesis in Schizosaccharomyces pombe and plays a key role in attaching the cytokinetic ring (CR) to the plasma membrane (PM). Cdc15's abilities to bind to the membrane and oligomerize via its F-BAR domain are inhibited by phosphorylation of its intrinsically disordered region (IDR). Multiple cell polarity kinases regulate Cdc15 IDR phosphostate, and of these the DYRK kinase Pom1 phosphorylation sites on Cdc15 have been shown in vivo to prevent CR formation at cell tips. Here, we compared the ability of Pom1 to control Cdc15 phosphostate and cortical localization to that of other Cdc15 kinases: Kin1, Pck1, and Shk1. We identified distinct but overlapping cohorts of Cdc15 phosphorylation sites targeted by each kinase, and the number of sites correlated with each kinases' abilities to influence Cdc15 PM localization. Coarse-grained simulations predicted that cumulative IDR phosphorylation moves the IDRs of a dimer apart and toward the F-BAR tips. Further, simulations indicated that the overall negative charge of phosphorylation masks positively charged amino acids necessary for F-BAR oligomerization and membrane interaction. Finally, simulations suggested that dephosphorylated Cdc15 undergoes phase separation driven by IDR interactions. Indeed, dephosphorylated but not phosphorylated Cdc15 undergoes liquid-liquid phase separation to form droplets in vitro that recruit Cdc15 binding partners. In cells, Cdc15 phosphomutants also formed PM-bound condensates that recruit other CR components. Together, we propose that a threshold of Cdc15 phosphorylation by assorted kinases prevents Cdc15 condensation on the PM and antagonizes CR assembly.

Imp2p forms actin-dependent clusters and imparts stiffness to the contractile ring

The contractile ring must anchor to the plasma membrane and cell wall to transmit its tension. F-BAR domain containing proteins including Imp2p and Cdc15p in fission yeast are likely candidate anchoring proteins based on their mutant phenotypes. Cdc15p is a node component, links the actin bundle to the plasma membrane, recruits Bgs1p to the division plane, prevents contractile ring sliding, and contributes to the stiffness of the contractile ring. Less is known about Imp2p. We found that similarly to Cdc15p, Imp2p contributes to the stiffness of the contractile ring and assembles into protein clusters. Imp2p clusters contain approximately eight Imp2p dimers and depend on the actin network for their stability at the division plane. Importantly, Imp2p and Cdc15p reciprocally affect the amount of each other in the contractile ring, indicating that the two proteins influence each other during cytokinesis, which may partially explain their similar phenotypes.

Crowdsourcing biocuration: The Community Assessment of Community Annotation with Ontologies (CACAO)

Experimental data about gene functions curated from the primary literature have enormous value for research scientists in understanding biology. Using the Gene Ontology (GO), manual curation by experts has provided an important resource for studying gene function, especially within model organisms. Unprecedented expansion of the scientific literature and validation of the predicted proteins have increased both data value and the challenges of keeping pace. Capturing literature-based functional annotations is limited by the ability of biocurators to handle the massive and rapidly growing scientific literature. Within the community-oriented wiki framework for GO annotation called the Gene Ontology Normal Usage Tracking System (GONUTS), we describe an approach to expand biocuration through crowdsourcing with undergraduates. This multiplies the number of high-quality annotations in international databases, enriches our coverage of the literature on normal gene function, and pushes the field in new directions. From an intercollegiate competition judged by experienced biocurators, Community Assessment of Community Annotation with Ontologies (CACAO), we have contributed nearly 5,000 literature-based annotations. Many of those annotations are to organisms not currently well-represented within GO. Over a 10-year history, our community contributors have spurred changes to the ontology not traditionally covered by professional biocurators. The CACAO principle of relying on community members to participate in and shape the future of biocuration in GO is a powerful and scalable model used to promote the scientific enterprise. It also provides undergraduate students with a unique and enriching introduction to critical reading of primary literature and acquisition of marketable skills.

Phosphorylation in the intrinsically disordered region of F-BAR protein Imp2 regulates its contractile ring recruitment

The F-BAR protein Imp2 is an important contributor to cytokinesis in the fission yeast, Schizosaccharomyces pombe. Because cell cycle regulated phosphorylation of the central intrinsically disordered region (IDR) of the Imp2 paralog, Cdc15, controls Cdc15 oligomerization state, localization, and ability to bind protein partners, we investigated whether Imp2 is similarly phosphoregulated. We found that Imp2 is endogenously phosphorylated on 28 sites within its IDR with the bulk of phosphorylation being constitutive. In vitro, casein kinase 1 (CK1) Hhp1 and Hhp2 can phosphorylate 17 sites and Cdk1 the remaining 11 sites. Mutations that prevent Cdk1 phosphorylation result in precocious Imp2 recruitment to the cell division site, and mutations designed to mimic these phosphorylation events delay Imp2 CR accumulation. Mutations that eliminated CK1 phosphorylation sites allowed CR sliding, and phosphomimetic substitutions at these sites reduced Imp2 protein levels and slowed CR constriction. Thus, like Cdc15, the Imp2 IDR is phosphorylated at many sites by multiple kinases. In contrast to Cdc15, for which phosphorylation plays a major cell cycle regulatory role, Imp2 phosphorylation is primarily constitutive with milder effects on localization and function.

Phosphorylation of Pal2 by the protein kinases Kin1 and Kin2 modulates HAC1 mRNA splicing in the unfolded protein response in yeast

During cellular stress in the budding yeast Saccharomyces cerevisiae, an endoplasmic reticulum (ER)-resident dual kinase and RNase Ire1 splices an intron from HAC1 mRNA in the cytosol, thereby releasing its translational block. Hac1 protein then activates an adaptive cellular stress response called the unfolded protein response (UPR) that maintains ER homeostasis. The polarity-inducing protein kinases Kin1 and Kin2 contribute to HAC1 mRNA processing. Here, we showed that an RNA-protein complex that included the endocytic proteins Pal1 and Pal2 mediated HAC1 mRNA splicing downstream of Kin1 and Kin2. We found that Pal1 and Pal2 bound to the 3' untranslated region (3'UTR) of HAC1 mRNA, and a yeast strain lacking both Pal1 and Pal2 was deficient in HAC1 mRNA processing. We also showed that Kin1 and Kin2 directly phosphorylated Pal2, and that a nonphosphorylatable Pal2 mutant could not rescue the UPR defect in a pal1Δ pal2Δ strain. Thus, our work uncovers a Kin1/2-Pal2 signaling pathway that coordinates HAC1 mRNA processing and ER homeostasis.

Fission yeast Pak1 phosphorylates anillin-like Mid1 for spatial control of cytokinesis

Protein kinases direct polarized growth by regulating the cytoskeleton in time and space and could play similar roles in cell division. We found that the Cdc42-activated polarity kinase Pak1 colocalizes with the assembling contractile actomyosin ring (CAR) and remains at the division site during septation. Mutations in pak1 led to defects in CAR assembly and genetic interactions with cytokinesis mutants. Through a phosphoproteomic screen, we identified novel Pak1 substrates that function in polarized growth and cytokinesis. For cytokinesis, we found that Pak1 regulates the localization of its substrates Mid1 and Cdc15 to the CAR. Mechanistically, Pak1 phosphorylates the Mid1 N-terminus to promote its association with cortical nodes that act as CAR precursors. Defects in Pak1-Mid1 signaling lead to misplaced and defective division planes, but these phenotypes can be rescued by synthetic tethering of Mid1 to cortical nodes. Our work defines a new signaling mechanism driven by a cell polarity kinase that promotes CAR assembly in the correct time and place.

Cell size: a key determinant of meristematic potential in plant protoplasts

Metabolic pathway reconstruction and gene edits for native natural product synthesis in single plant cells are considered to be less complicated when compared to the production of non-native metabolites. Being an efficient eukaryotic system, plants encompass suitable post-translational modifications. However, slow cell division rate and heterogeneous nature is an impediment for consistent product retrieval from plant cells. Plant cell synchrony can be attained in cultures developed in vitro. Isolated plant protoplasts capable of division, can potentially enhance the unimpaired yield of target bioactives, similar to microbes and unicellular eukaryotes. Evidence from yeast experiments suggests that 'critical cell size' and division rates for enhancement machinery, primarily depend on culture conditions and nutrient availability. The cell size control mechanisms in Arabidopsis shoot apical meristem is analogous to yeast notably, fission yeast. If protoplasts isolated from plants are subjected to cell size studies and cell cycle progression in culture, it will answer the underlying molecular mechanisms such as, unicellular to multicellular transition states, longevity, senescence, 'cell-size resetting' during organogenesis, and adaptation to external cues.

Connecting cell polarity signals to the cytokinetic machinery in yeast and metazoan cells

Polarized growth and cytokinesis are two fundamental cellular processes that exist in virtually all cell types. Mechanisms for asymmetric distribution of materials allow for cells to grow in a polarized manner. This gives rise to a variety of cell shapes seen throughout all cell types. Following polarized growth during interphase, dividing cells assemble a cytokinetic ring containing the protein machinery to constrict and separate daughter cells. Here, we discuss how cell polarity signaling pathways act on cytokinesis, with a focus on direct regulation of the contractile actomyosin ring (CAR). Recent studies have exploited phosphoproteomics to identify new connections between cell polarity kinases and CAR proteins. Existing evidence suggests that some polarity kinases guide the local organization of CAR proteins and structures while also contributing to global organization of the division plane within a cell. We provide several examples of this regulation from budding yeast, fission yeast, and metazoan cells. In some cases, kinase-substrate connections point to conserved processes in these different organisms. We point to several examples where future work can indicate the degree of conservation and divergence in the cell division process of these different organisms.

The phosphatase inhibitor Sds23 promotes symmetric spindle positioning in fission yeast

A hallmark of cell division in eukaryotic cells is the formation and elongation of a microtubule (MT)-based mitotic spindle. Proper positioning of the spindle is critical to ensure equal segregation of the genetic material to the resulting daughter cells. Both the timing of spindle elongation and constriction of the actomyosin contractile ring must be precisely coordinated to prevent missegregation or damage to the genetic material during cellular division. Here, we show that Sds23, an inhibitor of protein phosphatases, contributes to proper positioning of elongating spindles in fission yeast cells. We found that sds23∆ mutant cells exhibit asymmetric spindles that initially elongate asymmetrically toward one end of the dividing cell. Spindle asymmetry in sds23∆ cells results from a defect that is distinct from previously identified mechanisms, including MT protrusions and enlarged vacuoles. Combined with our previous work, this study demonstrates that Sds23, an inhibitor of PP2A-family protein phosphatases, promotes proper positioning of both the bipolar spindle and cytokinetic ring during fission yeast cell division. These two steps ensure the overall symmetry and fidelity of the cell division process.

Opposite Surfaces of the Cdc15 F-BAR Domain Create a Membrane Platform That Coordinates Cytoskeletal and Signaling Components for Cytokinesis

Many eukaryotes assemble an actin- and myosin-based cytokinetic ring (CR) on the plasma membrane (PM) for cell division, but how it is anchored there remains unclear. In Schizosaccharomyces pombe, the F-BAR protein Cdc15 links the PM via its F-BAR domain to proteins in the CR’s interior via its SH3 domain. However, Cdc15’s F-BAR domain also directly binds formin Cdc12, suggesting that Cdc15 may polymerize a protein network directly adjacent to the membrane. Here, we determine that the F-BAR domain binds Cdc12 using residues on the face opposite its membrane-binding surface. These residues also bind paxillin-like Pxl1, promoting its recruitment with calcineurin to the CR. Mutation of these F-BAR domain residues results in a shallower CR, with components localizing ~35% closer to the PM than in wild type, and aberrant CR constriction. Thus, F-BAR domains serve as oligomeric membrane-bound platforms that can modulate the architecture of an entire actin structure.

Multiple F-BAR domains link actin structures to membrane. Snider et al. show that the flat Cdc15 F-BAR domain utilizes opposite surfaces to bind the plasma membrane and cytokinetic ring proteins simultaneously. Disrupting Cdc15 F-BAR domain’s interaction with proteins results in an overall compression of the entire cytokinetic ring architecture.

FgPal1 regulates morphogenesis and pathogenesis in Fusarium graminearum

Ascospores are the primary inoculum in Fusarium graminearum, a causal agent of wheat head blight. In a previous study, FgPAL1 was found to be upregulated in the Fgama1 mutant and important for ascosporogenesis. However, the biological function of this well-conserved gene in filamentous ascomycetes is not clear. In this study, we characterized its functions in growth, differentiation and pathogenesis. The Fgpal1 mutant had severe growth defects and often displayed abnormal hyphal tips. It was defective in infectious growth in rachis tissues and spreading in wheat heads. The Fgpal1 mutant produced conidia with fewer septa and more nuclei per compartment than the wild type. In actively growing hyphal tips, FgPal1-GFP mainly localized to the subapical collar and septa. The FgPal1 and LifeAct partially co-localized at the subapical region in an interdependent manner. The Fgpal1 mutant was normal in meiosis with eight nuclei in developing asci but most asci were aborted. Taken together, our results showed that FgPal1 plays a role in maintaining polarized tip growth and coordination between nuclear division and cytokinesis, and it is also important for infectious growth and developments of ascospores by the free cell formation process.

Loss of FZO1 gene results in changes of cell dynamics in fission yeast

Mitochondrial fission and fusion dynamics are critical cellular processes, and abnormalities in these processes are associated with severe human disorders, such as Beckwith-Wiedemann syndrome, neurodegenerative diseases, Charcot-Marie-Tooth disease type 6, multiple symmetric lipomatosis and microcephaly. Fuzzy onions protein 1 (Fzo1p) regulates mitochondrial outer membrane fusion. In the present study, Schizosaccharomyces pombe (S. pombe) was used to explore the effect of FZO1 gene deletion on cell dynamics in mitosis. The mitochondrial morphology results showed that the mitochondria appeared to be fragmented and tubular in wild-type cells; however, they were observed to accumulate in fzo1Δ cells. The FZO1 gene deletion was demonstrated to result in slow proliferation, sporogenesis defects, increased microtubule (MT) number and actin contraction defects in S. pombe. The FZO1 gene deletion also affected the rate of spindle elongation and phase time at the metaphase and anaphase, as well as spindle MT organization. Live-cell imaging was performed on mutant strains to observe three distinct kinetochore behaviors (normal, lagging and mis-segregation), as well as abnormal spindle breakage. The FZO1 gene deletion resulted in coenzyme and intermediate metabolite abnormalities as determined via metabolomics analysis. It was concluded that the loss of FZO1 gene resulted in deficiencies in mitochondrial dynamics, which may result in deficiencies in spindle maintenance, chromosome segregation, spindle breakage, actin contraction, and coenzyme and intermediate metabolite levels.

Phosphoregulation of the cytokinetic protein Fic1 contributes to fission yeast growth polarity establishment

Cellular polarization underlies many facets of cell behavior, including cell growth. The rod-shaped fission yeast Schizosaccharomyces pombe is a well-established, genetically tractable system for studying growth polarity regulation. S. pombe cells elongate at their two cell tips in a cell cycle-controlled manner, transitioning from monopolar to bipolar growth in interphase when new ends established by the most recent cell division begin to extend. We previously identified cytokinesis as a critical regulator of new end growth and demonstrated that Fic1, a cytokinetic factor, is required for normal polarized growth at new ends. Here, we report that Fic1 is phosphorylated on two C-terminal residues, which are each targeted by multiple protein kinases. Endogenously expressed Fic1 phosphomutants cannot support proper bipolar growth, and the resultant defects facilitate the switch into an invasive pseudohyphal state. Thus, phosphoregulation of Fic1 links the completion of cytokinesis to the re-establishment of polarized growth in the next cell cycle. These findings broaden the scope of signaling events that contribute to regulating S. pombe growth polarity, underscoring that cytokinetic factors constitute relevant targets of kinases affecting new end growth.This article has an associated First Person interview with Anthony M. Rossi, joint first author of the paper.

Mutations in a Single Signaling Pathway Allow Cell Growth in Heavy Water

Life is completely dependent on water. To analyze the role of water as a solvent in biology, we replaced water with heavy water (D₂O) and investigated the biological effects by a wide range of techniques, using Schizosaccharomyces pombe as model organism. We show that high concentrations of D₂O lead to altered glucose metabolism and growth retardation. After prolonged incubation in D₂O, cells displayed gross morphological changes, thickened cell walls, and aberrant cytoskeletal organization. By transcriptomics and genetic screens, we show that the solvent replacement activates two signaling pathways: (1) the heat-shock response pathway and (2) the cell integrity pathway. Although the heat-shock response system upregulates various chaperones and other stress-relieving enzymes, we find that the activation of this pathway does not offer any fitness advantage to the cells under the solvent-replaced conditions. However, limiting the D₂O-triggered activation of the cell integrity pathway allows cell growth when H₂O is completely replaced with D₂O. The isolated D₂O-tolerant strains may aid biological production of deuterated biomolecules.

DYRK kinase Pom1 drives F-BAR protein Cdc15 from the membrane to promote medial division

In many organisms, positive and negative signals cooperate to position the division site for cytokinesis. In the rod-shaped fission yeast Schizosaccharomyces pombe, symmetric division is achieved through anillin/Mid1-dependent positive cues released from the central nucleus and negative signals from the DYRK-family polarity kinase Pom1 at cell tips. Here we establish that Pom1's kinase activity prevents septation at cell tips even if Mid1 is absent or mislocalized. We also find that Pom1 phosphorylation of F-BAR protein Cdc15, a major scaffold of the division apparatus, disrupts Cdc15's ability to bind membranes and paxillin, Pxl1, thereby inhibiting Cdc15's function in cytokinesis. A Cdc15 mutant carrying phosphomimetic versions of Pom1 sites or deletion of Cdc15 binding partners suppresses division at cell tips in cells lacking both Mid1 and Pom1 signals. Thus, inhibition of Cdc15-scaffolded septum formation at cell poles is a key Pom1 mechanism that ensures medial division.

The intrinsically disordered region of the cytokinetic F-BAR protein Cdc15 performs a unique essential function in maintenance of cytokinetic ring integrity

Successful separation of two daughter cells (i.e., cytokinesis) is essential for life. Many eukaryotic cells divide using a contractile apparatus called the cytokinetic ring (CR) that associates dynamically with the plasma membrane (PM) and generates force that contributes to PM ingression between daughter cells. In Schizosaccharomyces pombe, important membrane-CR scaffolds include the paralogous F-BAR proteins Cdc15 and Imp2. Their conserved protein structure consists of the archetypal F-BAR domain linked to an SH3 domain by an intrinsically disordered region (IDR). Functions have been assigned to the F-BAR and SH3 domains. In this study we probed the function of the central IDR. We found that the IDR of Cdc15 is essential for viability and cannot be replaced by that of Imp2, whereas the F-BAR domain of Cdc15 can be swapped with several different F-BAR domains, including that of Imp2. Deleting part of the IDR results in CR defects and abolishes calcineurin phosphatase localization to the CR. Together these results indicate that Cdc15's IDR has a nonredundant essential function that coordinates regulation of CR architecture.

Conserved NDR/LATS kinase controls RAS GTPase activity to regulate cell growth and chronological lifespan

Adaptation to the nutritional environment is critical for all cells. RAS GTPase is a highly conserved GTP-binding protein with crucial functions for cell growth and differentiation in response to environmental conditions. Here, we describe a novel mechanism connecting RAS GTPase to nutrient availability in fission yeast. We report that the conserved NDR/LATS kinase Orb6 responds to nutritional cues and regulates Ras1 GTPase activity. Orb6 increases the protein levels of an Ras1 GTPase activator, the guanine nucleotide exchange factor Efc25, by phosphorylating Sts5, a protein bound to efc25 mRNA. By manipulating the extent of Orb6-mediated Sts5 assembly into RNP granules, we can modulate Efc25 protein levels, Ras1 GTPase activity, and, as a result, cell growth and cell survival. Thus, we conclude that the Orb6-Sts5-Ras1 regulatory axis plays a crucial role in promoting cell adaptation, balancing the opposing demands of promoting cell growth and extending chronological lifespan.

Molecular Mechanism of Cytokinesis

Division of amoebas, fungi, and animal cells into two daughter cells at the end of the cell cycle depends on a common set of ancient proteins, principally actin filaments and myosin-II motors. Anillin, formins, IQGAPs, and many other proteins regulate the assembly of the actin filaments into a contractile ring positioned between the daughter nuclei by different mechanisms in fungi and animal cells. Interactions of myosin-II with actin filaments produce force to assemble and then constrict the contractile ring to form a cleavage furrow. Contractile rings disassemble as they constrict. In some cases, knowledge about the numbers of participating proteins and their biochemical mechanisms has made it possible to formulate molecularly explicit mathematical models that reproduce the observed physical events during cytokinesis by computer simulations.

Molecular Mechanism of Cytokinesis

Division of amoebas, fungi, and animal cells into two daughter cells at the end of the cell cycle depends on a common set of ancient proteins, principally actin filaments and myosin-II motors. Anillin, formins, IQGAPs, and many other proteins regulate the assembly of the actin filaments into a contractile ring positioned between the daughter nuclei by different mechanisms in fungi and animal cells. Interactions of myosin-II with actin filaments produce force to assemble and then constrict the contractile ring to form a cleavage furrow. Contractile rings disassemble as they constrict. In some cases, knowledge about the numbers of participating proteins and their biochemical mechanisms has made it possible to formulate molecularly explicit mathematical models that reproduce the observed physical events during cytokinesis by computer simulations.

Polarity, planes of cell division, and the evolution of plant multicellularity

Organisms as diverse as bacteria, fungi, plants, and animals manifest a property called "polarity." The literature shows that polarity emerges as a consequence of different mechanisms in different lineages. However, across all unicellular and multicellular organisms, polarity is evident when cells, organs, or organisms manifest one or more of the following: orientation, axiation, and asymmetry. Here, we review the relationships among these three features in the context of cell division and the evolution of multicellular polarity primarily in plants (defined here to include the algae). Data from unicellular and unbranched filamentous organisms (e.g., Chlamydomonas and Ulothrix) show that cell orientation and axiation are marked by cytoplasmic asymmetries. Branched filamentous organisms (e.g., Cladophora and moss protonema) require an orthogonal reorientation of axiation, or a localized cell asymmetry (e.g., "tip" growth in pollen tubes and fungal hyphae). The evolution of complex multicellular meristematic polarity required a third reorientation of axiation. These transitions show that polarity and the orientation of the future plane(s) of cell division are dyadic dynamical patterning modules that were critical for multicellular eukaryotic organisms.

Substrate priming enhances phosphorylation by the budding yeast kinases Kin1 and Kin2

Multisite phosphorylation of proteins is a common mechanism for signal integration and amplification in eukaryotic signaling networks. Proteins are commonly phosphorylated at multiple sites in an ordered manner, whereby phosphorylation by one kinase primes the substrate by generating a recognition motif for a second kinase. Here we show that substrate priming promotes phosphorylation by Saccharomyces cerevisiae Kin1 and Kin2, kinases that regulate cell polarity, exocytosis, and the endoplasmic reticulum (ER) stress response. Kin1/Kin2 phosphorylated substrates within the context of a sequence motif distinct from those of their most closely related kinases. In particular, the rate of phosphorylation of a peptide substrate by Kin1/Kin2 increased >30-fold with incorporation of a phosphoserine residue two residues downstream of the phosphorylation site. Recognition of phosphorylated substrates by Kin1/Kin2 was mediated by a patch of basic residues located in the region of the kinase αC helix. We identified a set of candidate Kin1/Kin2 substrates reported to be dually phosphorylated at sites conforming to the Kin1/Kin2 consensus sequence. One of these proteins, the t-SNARE protein Sec9, was confirmed to be a Kin1/Kin2 substrate both in vitro and in vivo Sec9 phosphorylation by Kin1 in vitro was enhanced by prior phosphorylation at the +2 position. Recognition of primed substrates was not required for the ability of Kin2 to suppress the growth defect of secretory pathway mutants but was necessary for optimal growth under conditions of ER stress. These results suggest that at least some endogenous protein substrates of Kin1/Kin2 are phosphorylated in a priming-dependent manner.

Kin1 kinase localizes at the hyphal septum and is dephosphorylated by calcineurin but is dispensable for septation and virulence in the human pathogen Aspergillus fumigatus

Studies in yeasts have implicated the importance of Kin1 protein kinase, a member of the eukaryotic PAR1/MARK/MELK family, in polarized growth, cell division and septation through coordinated activity with the phosphatase, calcineurin. Kin1 is also required for virulence of the fungal pathogens Cryptococcus neoformans and Fusarium graminearum. Here we show that kin1 deletion in the human fungal pathogen Aspergillus fumigatus does not affect hyphal growth and septation but results in differential susceptibility to antifungals targeting the cell wall and cell membrane. The Δkin1 strain remained virulent in a Galleria mellonella model of invasive aspergillosis. Expression of Kin1 tagged to GFP or RFP showed its stable localization at the septum. Co-localization experiments revealed calcineurin (CnaA) localization on either side of Kin1 at the septum suggesting possible interaction. Bimolecular fluorescence complementation assay confirmed the interaction of Kin1 with CnaA at the hyphal tips and septa in the presence of the antifungal caspofungin. Furthermore, phosphoproteomic analyses for the first time revealed Kin1 as a substrate of calcineurin providing novel insight into Kin1 regulation through calcineurin-mediated dephosphorylation mechanism.