The yeast par-1 homologs kin1 and kin2 show genetic and physical interactions with components of the exocytic machinery

TOR complex 1 negatively regulates NDR kinase Cbk1 to control cell separation in budding yeast

The target of rapamycin (TOR) signalling pathway plays a key role in the coordination between cellular growth and the cell cycle machinery in eukaryotes. The underlying molecular mechanisms by which TOR might regulate events after anaphase remain unknown. We show for the first time that one of the 2 TOR complexes in budding yeast, TORC1, blocks the separation of cells following cytokinesis by phosphorylation of a member of the NDR (nuclear Dbf2-related) protein-kinase family, the protein Cbk1. We observe that TORC1 alters the phosphorylation pattern of Cbk1 and we identify a residue within Cbk1 activation loop, T574, for which a phosphomimetic substitution makes Cbk1 catalytically inactive and, indeed, reproduces TORC1 control over cell separation. In addition, we identify the exocyst component Sec3 as a key substrate of Cbk1, since Sec3 activates the SNARE complex to promote membrane fusion. TORC1 activity ultimately compromises the interaction between Sec3 and a t-SNARE component. Our data indicate that TORC1 negatively regulates cell separation in budding yeast by participating in Cbk1 phosphorylation, which in turn controls the fusion of secretory vesicles transporting hydrolase at the site of division.

Cell polarity in the protist-to-animal transition

A signature feature of the animal kingdom is the presence of epithelia: sheets of polarized cells that both insulate the organism from its environment and mediate interactions with it. Epithelial cells display a marked apico-basal polarity, which is highly conserved across the animal kingdom, both in terms of morphology and of molecular regulators. How did this architecture first evolve? Although the last eukaryotic common ancestor almost certainly possessed a simple form of apico-basal polarity (marked by the presence of one or several flagella at a single cellular pole), comparative genomics and evolutionary cell biology reveal that the polarity regulators of animal epithelial cells have a surprisingly complex and stepwise evolutionary history. Here, we retrace their evolutionary assembly. We suggest that the "polarity network" that polarized animal epithelial cells evolved by integration of initially independent cellular modules that evolved at distinct steps of our evolutionary ancestry. The first module dates back to the last common ancestor of animals and amoebozoans and involved Par1, extracellular matrix proteins, and the integrin-mediated adhesion complex. Other regulators, such as Cdc42, Dlg, Par6 and cadherins evolved in ancient unicellular opisthokonts, and might have first been involved in F-actin remodeling and filopodial dynamics. Finally, the bulk of "polarity proteins" as well as specialized adhesion complexes evolved in the metazoan stem-line, in concert with the newly evolved intercellular junctional belts. Thus, the polarized architecture of epithelia can be understood as a palimpsest of components of distinct histories and ancestral functions, which have become tightly integrated in animal tissues.

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.

Phospholipids of the Plasma Membrane - Regulators or Consequence of Cell Polarity?

Cell polarity is a key feature of many eukaryotic cells, including neurons, epithelia, endothelia and asymmetrically dividing stem cells. Apart from the specific localization of proteins to distinct domains of the plasma membrane, most of these cells exhibit an asymmetric distribution of phospholipids within the plasma membrane too. Notably, research over the last years has revealed that many known conserved regulators of apical-basal polarity in epithelial cells are capable of binding to phospholipids, which in turn regulate the localization and to some extent the function of these proteins. Conversely, phospholipid-modifying enzymes are recruited and controlled by polarity regulators, demonstrating an elaborated balance between asymmetrically localized proteins and phospholipids, which are enriched in certain (micro)domains of the plasma membrane. In this review, we will focus on our current understanding of apical-basal polarity and the implication of phospholipids within the plasma membrane during the cell polarization of epithelia and migrating cells.

A Snf1-related nutrient-responsive kinase antagonizes endocytosis in yeast

Endocytosis is regulated in response to changing environmental conditions to adjust plasma membrane (PM) protein composition for optimal cell growth. Protein networks involved in cargo capture and sorting, membrane sculpting and deformation, and vesicle scission have been well-characterized, but less is known about the networks that sense extracellular cues and relay signals to trigger endocytosis of specific cargo. Hal4 and Hal5 are yeast Snf1-related kinases that were previously reported to regulate nutrient transporter stability by an unknown mechanism. Here we demonstrate that loss of Hal4 and Hal5 activates endocytosis of many different kinds of PM proteins, including Art1-mediated and Art1-independent endocytic events. Acute inhibition of Hal5 in the absence of Hal4 triggers rapid endocytosis, suggesting that Hal kinases function in a nutrient-sensing relay upstream of the endocytic response. Interestingly, Hal5 localizes to the PM, but shifts away from the cell surface in response to stimulation with specific nutrients. We propose that Hal5 functions as a nutrient-responsive regulator of PM protein stability, antagonizing endocytosis and promoting stability of endocytic cargos at the PM in nutrient-limiting conditions.

Cellular homeostasis, a fundamental requirement for all living organisms, is maintained in part through evolutionarily conserved mechanisms that regulate the abundance and activity of ion and nutrient transporters at the cell surface. These mechanisms often incorporate signaling networks that sense changes in the environment and relay signals to alter protein composition at the plasma membrane, often by inducing endocytosis of specific transporters in order to adjust and optimize transport activities at the cell surface. Here, we investigate two kinases in yeast–Hal4 and Hal5 –that are related to the yeast and human AMP sensing kinases. Loss of both Hal4 and Hal5 was previously reported to result in destabilization of ion and nutrient transporters by an unknown mechanism. Our data indicates that Hal kinases function broadly in the regulation of many different classes of endocytic cargo. Hal5 localizes to the plasma membrane in a manner that is responsive to nutrient availability and acute loss of Hal5 activity triggers rapid internalization of endocytic cargo. By uncovering a role for Hal5 as a nutrient-responsive regulator of endocytosis, this research sheds light on how signaling molecules regulate membrane trafficking events to coordinate adaptive growth responses.

Tunicamycin Sensitivity-Suppression by High Gene Dosage Reveals New Functions of the Yeast Hog1 MAP Kinase

In the yeast Saccharomyces cerevisiae, components of the High Osmolarity Glycerol (HOG) pathway are important for the response to diverse stresses including response to endoplasmic reticulum stress (ER stress), which is produced by the accumulation of unfolded proteins in the lumen of this organelle. Accumulation of unfolded proteins may be due to the inhibition of protein N-glycosylation, which can be achieved by treatment with the antibiotic tunicamycin (Tn). In this work we were interested in finding proteins involved in the ER stress response regulated by Hog1, the mitogen activated protein kinase (MAPK) of the HOG pathway. A high gene dosage suppression screening allowed us to identify genes that suppressed the sensitivity to Tn shown by a hog1Δ mutant. The suppressors participate in a limited number of cellular processes, including lipid/carbohydrate biosynthesis and protein glycosylation, vesicle-mediated transport and exocytosis, cell wall organization and biogenesis, and cell detoxification processes. The finding of suppressors Rer2 and Srt1, which participate in the dolichol biosynthesis pathway revealed that the hog1Δ strain has a defective polyprenol metabolism. This work uncovers new genetic and functional interactors of Hog1 and contributes to a better understanding of the participation of this MAPK in the ER stress response.

Fission Yeast NDR/LATS Kinase Orb6 Regulates Exocytosis via Phosphorylation of the Exocyst Complex

NDR/LATS kinases regulate multiple aspects of cell polarity and morphogenesis from yeast to mammals. Fission yeast NDR/LATS kinase Orb6 has been proposed to control cell polarity by regulating the Cdc42 guanine nucleotide exchange factor Gef1. Here, we show that Orb6 regulates polarity largely independently of Gef1 and that Orb6 positively regulates exocytosis. Through Orb6 inhibition in vivo and quantitative global phosphoproteomics, we identify Orb6 targets, including proteins involved in membrane trafficking. We confirm Sec3 and Sec5, conserved components of the exocyst complex, as substrates of Orb6 both in vivo and in vitro, and we show that Orb6 kinase activity is important for exocyst localization to cell tips and for exocyst activity during septum dissolution after cytokinesis. We further find that Orb6 phosphorylation of Sec3 contributes to exocyst function in concert with exocyst protein Exo70. We propose that Orb6 contributes to polarized growth by regulating membrane trafficking at multiple levels.

Adaptation to Endoplasmic Reticulum Stress Requires Transphosphorylation within the Activation Loop of Protein Kinases Kin1 and Kin2, Orthologs of Human Microtubule Affinity-Regulating Kinase

Perturbations in endoplasmic reticulum (ER) homeostasis, a condition termed ER stress, activate the unfolded protein response (UPR), an intracellular network of signaling pathways. Recently, we have shown that protein kinase Kin1 and its paralog, Kin2, in the budding yeast Saccharomyces cerevisiae (orthologs of microtubule affinity-regulating kinase in humans) contribute to the UPR function. These Kin kinases contain a conserved kinase domain and an autoinhibitory kinase-associated 1 (KA1) domain separated by a long undefined domain. Here, we show that Kin1 or Kin2 protein requires minimally a kinase domain and an adjacent kinase extension region (KER) for UPR function. We also show that the functional mini-Kin2 protein is predominantly visualized inside the cells and precipitated with the cellular membrane fraction, suggesting its association with the cellular endomembrane system. Furthermore, we show that transphosphorylation of the Kin1 residue T302 and the analogous Kin2 residue T281 within the activation loop are important for full kinase activity. Collectively, our data suggest that, during ER stress, the Kin kinase domain is released from its autoinhibitory KA1 domain and is activated by transphosphorylation.

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.

Identification of stress responsive genes by studying specific relationships between mRNA and protein abundance

Protein expression is regulated by the production and degradation of mRNAs and proteins but the specifics of their relationship are controversial. Although technological advances have enabled genome-wide and time-series surveys of mRNA and protein abundance, recent studies have shown paradoxical results, with most statistical analyses being limited to linear correlation, or analysis of variance applied separately to mRNA and protein datasets. Here, using recently analyzed genome-wide time-series data, we have developed a statistical analysis framework for identifying which types of genes or biological gene groups have significant correlation between mRNA and protein abundance after accounting for potential time delays. Our framework stratifies all genes in terms of the extent of time delay, conducts gene clustering in each stratum, and performs a non-parametric statistical test of the correlation between mRNA and protein abundance in a gene cluster. Consequently, we revealed stronger correlations than previously reported between mRNA and protein abundance in two metabolic pathways. Moreover, we identified a pair of stress responsive genes (ADC17 and KIN1) that showed a highly similar time series of mRNA and protein abundance. Furthermore, we confirmed robustness of the analysis framework by applying it to another genome-wide time-series data and identifying a cytoskeleton-related gene cluster (keratin 18, keratin 17, and mitotic spindle positioning) that shows similar correlation. The significant correlation and highly similar changes of mRNA and protein abundance suggests a concerted role of these genes in cellular stress response, which we consider provides an answer to the question of the specific relationships between mRNA and protein in a cell. In addition, our framework for studying the relationship between mRNAs and proteins in a cell will provide a basis for studying specific relationships between mRNA and protein abundance after accounting for potential time delays.

Phosphoinositides and Membrane Targeting in Cell Polarity

Selective enrichment of the polyphosphoinositides (PPIn), such as PtdIns(4,5)P2 and PtdIns4P, helps to determine the identity of the plasma membrane (PM) and regulates many aspects of cell biology through a vast number of protein effectors. Polarity proteins had long been assumed to be non-PPIn-binding proteins that mainly associate with PM/cell cortex through their extensive protein-protein interaction network. However, recent studies began to reveal that several key polarity proteins electrostatically bind to PPIn through their positively charged protein domains or structures and such PPIn-binding property is essential for their direct and specific attachment to PM. Although the physical nature of the charge-based PPIn binding appears to be simple and nonspecific, it serves as an elegant mechanism that can be efficiently and specifically regulated for achieving polarized PM targeting of polarity proteins. As an unexpected consequence, subcellular localization of PPIn-binding polarity proteins are also subject to regulations by physiological conditions such as hypoxia and ischemia that acutely and reversibly depletes PPIn from PM.

Basidiomycete-specific PsCaMKL1 encoding a CaMK-like protein kinase is required for full virulence of Puccinia striiformis f. sp. tritici

Calcium/calmodulin-dependent kinases (CaMKs) are Ser/Thr protein kinases (PKs) that respond to changes in cytosolic free Ca²⁺ and play diverse roles in eukaryotes. In fungi, CAMKs are generally classified into four families CAMK1, CAMKL, RAD53 and CAMK-Unique. Among these, CAMKL constitutes the largest family. In some fungal plant pathogens, members of the CaMKL family have been shown to be responsible for pathogenesis. However, little is known about their role(s) in rust fungi. In this study, we functionally characterized a novel PK gene, PsCaMKL1, from Puccinia striiformis f. sp. tritici (Pst). PsCaMKL1 belongs to a group of PKs that is evolutionarily specific to basidiomyceteous fungi. PsCaMKL1 shows little intra-species polymorphism between Pst isolates. PsCaMKL1 transcripts are highly elevated at early infection stages, whereas gene expression is downregulated in barely germinated urediospores under KN93 treatment. Overexpression of PsCaMKL1 in fission yeast increased resistance to environmental stresses. Knock down of PsCaMKL1 using host-induced gene silencing (HIGS) reduced the virulence of Pst accompanied by reactive oxygen species (ROS) accumulation and a hypersensitive response. These results suggest that PsCaMKL1 is a novel pathogenicity factor that exerts it virulence function by regulating ROS production in wheat.

Identifying protein kinase-specific effectors of the osmostress response in yeast

The budding yeast Saccharomyces cerevisiae reacts to increased external osmolarity by modifying many cellular processes. Adaptive signaling relies primarily on the high-osmolarity glycerol (HOG) pathway, which is closely related to the mammalian p38 mitogen-activated protein kinase (MAPK) pathway in core architecture. To identify target proteins of the MAPK Hog1, we designed a mass spectrometry-based high-throughput experiment to measure the impact of Hog1 activation or inhibition on the Scerevisiae phosphoproteome. In addition, we analyzed how deletion of RCK2, which encodes a known effector protein kinase target of Hog1, modulated osmotic stress-induced phosphorylation. Our results not only provide an overview of the diversity of cellular functions that are directly and indirectly affected by the activity of the HOG pathway but also enabled an assessment of the Hog1-independent events that occur under osmotic stress conditions. We extended the number of putative Hog1 direct targets by analyzing the modulation of motifs consisting of serine or threonine followed by a proline (S/T-P motif) and subsequently validated these with an in vivo interaction assay. Rck2 appears to act as a central hub for many Hog1-mediated secondary phosphorylation events. This study clarifies many of the direct and indirect effects of HOG signaling and its stress-adaptive functions.

Rab11-FIP1 phosphorylation by MARK2 regulates polarity in MDCK cells

MARK2/Par1b/EMK1, a serine/threonine kinase, is required for correct apical/basolateral membrane polarization in epithelial cells. However, the specific substrates mediating MARK2 action are less well understood. We have now found that MARK2 phosphorylates Rab11-FIP1B/C at serine 234 in a consensus site similar to that previously identified in Rab11-FIP2. In MDCK cells undergoing repolarization after a calcium switch, antibodies specific for pS234-Rab11-FIP1 or pS227-Rab11-FIP2 demonstrate that the spatial and temporal activation of Rab11-FIP1 phosphorylation is distinct from that for Rab11-FIP2. Phosphorylation of Rab11-FIP1 persists through calcium switch and remains high after polarity has been reestablished whereas FIP2 phosphorylation is highest early in reestablishment of polarity but significantly reduced once polarity has been re-established. MARK2 colocalized with FIP1B/C/D and p(S234)-FIP1 in vivo. Overexpression of GFP-Rab11-FIP1C wildtype or non-phosphorylatable GFP-Rab11-FIP1C(S234A) induced two significant phenotypes following calcium switch. Overexpression of FIP1C wildtype and FIP1C(S234A) caused a psuedo-stratification of cells in early time points following calcium switch. At later time points most prominently observed in cells expressing FIP1C(S234A) a significant lateral lumen phenotype was observed, where F-actin-rich lateral lumens appeared demarcated by a ring of ZO1 and also containing ezrin, syntaxin 3 and podocalyxin. In contrast, p120 and E-Cadherin were excluded from the new apical surface at the lateral lumens and now localized to the new lateral surface oriented toward the media. GFP-FIP1C(S234A) localized to membranes deep to the lateral lumens, and immunostaining demonstrated the reorientation of the centrosome and the Golgi apparatus toward the lateral lumen. These results suggest that both Rab11-FIP1B/C and Rab11-FIP2 serve as critical substrates mediating aspects of MARK2 regulation of epithelial polarity.

Regulation of Cell Polarity by PAR-1/MARK Kinase

PAR-1/MARK kinases are conserved serine/threonine kinases that are essential regulators of cell polarity. PAR-1/MARK kinases localize and function in opposition to the anterior PAR proteins to control the asymmetric distribution of factors in a wide variety polarized cells. In this review, we discuss the mechanisms that control the localization and activity of PAR-1/MARK kinases, including their antagonistic interactions with the anterior PAR proteins. We focus on the role PAR-1 plays in the asymmetric division of the Caenorhabditis elegans zygote, in the establishment of the anterior/posterior axis in the Drosophila oocyte and in the control of microtubule dynamics in mammalian neurons. In addition to conserved aspects of PAR-1 biology, we highlight the unique ways in which PAR-1 acts in these distinct cell types to orchestrate their polarization. Finally, we review the connections between disruptions in PAR-1/MARK function and Alzheimer's disease and cancer.

Molecular determinants of KA1 domain-mediated autoinhibition and phospholipid activation of MARK1 kinase

Protein kinases are frequently regulated by intramolecular autoinhibitory interactions between protein modules that are reversed when these modules bind other 'activating' protein or membrane-bound targets. One group of kinases, the MAP/microtubule affinity-regulating kinases (MARKs) contain a poorly understood regulatory module, the KA1 (kinase associated-1) domain, at their C-terminus. KA1 domains from MARK1 and several related kinases from yeast to humans have been shown to bind membranes containing anionic phospholipids, and peptide ligands have also been reported. Deleting or mutating the C-terminal KA1 domain has been reported to activate the kinase in which it is found - also suggesting an intramolecular autoinhibitory role. Here, we show that the KA1 domain of human MARK1 interacts with, and inhibits, the MARK1 kinase domain. Using site-directed mutagenesis, we identify residues in the KA1 domain required for this autoinhibitory activity, and find that residues involved in autoinhibition and in anionic phospholipid binding are the same. We also demonstrate that a 'mini' MARK1 becomes activated upon association with vesicles containing anionic phospholipids, but only if the protein is targeted to these vesicles by a second signal. These studies provide a mechanistic basis for understanding how MARK1 and its relatives may require more than one signal at the membrane surface to control their activation at the correct location and time. MARK family kinases have been implicated in a plethora of disease states including Alzheimer's, cancer, and autism, so advancing our understanding of their regulatory mechanisms may ultimately have therapeutic value.

The Kinome of Edible and Medicinal Fungus Wolfiporia cocos

Wolfiporia cocos is an edible and medicinal fungus that grows in association with pine trees, and its dried sclerotium, known as Fuling in China, has been used as a traditional medicine in East Asian countries for centuries. Nearly 10% of the traditional Chinese medicinal preparations contain W. cocos. Currently, the commercial production of Fuling is limited because of the lack of pine-based substrate and paucity of knowledge about the sclerotial development of the fungus. Since protein kinase (PKs) play significant roles in the regulation of growth, development, reproduction, and environmental responses in filamentous fungi, the kinome of W. cocos was analyzed by identifying the PKs genes, studying transcript profiles and assigning PKs to orthologous groups. Of the 10 putative PKs, 11 encode atypical PKs, and 13, 10, 2, 22, and 11 could encoded PKs from the AGC, CAMK, CK, CMGC, STE, and TLK Groups, respectively. The level of transcripts from PK genes associated with sclerotia formation in the mycelium and sclerotium stages were analyzed by qRT-PCR. Based on the functions of the orthologs in Sclerotinia sclerotiorum (a sclerotia-formation fungus) and Saccharomyces cerevisiae, the potential roles of these W. cocos PKs were assigned. To the best of our knowledge, our study is the first identification and functional discussion of the kinome in the edible and medicinal fungus W. cocos. Our study systematically suggests potential roles of W. cocos PKs and provide comprehensive and novel insights into W. cocos sclerotial development and other economically important traits. Additionally, based on our result, genetic engineering can be employed for over expression or interference of some significant PKs genes to promote sclerotial growth and the accumulation of active compounds.

The eukaryotic protein kinase superfamily of the necrotrophic fungal plant pathogen, Sclerotinia sclerotiorum

Protein kinases have been implicated in the regulation of many processes that guide pathogen development throughout the course of infection. A survey of the Sclerotinia sclerotiorum genome for genes encoding proteins containing the highly conserved eukaryotic protein kinase (ePK) domain, the largest protein kinase superfamily, revealed 92 S. sclerotiorum ePKs. This review examines the composition of the S. sclerotiorum ePKs based on conserved motifs within the ePK domain family, and relates this to orthologues found in other filamentous fungi and yeasts. The ePKs are also discussed in terms of their proposed role(s) in aspects of host pathogenesis, including the coordination of mycelial growth/development and deployment of pathogenicity determinants in response to environmental stimuli, nutrients and stress.

Kin2, the Budding Yeast Ortholog of Animal MARK/PAR-1 Kinases, Localizes to the Sites of Polarized Growth and May Regulate Septin Organization and the Cell Wall

MARK/PAR-1 protein kinases play important roles in cell polarization in animals. Kin1 and Kin2 are a pair of MARK/PAR-1 orthologs in the budding yeast Saccharomyces cerevisiae. They participate in the regulation of secretion and ER stress response. However, neither the subcellular localization of these two kinases nor whether they may have other cellular functions is clear. Here, we show that Kin2 localizes to the sites of polarized growth in addition to localization on the plasma membrane. The localization to polarity sites is mediated by two targeting domains-TD1 and TD2. TD1 locates in the N-terminal region that spans the protein kinase domain whereas TD2 locates in the C-terminal end that covers the KA1 domain. We also show that an excess of Kin2 activity impaired growth, septin organization, and chitin deposition in the cell wall. Both TD1 and TD2 contribute to this function. Moreover, we find that the C-terminal region of Kin2 interacts with Cdc11, a septin subunit, and Pea2, a component of the polarisome that is known to play a role in septin organization. These findings suggest that Kin2 may play a role in the regulation of the septin cytoskeleton and the cell wall. Finally, we show that the C-terminal region of Kin2 interacts with Rho3, a Rho GTPase, whereas the N-terminal region of Kin2 interacts with Bmh1, a 14-3-3 protein. We speculate that Kin2 may be regulated by Bmh1, Rho3, or Pea2 in vivo. Our study provides new insight in the localization, function, and regulation of Kin2.

Structural insight into the mechanism of synergistic autoinhibition of SAD kinases

The SAD/BRSK kinases participate in various important life processes, including neural development, cell cycle and energy metabolism. Like other members of the AMPK family, SAD contains an N-terminal kinase domain followed by the characteristic UBA and KA1 domains. Here we identify a unique autoinhibitory sequence (AIS) in SAD kinases, which exerts autoregulation in cooperation with UBA. Structural studies of mouse SAD-A revealed that UBA binds to the kinase domain in a distinct mode and, more importantly, AIS nestles specifically into the KD-UBA junction. The cooperative action of AIS and UBA results in an ‘αC-out' inactive kinase, which is conserved across species and essential for presynaptic vesicle clustering in C. elegans. In addition, the AIS, along with the KA1 domain, is indispensable for phospholipid binding. Taken together, these data suggest a model for synergistic autoinhibition and membrane activation of SAD kinases.

The SAD kinases contain a UBA domain that binds to the kinase domain and has a role in autoinhibition and allosteric activation of the AMPK homoenzyme. Here, the authors identify an autoinhibitory sequence in SAD and show that the UBA domain synergistically functions as an autoinhibitory domain.

Suggestive evidence on the involvement of polypyrimidine-tract binding protein in regulating alternative splicing of MAP/microtubule affinity-regulating kinase 4 in glioma

MAP/microtubule affinity-regulating kinase 4 (MARK4) is a serine-threonine kinase that phosphorylates microtubule-associated proteins taking part in the regulation of microtubule dynamics. MARK4 is expressed in two spliced isoforms characterized by inclusion (MARK4S) or exclusion (MARK4L) of exon 16. The distinct expression profiles in the central nervous system and their imbalance in gliomas point to roles of MARK4L and MARK4S in cell proliferation and cell differentiation, respectively. Having ruled out mutations and transcription defects, we hypothesized that alterations in the expression of splicing factors may underlie deregulated MARK4 expression in gliomas. Bioinformatic analysis revealed four putative polypyrimidine-tract binding (PTB) protein binding sites in MARK4 introns 15 and 16. Glioma tissues and glioblastoma-derived cancer stem cells showed, compared with normal brain, significant overexpression of PTB, correlated with high MARK4L mRNA expression. Splicing minigene assays revealed a functional intronic splicing silencer in MARK4 intron 15, but mutagenesis of the PTB binding site in this region did not affect minigene splicing, suggesting that PTB may bind to a splicing silencer other than the predicted one and synergistically acting with the other predicted PTB sites. Electrophoretic mobility shift assays coupled with mass spectrometry confirmed binding of PTB to the polypyrimidine tract of intron 15, and thus its involvement in MARK4 alternative splicing. This finding, along with evidence of PTB overexpression in gliomas and glioblastoma-derived cancer stem cells and differentiated progeny, merged in pointing out the involvement of PTB in the switch to MARK4L, consistent with its established role in driving oncogenic splicing in brain tumors.

A novel role for protein kinase Kin2 in regulating HAC1 mRNA translocation, splicing, and translation

A signaling network called the unfolded protein response (UPR) resolves the protein-folding defects in the endoplasmic reticulum (ER) from yeasts to humans. In the yeast Saccharomyces cerevisiae, the UPR activation involves (i) aggregation of the ER-resident kinase/RNase Ire1 to form an Ire1 focus, (ii) targeting HAC1 pre-mRNA toward the Ire1 focus that cleaves out an inhibitory intron from the mRNA, and (iii) translation of Hac1 protein from the spliced mRNA. Targeting HAC1 mRNA to the Ire1 focus requires a cis-acting bipartite element (3'BE) located at the 3' untranslated leader. Here, we report that the 3'BE plays an additional role in promoting translation from the spliced mRNA. We also report that a high dose of either of two paralogue kinases, Kin1 and Kin2, overcomes the defective UPR caused by a mutation in the 3'BE. These results define a novel role for Kin kinases in the UPR beyond their role in cell polarity and exocytosis. Consistently, targeting, splicing, and translation of HAC1 mRNA are substantially reduced in the kin1Δ kin2Δ strain. Furthermore, we show that Kin2 kinase domain itself is sufficient to activate the UPR, suggesting that Kin2 initiates a signaling cascade to ensure an optimum UPR.

FgKin1 kinase localizes to the septal pore and plays a role in hyphal growth, ascospore germination, pathogenesis, and localization of Tub1 beta-tubulins in Fusarium graminearum

The Kin1/Par-1/MARK kinases regulate various cellular processes in eukaryotic organisms. Kin1 orthologs are well conserved in fungal pathogens but none of them have been functionally characterized. Here, we show that KIN1 is important for pathogenesis and growth in two phytopathogenic fungi and that FgKin1 regulates ascospore germination and the localization of Tub1 β-tubulins in Fusarium graminearum. The Fgkin1 mutant and putative FgKIN1(S172A) kinase dead (nonactivatable) transformants were characterized for defects in plant infection, sexual and asexual reproduction, and stress responses. The localization of FgKin1 and two β-tubulins were examined in the wild-type and mutant backgrounds. Deletion of FgKIN1 resulted in reduced virulence and defects in ascospore germination and release. FgKin1 localized to the center of septal pores. FgKIN1 deletion had no effect on Tub2 microtubules but disrupted Tub1 localization. In the mutant, Tub1 appeared to be enriched in the nucleolus. In Magnaporthe oryzae, MoKin1 has similar functions in growth and infection and it also localizes to septal pores. The S172A mutation had no effect on the localization and function of FgKIN1 during sexual reproduction. These results indicate that FgKIN1 has kinase-dependent and independent functions and it specifically regulates Tub1 β-tubulins. FgKin1 plays a critical role in ascospore discharge, germination, and plant infection.

The phosphoproteome of Aspergillus nidulans reveals functional association with cellular processes involved in morphology and secretion

We describe the first phosphoproteome of the model filamentous fungus Aspergillus nidulans. Phosphopeptides were enriched using titanium dioxide, separated using a convenient ultra-long reverse phase gradient, and identified using a "high-high" strategy (high mass accuracy on the parent and fragment ions) with higher-energy collisional dissociation. Using this approach 1801 phosphosites, from 1637 unique phosphopeptides, were identified. Functional classification revealed phosphoproteins were overrepresented under GO categories related to fungal morphogenesis: "sites of polar growth," "vesicle mediated transport," and "cytoskeleton organization." In these same GO categories, kinase-substrate analysis of phosphoproteins revealed the majority were target substrates of CDK and CK2 kinase families, indicating these kinase families play a prominent role in fungal morphogenesis. Kinase-substrate analysis also identified 57 substrates for kinases known to regulate secretion of hydrolytic enzymes (e.g. PkaA, SchA, and An-Snf1). Altogether this data will serve as a benchmark that can be used to elucidate regulatory networks functionally associated with fungal morphogenesis and secretion. All MS data have been deposited in the ProteomeXchange with identifier PXD000715 (http://proteomecentral.proteomexchange.org/dataset/PXD000715).

Pichia pastoris Aft1--a novel transcription factor, enhancing recombinant protein secretion

BACKGROUND

The methylotrophic yeast Pichia pastoris is frequently used for the production of recombinant proteins. However, expression levels can vary depending on the target protein. Allowing for simultaneous regulation of many genes, which may elicit a desired phenotype like increased protein production, overexpression of transcription factors can be used to overcome expression bottlenecks. Here, we present a novel P. pastoris transcription factor currently annotated as Aft1, activator of ferrous transport.

RESULTS

The promoter regions of key secretory P. pastoris genes were screened for fungal transcription factor binding sites, revealing Aft1 as an interesting candidate for improving secretion. Genome wide analysis of transcription factor binding sites suggested Aft1 to be involved in the regulation of many secretory genes, but also indicated possible novel functions in carbohydrate metabolism. No Aft binding sites were found in promoters of characteristic iron homeostasis genes in P. pastoris. Microarrays were used to study the Aft1 regulon in detail, confirming Aft1 involvement in the regulation of carbon-responsive genes, and showing that iron regulation is dependent on FEP1, but not AFT1 expression levels. The positive effect of AFT1 overexpression on recombinant protein secretion was demonstrated for a carboxylesterase from Sphingopyxis sp. MTA144, for which secretion was improved 2.5-fold in fed batch bioreactor cultivations.

CONCLUSION

This study demonstrates that the transcription factor Aft1 can be used to improve recombinant protein secretion in P. pastoris. Furthermore, we discovered possible novel functions of Aft1 in carbohydrate metabolism and provide evidence arguing against a direct role of Aft1 in P. pastoris iron regulation.

Dueling kinases regulate cell size at division through the SAD kinase Cdr2

Cell size control requires mechanisms that integrate cell growth and division. Key to this integration in fission yeast is the SAD family kinase Cdr2, which organizes a set of cortical nodes in the cell middle to promote mitotic entry through Wee1 and Cdk1. Cdr2 is inhibited by a spatial gradient of the DYRK kinase Pom1 emanating from cell tips in a cell-size-dependent manner, but how the Pom1 gradient inhibits Cdr2 activity during cell growth is unknown. Here, we show that Pom1 acts to prevent activation of Cdr2 kinase activity by the CaMKK Ssp1. We found that Ssp1 activates Cdr2 through phosphorylation of a conserved threonine residue (Thr166) in the activation loop of the Cdr2 N-terminal kinase domain both in vitro and in cells. The levels of this activating phosphorylation increased with cell-cycle progression, and genetic epistasis demonstrated that Ssp1 promotes mitotic entry through Cdr2. Intriguingly, Pom1 phosophorylated the C-terminal domain of Cdr2, and this modification reduced Cdr2-T166 phosphorylation by Ssp1. These findings show how activation of the conserved mitotic inducer Cdr2 is integrated with an inhibitory spatial gradient to ensure proper cell size control at mitosis.

par-1, atypical pkc, and PP2A/B55 sur-6 are implicated in the regulation of exocyst-mediated membrane trafficking in Caenorhabditis elegans

The exocyst is a conserved protein complex that is involved in tethering secretory vesicles to the plasma membrane and regulating cell polarity. Despite a large body of work, little is known how exocyst function is controlled. To identify regulators for exocyst function, we performed a targeted RNA interference (RNAi) screen in Caenorhabditis elegans to uncover kinases and phosphatases that genetically interact with the exocyst. We identified seven kinase and seven phosphatase genes that display enhanced phenotypes when combined with hypomorphic alleles of exoc-7 (exo70), exoc-8 (exo84), or an exoc-7;exoc-8 double mutant. We show that in line with its reported role in exocytotic membrane trafficking, a defective exoc-8 caused accumulation of exocytotic soluble NSF attachment protein receptor (SNARE) proteins in both intestinal and neuronal cells in C. elegans. Down-regulation of the phosphatase protein phosphatase 2A (PP2A) phosphatase regulatory subunit sur-6/B55 gene resulted in accumulation of exocytic SNARE proteins SNB-1 and SNAP-29 in wild-type and in exoc-8 mutant animals. In contrast, RNAi of the kinase par-1 caused reduced intracellular green fluorescent protein signal for the same proteins. Double RNAi experiments for par-1, pkc-3, and sur-6/B55 in C. elegans suggest a possible cooperation and involvement in postembryo lethality, developmental timing, as well as SNARE protein trafficking. Functional analysis of the homologous kinases and phosphatases in Drosophila median neurosecretory cells showed that atypical protein kinase C kinase and phosphatase PP2A regulate exocyst-dependent, insulin-like peptide secretion. Collectively, these results characterize kinases and phosphatases implicated in the regulation of exocyst function, and suggest the possibility for interplay between the par-1 and pkc-3 kinases and the PP2A phosphatase regulatory subunit sur-6 in this process.

MAP/microtubule affinity-regulating kinases, microtubule dynamics, and spermatogenesis

During spermatogenesis, spermatids derived from meiosis simultaneously undergo extensive morphological transformation, to become highly specialized and metabolically quiescent cells, and transport across the seminiferous epithelium. Spermatids are also transported back-and-forth across the seminiferous epithelium during the epithelial cycle until they line up at the luminal edge of the tubule to prepare for spermiation at stage VIII of the cycle. Spermatid transport thus requires the intricate coordination of the cytoskeletons in Sertoli cells (SCs) as spermatids are nonmotile cells lacking the ultrastructures of lamellipodia and filopodia, as well as the organized components of the cytoskeletons. In the course of preparing this brief review, we were surprised to see that, except for some earlier eminent morphological studies, little is known about the regulation of the microtubule (MT) cytoskeleton and the coordination of MT with the actin-based cytoskeleton to regulate spermatid transport during the epithelia cycle, illustrating that this is a largely neglected area of research in the field. Herein, we summarize recent findings in the field regarding the significance of actin- and tubulin-based cytoskeletons in SCs that support spermatid transport; we also highlight specific areas of research that deserve attention in future studies.

Regulatory motifs in Chk1

Chk1 is the effector kinase of the G 2 DNA damage checkpoint. Chk1 homologs possess a highly conserved N-terminal kinase domain and a less conserved C-terminal regulatory domain. In response to DNA damage, Chk1 is recruited to mediator proteins assembled at lesions on replication protein A (RPA)-coated single-stranded DNA (ssDNA). Chk1 is then activated by phosphorylation on S345 in the C-terminal regulatory domain by the PI3 kinase-related kinases ATM and ATR to enforce a G 2 cell cycle arrest to allow time for DNA repair. Models have emerged in which this C-terminal phosphorylation relieves auto-inhibitory regulation of the kinase domain by the regulatory domain. However, experiments in fission yeast have shown that deletion of this putative auto-inhibitory domain actually inactivates Chk1 function. We show here that Chk1 homologs possess a kinase-associated 1 (KA1) domain that possesses residues previously implicated in Chk1 auto-inhibition. In addition, all Chk1 homologs have a small and highly conserved C-terminal extension (CTE domain). In fission yeast, both of these motifs are essential for Chk1 activation through interaction with the mediator protein Crb2, the homolog of human 53BP1. Thus, through different intra- and intermolecular interactions, these motifs explain why the regulatory domain exerts both positive and negative control over Chk1 activation. Such motifs may provide alternative targets to the ATP-binding pocket on which to dock Chk1 inhibitors as anticancer therapeutics.

The Kin1 kinase and the calcineurin phosphatase cooperate to link actin ring assembly and septum synthesis in fission yeast

BACKGROUND INFORMATION

The Kin1 protein kinase of fission yeast, which regulates cell surface cohesiveness during interphase cell growth, is also present at the cell division site during mitosis; however, its function in cell division has remained elusive.

RESULTS

In FK506-mediated calcineurin deficient cells, mitosis is extended and ring formation is transiently compromised but septation remains normal. Here, we show that Kin1 inhibition in these cells leads to polyseptation and defects in membrane closure. Actomyosin ring disassembly is prevented and ultimately the daughter cells fail to separate. We show that the Pmk1 MAP kinase pathway and the type V myosin Myo4 act downstream of the cytokinetic function of Kin1. Kin1 inhibition also promotes polyseptation in myo3Δ, a type II myosin heavy-chain mutant defective in ring assembly. In contrast, Kin1 inactivation rescues septation in a myosin light-chain cdc4-8 thermosensitive mutant. A structure/function analysis of the Kin1 protein sequence identified a novel motif outside the kinase domain that is important for its polarised localisation and its catalytic activity. This motif is remarkably conserved in all fungal Kin1 homologues but is absent in related kinases of metazoans.

CONCLUSIONS

We conclude that calcineurin and Kin1 activities must be tightly coordinated to link actomyosin ring assembly with septum synthesis and membrane closure and to ensure separation of the daughter cells.

Roles of cadherins and catenins in cell-cell adhesion and epithelial cell polarity

A simple epithelium is the building block of all metazoans and a multicellular stage of a nonmetazoan. It comprises a closed monolayer of quiescent cells that surround a luminal space. Cells are held together by cell-cell adhesion complexes and surrounded by extracellular matrix. These extracellular contacts are required for the formation of a polarized organization of plasma membrane proteins that regulate the directional absorption and secretion of ions, proteins, and other solutes. While advances have been made in understanding how proteins are sorted to different plasma membrane domains, less is known about how cell-cell adhesion is regulated and linked to the development of epithelial cell polarity and regulation of homeostasis.

Polarization and myelination in myelinating glia

Myelinating glia, oligodendrocytes in central nervous system and Schwann cells in peripheral nervous system, form myelin sheath, a multilayered membrane system around axons enabling salutatory nerve impulse conduction and maintaining axonal integrity. Myelin sheath is a polarized structure localized in the axonal side and therefore is supposed to be formed based on the preceding polarization of myelinating glia. Thus, myelination process is closely associated with polarization of myelinating glia. However, cell polarization has been less extensively studied in myelinating glia than other cell types such as epithelial cells. The ultimate goal of this paper is to provide insights for the field of myelination research by applying the information obtained in polarity study in other cell types, especially epithelial cells, to cell polarization of myelinating glia. Thus, in this paper, the main aspects of cell polarization study in general are summarized. Then, they will be compared with polarization in oligodendrocytes. Finally, the achievements obtained in polarization study for epithelial cells, oligodendrocytes, and other types of cells will be translated into polarization/myelination process by Schwann cells. Then, based on this model, the perspectives in the study of Schwann cell polarization/myelination will be discussed.

Caenorhabditis elegans PIG-1/MELK acts in a conserved PAR-4/LKB1 polarity pathway to promote asymmetric neuroblast divisions

Asymmetric cell divisions produce daughter cells with distinct sizes and fates, a process important for generating cell diversity during development. Many Caenorhabditis elegans neuroblasts, including the posterior daughter of the Q cell (Q.p), divide to produce a larger neuron or neuronal precursor and a smaller cell that dies. These size and fate asymmetries require the gene pig-1, which encodes a protein orthologous to vertebrate MELK and belongs to the AMPK-related family of kinases. Members of this family can be phosphorylated and activated by the tumor suppressor kinase LKB1, a conserved polarity regulator of epithelial cells and neurons. In this study, we present evidence that the C. elegans orthologs of LKB1 (PAR-4) and its partners STRAD (STRD-1) and MO25 (MOP-25.2) regulate the asymmetry of the Q.p neuroblast division. We show that PAR-4 and STRD-1 act in the Q lineage and function genetically in the same pathway as PIG-1. A conserved threonine residue (T169) in the PIG-1 activation loop is essential for PIG-1 activity, consistent with the model that PAR-4 (or another PAR-4-regulated kinase) phosphorylates and activates PIG-1. We also demonstrate that PIG-1 localizes to centrosomes during cell divisions of the Q lineage, but this localization does not depend on T169 or PAR-4. We propose that a PAR-4-STRD-1 complex stimulates PIG-1 kinase activity to promote asymmetric neuroblast divisions and the generation of daughter cells with distinct fates. Changes in cell fate may underlie many of the abnormal behaviors exhibited by cells after loss of PAR-4 or LKB1.

The signaling adaptor GAB1 regulates cell polarity by acting as a PAR protein scaffold

Cell polarity plays a key role in development and is disrupted in tumors, yet the molecules and mechanisms that regulate polarity remain poorly defined. We found that the scaffolding adaptor GAB1 interacts with two polarity proteins, PAR1 and PAR3. GAB1 binds PAR1 and enhances its kinase activity. GAB1 brings PAR1 and PAR3 into a transient complex, stimulating PAR3 phosphorylation by PAR1. GAB1 and PAR6 bind the PAR3 PDZ1 domain and thereby compete for PAR3 binding. Consequently, GAB1 depletion causes PAR3 hypophosphorylation and increases PAR3/PAR6 complex formation, resulting in accelerated and enhanced tight junction formation, increased transepithelial resistance, and lateral domain shortening. Conversely, GAB1 overexpression, in a PAR1/PAR3-dependent manner, disrupts epithelial apical-basal polarity, promotes multilumen cyst formation, and enhances growth factor-induced epithelial cell scattering. Our results identify GAB1 as a negative regulator of epithelial cell polarity that functions as a scaffold for modulating PAR protein complexes on the lateral membrane.

Microtubule affinity-regulating kinase 2 (MARK2) turns on phosphatase and tensin homolog (PTEN)-induced kinase 1 (PINK1) at Thr-313, a mutation site in Parkinson disease: effects on mitochondrial transport

The kinase MARK2/Par-1 plays key roles in several cell processes, including neurodegeneration such as Alzheimer disease by phosphorylating tau and detaching it from microtubules. In search of interaction partners of MARK2, we identified phosphatase and tensin homolog (PTEN)-induced kinase 1 (PINK1), which is important for the survival of neurons and whose mutations are linked to familial Parkinson disease (PD). MARK2 phosphorylated and activated the cleaved form of PINK1 (ΔN-PINK1; amino acids 156-581). Thr-313 was the primary phosphorylation site, a residue mutated to a non-phosphorylatable form (T313M) in a frequent variant of PD. Mutation of Thr-313 to Met or Glu in PINK1 showed toxic effects with abnormal mitochondrial distribution in neurons. MARK2 and PINK1 were found to colocalize with mitochondria and regulate their transport. ΔN-PINK1 promoted anterograde transport and increased the fraction of stationary mitochondria, whereas full-length PINK1 promoted retrograde transport. In both cases, MARK2 enhanced the effects. The results identify MARK2 as an upstream regulator of PINK1 and ΔN-PINK1 and provide insights into the regulation of mitochondrial trafficking in neurons and neurodegeneration in PD.

A functional analysis of MELK in cell division reveals a transition in the mode of cytokinesis during Xenopus development

MELK is a serine/threonine kinase involved in several cell processes, including the cell cycle, proliferation, apoptosis and mRNA processing. However, its function remains elusive. Here, we explored its role in the Xenopus early embryo and show by knockdown that xMELK (Xenopus MELK) is necessary for completion of cell division. Consistent with a role in cell division, endogenous xMELK accumulates at the equatorial cortex of anaphase blastomeres. Its relocalization is highly dynamic and correlates with a conformational rearrangement in xMELK. Overexpression of xMELK leads to failure of cytokinesis and impairs accumulation at the division furrow of activated RhoA - a pivotal regulator of cytokinesis. Furthermore, endogenous xMELK associates and colocalizes with the cytokinesis organizer anillin. Unexpectedly, our study reveals a transition in the mode of cytokinesis correlated to cell size and that implicates xMELK. Collectively, our findings disclose the importance of xMELK in cytokinesis during early development and show that the mechanism of cytokinesis changes during Xenopus early development.

Origin of animal epithelia: insights from the sponge genome

Epithelial tissues are a key metazoan cell type, providing a basic structural unit for the construction of diverse animal body plans. Historically, an epithelial grade of organization was considered to be restricted to the Eumetazoa, with the majority of cell layers described for Porifera lacking any of the conserved ultrastructural characteristics of epithelia. Now with the use of genomic information from the demosponge, Amphimedon queenslandica, we identify orthologs of bilaterian genes that determine epithelial cell polarity or encode components of specialized epithelial junctions and extracellular matrix structures. Amphimedon possesses orthologs of most bilaterian epithelial polarity and adherens junction genes but few or no tight junction, septate junction, or basal lamina genes. To place this information in an evolutionary context, we extended these analyses to the completed genomes of various fungi, the choanoflagellate, Monosiga brevicollis, the placozoan, Trichoplax adhaerens, and the cnidarian, Nematostella vectensis. The results indicate that the majority of "epithelial" genes originated in metazoan or eumetazoan lineages, with only two genes, Par-1 and Discs large, antedating the choanoflagellate-metazoan split. We further explored the mechanism of evolution for each of these genes by tracking the origin of constituent domains and domain combinations. In general, domain configurations found in contemporary bilaterians are inferred to have evolved early in metazoan evolution and are identical or similar to those present in representatives of modern cnidarians, placozoans, and demosponges.

Kin1 is a plasma membrane-associated kinase that regulates the cell surface in fission yeast

Cell morphogenesis is a complex process that depends on cytoskeleton and membrane organization, intracellular signalling and vesicular trafficking. The rod shape of the fission yeast Schizosaccharomyces pombe and the availability of powerful genetic tools make this species an excellent model to study cell morphology. Here we have investigated the function of the conserved Kin1 kinase. Kin1-GFP associates dynamically with the plasma membrane at sites of active cell surface remodelling and is present in the membrane fraction. Kin1Δ null cells show severe defects in cell wall structure and are unable to maintain a rod shape. To explore Kin1 primary function, we constructed an ATP analogue-sensitive allele kin1-as1. Kin1 inhibition primarily promotes delocalization of plasma membrane-associated markers of actively growing cell surface regions. Kin1 itself is depolarized and its mobility is strongly reduced. Subsequently, amorphous cell wall material accumulates at the cell surface, a phenotype that is dependent on vesicular trafficking, and the cell wall integrity mitogen-activated protein kinase pathway is activated. Deletion of cell wall integrity mitogen-activated protein kinase components reduces kin1Δ hypersensitivity to stresses such as those induced by Calcofluor white and SDS. We propose that Kin1 is required for a tight link between the plasma membrane and the cell wall.

DAPK activates MARK1/2 to regulate microtubule assembly, neuronal differentiation, and tau toxicity

Death-associated protein kinase (DAPK) is a key player in several modes of neuronal death/injury and has been implicated in the late-onset Alzheimer's disease (AD). DAPK promotes cell death partly through its effect on regulating actin cytoskeletons. In this study, we report that DAPK inhibits microtubule (MT) assembly by activating MARK/PAR-1 family kinases MARK1/2, which destabilize MT by phosphorylating tau and related MAP2/4. DAPK death domain, but not catalytic activity, is responsible for this activation by binding to MARK1/2 spacer region, thereby disrupting an intramolecular interaction that inhibits MARK1/2. Accordingly, DAPK(-/-) mice brain displays a reduction of tau phosphorylation and DAPK enhances the effect of MARK2 on regulating polarized neurite outgrowth. Using a well-characterized Drosophila model of tauopathy, we show that DAPK exerts an effect in part through MARK Drosophila ortholog PAR-1 to induce rough eye and loss of photoreceptor neurons. Furthermore, DAPK enhances tau toxicity through a PAR-1 phosphorylation-dependent mechanism. Together, our study reveals a novel mechanism of MARK activation, uncovers DAPK functions in modulating MT assembly and neuronal differentiation, and provides a molecular link of DAPK to tau phosphorylation, an event associated with AD pathology.

Kinase associated-1 domains drive MARK/PAR1 kinases to membrane targets by binding acidic phospholipids

Phospholipid-binding modules such as PH, C1, and C2 domains play crucial roles in location-dependent regulation of many protein kinases. Here, we identify the KA1 domain (kinase associated-1 domain), found at the C terminus of yeast septin-associated kinases (Kcc4p, Gin4p, and Hsl1p) and human MARK/PAR1 kinases, as a membrane association domain that binds acidic phospholipids. Membrane localization of isolated KA1 domains depends on phosphatidylserine. Using X-ray crystallography, we identified a structurally conserved binding site for anionic phospholipids in KA1 domains from Kcc4p and MARK1. Mutating this site impairs membrane association of both KA1 domains and intact proteins and reveals the importance of phosphatidylserine for bud neck localization of yeast Kcc4p. Our data suggest that KA1 domains contribute to "coincidence detection," allowing kinases to bind other regulators (such as septins) only at the membrane surface. These findings have important implications for understanding MARK/PAR1 kinases, which are implicated in Alzheimer's disease, cancer, and autism.

Remodeling epithelial cell organization: transitions between front-rear and apical-basal polarity

Polarized epithelial cells have a distinctive apical-basal axis of polarity for vectorial transport of ions and solutes across the epithelium. In contrast, migratory mesenchymal cells have a front-rear axis of polarity. During development, mesenchymal cells convert to epithelia by coalescing into aggregates that undergo epithelial differentiation. Signaling networks and protein complexes comprising Rho family GTPases, polarity complexes (Crumbs, PAR, and Scribble), and their downstream effectors, including the cytoskeleton and the endocytic and exocytic vesicle trafficking pathways, together regulate the distributions of plasma membrane and cytoskeletal proteins between front-rear and apical-basal polarity. The challenge is to understand how these regulators and effectors are adapted to regulate symmetry breaking processes that generate cell polarities that are specialized for different cellular activities and functions.

LKB1 deletion with the RIP2.Cre transgene modifies pancreatic beta-cell morphology and enhances insulin secretion in vivo

The tumor suppressor liver kinase B1 (LKB1), also called STK11, is a protein kinase mutated in Peutz-Jeghers syndrome. LKB1 phosphorylates AMP-activated protein kinase (AMPK) and several related protein kinases. Whereas deletion of both catalytic isoforms of AMPK from the pancreatic beta-cell and hypothalamic neurons using the rat insulin promoter (RIP2).Cre transgene (betaAMPKdKO) diminishes insulin secretion in vivo, deletion of LKB1 in the beta-cell with an inducible Pdx-1.CreER transgene enhances insulin secretion in mice. To determine whether the differences between these models reflect genuinely distinct roles for the two kinases in the beta-cell or simply differences in the timing and site(s) of deletion, we have therefore created mice deleted for LKB1 with the RIP2.Cre transgene. In marked contrast to betaAMPKdKO mice, betaLKB1KO mice showed diminished food intake and weight gain, enhanced insulin secretion, unchanged insulin sensitivity, and improved glucose tolerance. In line with the phenotype of Pdx1-CreER mice, total beta-cell mass and the size of individual islets and beta-cells were increased and islet architecture was markedly altered in betaLKB1KO islets. Signaling by mammalian target of rapamycin (mTOR) to eIF4-binding protein-1 and ribosomal S6 kinase was also enhanced. In contrast to Pdx1-CreER-mediated deletion, the expression of Glut2, glucose-induced changes in membrane potential and intracellular Ca(2+) were sharply reduced in betaLKB1KO mouse islets and the stimulation of insulin secretion was modestly inhibited. We conclude that LKB1 and AMPK play distinct roles in the control of insulin secretion and that the timing of LKB1 deletion, and/or its loss from extrapancreatic sites, influences the final impact on beta-cell function.

The tau of MARK: a polarized view of the cytoskeleton

Microtubule-affinity regulating kinases (MARKs) were originally discovered by their ability to phosphorylate tau protein and related microtubule-associated proteins (MAPs), and thereby to regulate microtubule dynamics in neurons. Members of the MARK (also known as partition-defective [Par]-1 kinase) family were subsequently found to be highly conserved and to have key roles in cell processes such as determination of polarity, cell-cycle control, intracellular signal transduction, transport and cytoskeleton. This is important for neuronal differentiation, but is also prominent in neurodegenerative 'tauopathies' such as Alzheimer's disease. The identified functions of MARK/Par-1 are diverse and require accurate regulation. Recent discoveries including the x-ray structure of human MARKs contributed to an increased understanding of the mechanisms that control the kinase activity and, thus, the actin and microtubule cytoskeleton.

RalA and the exocyst complex influence neuronal polarity through PAR-3 and aPKC

Neuronal polarization requires localized cytoskeletal changes and polarized membrane traffic. Here, I report that the small GTPase RalA, previously shown to control neurite branching, also regulates neuronal polarity. RalA depletion, or ectopic expression of constitutively active RalA in cultured neurons inhibit axon formation. However, expression of a constitutively active RalA mutant that is unable to interact with the exocyst complex has no effect on neuronal polarization. Furthermore, depletion of the Sec6, Sec8 or Exo84 subunits of the exocyst complex also leads to unpolarized neurons. Early stages of neuronal polarization are accompanied by increasing levels of interaction of the exocyst complex with PAR-3 and atypical protein kinase C (aPKC), and by the RalA-dependent association of the exocyst complex with PAR-3. Thus, neuronal polarization involves a RalA-regulated association between mediators of vesicle trafficking (exocyst complex) and cell polarity (PAR-3).