Ribosomes are the molecular machines that translate mRNAs into proteins. The synthesis of ribosomes is therefore a fundamental cellular process and consists in the ordered assembly of 79 ribosomal proteins (r-proteins) and four ribosomal RNAs (rRNAs) into a small 40S and a large 60S ribosomal subunit that form the translating 80S ribosomes. Most of our knowledge concerning this dynamic multi-step process comes from studies with the yeast Saccharomyces cerevisiae, which have shown that assembly and maturation of pre-ribosomal particles, as they travel from the nucleolus to the cytoplasm, relies on a multitude (>200) of biogenesis factors. Amongst these are many energy-consuming enzymes, including 19 ATP-dependent RNA helicases and three AAA-ATPases. We have previously shown that the AAA-ATPase Rix7 promotes the release of the essential biogenesis factor Nsa1 from late nucleolar pre-60S particles. Here we show that mutant alleles of genes encoding the DEAD-box RNA helicase Mak5, the C/D-box snoRNP component Nop1 and the rRNA-binding protein Nop4 bypass the requirement for Nsa1. Interestingly, dominant-negative alleles of RIX7 retain their phenotype in the absence of Nsa1, suggesting that Rix7 may have additional nuclear substrates besides Nsa1. Mak5 is associated with the Nsa1 pre-60S particle and synthetic lethal screens with mak5 alleles identified the r-protein Rpl14 and the 60S biogenesis factors Ebp2, Nop16 and Rpf1, which are genetically linked amongst each other. We propose that these ’Mak5 cluster’ factors orchestrate the structural arrangement of a eukaryote-specific 60S subunit surface composed of Rpl6, Rpl14 and Rpl16 and rRNA expansion segments ES7L and ES39L. Finally, over-expression of Rix7 negatively affects growth of mak5 and ebp2 mutant cells both in the absence and presence of Nsa1, suggesting that Rix7, at least when excessively abundant, may act on structurally defective pre-60S subunits and may subject these to degradation.
Ribosome biogenesis in eukaryotes depends on the coordinated action of ribosomal and nonribosomal proteins that guide the assembly of preribosomal particles. These intermediate particles follow a maturation pathway in which important changes in their protein composition occur. The mechanisms involved in the coordinated assembly of the ribosomal particles are poorly understood. We show here that the association of preribosomal factors with pre-60S complexes depends on the presence of earlier factors, a phenomenon essential for ribosome biogenesis. The analysis of the composition of purified preribosomal complexes blocked in maturation at specific steps allowed us to propose a model of sequential protein association with, and dissociation from, early pre-60S complexes for several preribosomal factors such as Mak11, Ssf1, Rlp24, Nog1, and Nog2. The presence of either Ssf1 or Nog2 in complexes that contain the 27SB pre-rRNA defines novel, distinct pre-60S particles that contain the same pre-rRNA intermediates and that differ only by the presence or absence of specific proteins. Physical and functional interactions between Rlp24 and Nog1 revealed that the assembly steps are, at least in part, mediated by direct protein-protein interactions.
Cell fusion in genetically identical Neurospora crassa germlings and in hyphae is a highly regulated process involving the activation of a conserved MAP kinase cascade that includes NRC-1, MEK-2 and MAK-2. During chemotrophic growth in germlings, the MAP kinase cascade members localize to conidial anastomosis tube (CAT) tips every ∼8 minutes, perfectly out of phase with another protein that is recruited to the tip: SOFT, a recently identified scaffold for the MAK-1 MAP kinase pathway in Sordaria macrospora. How the MAK-2 oscillation process is initiated, maintained and what proteins regulate the MAP kinase cascade is currently unclear. A global phosphoproteomics approach using an allele of mak-2 (mak-2Q100G) that can be specifically inhibited by the ATP analog 1NM-PP1 was utilized to identify MAK-2 kinase targets in germlings that were potentially involved in this process. One such putative target was HAM-5, a protein of unknown biochemical function. Previously, Δham-5 mutants were shown to be deficient for hyphal fusion. Here we show that HAM-5-GFP co-localized with NRC-1, MEK-2 and MAK-2 and oscillated with identical dynamics from the cytoplasm to CAT tips during chemotropic interactions. In the Δmak-2 strain, HAM-5-GFP localized to punctate complexes that did not oscillate, but still localized to the germling tip, suggesting that MAK-2 activity influences HAM-5 function/localization. However, MAK-2-GFP showed cytoplasmic and nuclear localization in a Δham-5 strain and did not localize to puncta. Via co-immunoprecipitation experiments, HAM-5 was shown to physically interact with NRC-1, MEK-2 and MAK-2, suggesting that it functions as a scaffold/transport hub for the MAP kinase cascade members for oscillation and chemotropic interactions during germling and hyphal fusion in N. crassa. The identification of HAM-5 as a scaffold-like protein will help to link the activation of MAK-2 cascade to upstream factors and proteins involved in this intriguing process of fungal communication.
Cell fusion between genetically identical cells of the fungus Neurospora crassa occurs when germinating asexual cells (conidia) sense each other's proximity and redirect their growth. Chemotropic growth is dependent upon the assembly of a MAPK cascade (NRC-1/MEK-2/MAK-2) at the cell cortex (conidial anastomosis tubes; CATs), followed by disassembly over an ∼8 min cycle. A second protein required for fusion, SO, also assembles and disassembles at CAT tips during chemotropic growth, but with perfectly opposite dynamics to the MAK-2 complex. This process of germling chemotropism, oscillation and cell fusion is regulated by many genes and is poorly understood. Via a phosphoproteomics approach, we identify HAM-5, which functions as a scaffold for the MAK-2 signal transduction complex. HAM-5 is required for assembly/disassembly and oscillation of the MAK-2 complex during chemotropic growth. Our data supports a model whereby regulated modification of HAM-5 controls the disassembly of the MAK-2 MAPK complex and is essential for modulating the tempo of oscillation during chemotropic interactions.
The Schizosaccharomyces pombe stress-activated
Sty1p/Spc1p mitogen-activated protein (MAP) kinase regulates gene
expression through the Atf1p and Pap1p transcription factors, homologs
of human ATF2 and c-Jun, respectively. Mcs4p, a response
regulator protein, acts upstream of Sty1p by binding the Wak1p/Wis4p
MAP kinase kinase kinase. We show that phosphorylation of Mcs4p on a
conserved aspartic acid residue is required for activation of Sty1p
only in response to peroxide stress. Mcs4p acts in a conserved
phospho-relay system initiated by two PAS/PAC domain-containing
histidine kinases, Mak2p and Mak3p. In the absence of Mak2p or Mak3p,
Sty1p fails to phosphorylate the Atf1p transcription factor or induce
Atf1p-dependent gene expression. As a consequence, cells lacking Mak2p
and Mak3p are sensitive to peroxide attack in the absence of Prr1p, a
distinct response regulator protein that functions in association with
Pap1p. The Mak1p histidine kinase, which also contains PAS/PAC repeats,
does not regulate Sty1p or Atf1p but is partially required for Pap1p-
and Prr1p-dependent transcription. We conclude that the transcriptional
response to free radical attack is initiated by at least two distinct
phospho-relay pathways in fission yeast.
Intercellular communication is critical for the survival of unicellular organisms as well as for the development and function of multicellular tissues. Cell-to-cell signaling is also required to develop the interconnected mycelial network characteristic of filamentous fungi and is a prerequisite for symbiotic and pathogenic host colonization achieved by molds. Somatic cell–cell communication and subsequent cell fusion is governed by the MAK-2 mitogen activated protein kinase (MAPK) cascade in the filamentous ascomycete model Neurospora crassa, yet the composition and mode of regulation of the MAK-2 pathway are currently unclear. In order to identify additional components involved in MAK-2 signaling we performed affinity purification experiments coupled to mass spectrometry with strains expressing functional GFP-fusion proteins of the MAPK cascade. This approach identified STE-50 as a regulatory subunit of the Ste11p homolog NRC-1 and HAM-5 as cell-communication-specific scaffold protein of the MAPK cascade. Moreover, we defined a network of proteins consisting of two Ste20-related kinases, the small GTPase RAS-2 and the adenylate cyclase capping protein CAP-1 that function upstream of the MAK-2 pathway and whose signals converge on the NRC-1/STE-50 MAP3K complex and the HAM-5 scaffold. Finally, our data suggest an involvement of the striatin interacting phosphatase and kinase (STRIPAK) complex, the casein kinase 2 heterodimer, the phospholipid flippase modulators YPK-1 and NRC-2 and motor protein-dependent vesicle trafficking in the regulation of MAK-2 pathway activity and function. Taken together, these data will have significant implications for our mechanistic understanding of MAPK signaling and for homotypic cell–cell communication in fungi and higher eukaryotes.
Appropriate cellular responses to external stimuli depend on the highly orchestrated activity of interconnected signaling cascades. One crucial level of control arises from the formation of discrete complexes through scaffold proteins that bind multiple components of a given pathway. Central for our understanding of these signaling platforms is the archetypical MAP kinase scaffold Ste5p, a protein that is restricted to budding yeast and close relatives. We identified HAM-5, a protein highly conserved in filamentous ascomycete fungi, as cell–cell communication-specific scaffold protein of the Neurospora crassa MAK-2 cascade (homologous to the budding yeast pheromone pathway). We also describe a network of upstream acting proteins, consisting of two Ste20-related kinases, the small G-protein RAS-2 and the adenylate cyclase capping protein CAP-1, whose signals converge on HAM-5. Our work has implications for the mechanistic understanding of MAP kinase scaffold proteins and their function during intercellular communication in eukaryotic microbes as well as higher eukaryotes.
The circadian clock regulates the expression of many genes involved in a wide range of biological functions through output pathways such as mitogen-activated protein kinase (MAPK) pathways. We demonstrate here that the clock regulates the phosphorylation, and thus activation, of the MAPKs MAK-1 and MAK-2 in the filamentous fungus Neurospora crassa. In this study, we identified genetic targets of the MAK-1 pathway, which is homologous to the cell wall integrity pathway in Saccharomyces cerevisiae and the extracellular signal-regulated kinase 1/2 (ERK1/2) pathway in mammals. When MAK-1 was deleted from Neurospora cells, vegetative growth was reduced and the transcript levels for over 500 genes were affected, with significant enrichment for genes involved in protein synthesis, biogenesis of cellular components, metabolism, energy production, and transcription. Additionally, of the ∼500 genes affected by the disruption of MAK-1, more than 25% were previously identified as putative clock-controlled genes. We show that MAK-1 is necessary for robust rhythms of two morning-specific genes, i.e., ccg-1 and the mitochondrial phosphate carrier protein gene NCU07465. Additionally, we show clock regulation of a predicted chitin synthase gene, NCU04352, whose rhythmic accumulation is also dependent upon MAK-1. Together, these data establish a role for the MAK-1 pathway as an output pathway of the circadian clock and suggest a link between rhythmic MAK-1 activity and circadian control of cellular growth.
Mitogen-activated protein (MAP) kinase signaling pathways are ubiquitous and evolutionarily conserved in eukaryotic organisms. MAP kinase pathways are composed of a MAP kinase, a MAP kinase kinase, and a MAP kinase kinase kinase; activation is regulated by sequential phosphorylation. Components of three MAP kinase pathways have been identified by genome sequence analysis in the filamentous fungus Neurospora crassa. One of the predicted MAP kinases in N. crassa, MAK-2, shows similarity to Fus3p and Kss1p of Saccharomyces cerevisiae, which are involved in sexual reproduction and filamentation, respectively. In this study, we show that an N. crassa mutant disrupted in mak-2 exhibits a pleiotropic phenotype: derepressed conidiation, shortened aerial hyphae, lack of vegetative hyphal fusion, female sterility, and autonomous ascospore lethality. We assessed the phosphorylation of MAK-2 during conidial germination and early colony development. Peak levels of MAK-2 phosphorylation were most closely associated with germ tube elongation, branching, and hyphal fusion events between conidial germlings. A MAP kinase kinase kinase (NRC-1) is the predicted product of N. crassa nrc-1 locus and is a homologue of STE11 in S. cerevisiae. An nrc-1 mutant shares many of the same phenotypic traits as the mak-2 mutant and, in particular, is a hyphal fusion mutant. We show that MAK-2 phosphorylation during early colony development is dependent upon the presence of NRC-1 and postulate that phosphorylation of MAK-2 is required for hyphal fusion events that occur during conidial germination.
Protein Nα-terminal acetylation is one of the most common protein modifications in eukaryotic cells. In yeast, three major complexes, NatA, NatB, and NatC, catalyze nearly all N-terminal acetylation, acetylating specific subsets of protein N termini. In human cells, only the NatA and NatB complexes have been described. We here identify and characterize the human NatC (hNatC) complex, containing the catalytic subunit hMak3 and the auxiliary subunits hMak10 and hMak31. This complex associates with ribosomes, and hMak3 acetylates Met-Leu protein N termini in vitro, suggesting a model in which the human NatC complex functions in cotranslational N-terminal acetylation. Small interfering RNA-mediated knockdown of NatC subunits results in p53-dependent cell death and reduced growth of human cell lines. As a consequence of hMAK3 knockdown, p53 is stabilized and phosphorylated and there is a significant transcriptional activation of proapoptotic genes downstream of p53. Knockdown of hMAK3 alters the subcellular localization of the Arf-like GTPase hArl8b, supporting that hArl8b is a hMak3 substrate in vivo. Taken together, hNatC-mediated N-terminal acetylation is important for maintenance of protein function and cell viability in human cells.
Allelic forms of DRG1/AFG2 confer resistance to the drug diazaborine, an inhibitor of ribosome biogenesis in Saccharomyces cerevisiae. Our results show that the AAA-ATPase Drg1 is essential for 60S maturation and associates with 60S precursor particles in the cytoplasm. Functional inactivation of Drg1 leads to an increased cytoplasmic localization of shuttling pre-60S maturation factors like Rlp24, Arx1, and Tif6. Surprisingly, Nog1, a nuclear pre-60S factor, was also relocalized to the cytoplasm under these conditions, suggesting that it is a previously unsuspected shuttling preribosomal factor that is exported with the precursor particles and very rapidly reimported. Proteins that became cytoplasmic under drg1 mutant conditions were blocked on pre-60S particles at a step that precedes the association of Rei1, a later-acting preribosomal factor. A similar cytoplasmic accumulation of Nog1 and Rlp24 in pre-60S-bound form could be seen after overexpression of a dominant-negative Drg1 variant mutated in the D2 ATPase domain. We conclude that the ATPase activity of Drg1 is required for the release of shuttling proteins from the pre-60S particles shortly after their nuclear export. This early cytoplasmic release reaction defines a novel step in eukaryotic ribosome maturation.
Over 30 MAK (maintenance of killer) genes are necessary for propagation of the killer toxin-encoding M1 satellite double-stranded RNA of the L-A virus. Sequence analysis revealed that MAK7 is RPL4A, one of the two genes encoding ribosomal protein L4 of the 60S subunit. We further found that mutants with mutations in 18 MAK genes (including mak1 [top1], mak7 [rpl4A], mak8 [rpl3], mak11, and mak16) had decreased free 60S subunits. Mutants with another three mak mutations had half-mer polysomes, indicative of poor association of 60S and 40S subunits. The rest of the mak mutants, including the mak3 (N-acetyltransferase) mutant, showed a normal profile. The free 60S subunits, L-A copy number, and the amount of L-A coat protein in the mak1, mak7, mak11, and mak16 mutants were raised to the normal level by the respective normal single-copy gene. Our data suggest that most mak mutations affect M1 propagation by their effects on the supply of proteins from the L-A virus and that the translation of the non-poly(A) L-A mRNA depends critically on the amount of free 60S ribosomal subunits, probably because 60S association with the 40S subunit waiting at the initiator AUG is facilitated by the 3' poly(A).
ETOC: Ribosome synthesis is a multistep process initiated in the nucleolus with the transcription of a precursor rRNA that is subjected to a series of modification and processing steps to generate the mature rRNA. In this paper, we describe a novel 60S ribosome biogenesis complex associating with LAS1L that controls rRNA processing and synthesis of the 28S rRNA.
The coordination of RNA polymerase I transcription with pre-rRNA processing, preribosomal particle assembly, and nuclear export is a finely tuned process requiring the concerted actions of a number of accessory factors. However, the exact functions of some of these proteins and how they assemble in subcomplexes remain poorly defined. LAS1L was first described as a nucleolar protein required for maturation of the 60S preribosomal subunit. In this paper, we demonstrate that LAS1L interacts with PELP1, TEX10, and WDR18, the mammalian homologues of the budding yeast Rix1 complex, along with NOL9 and SENP3, to form a novel nucleolar complex that cofractionates with the 60S preribosomal subunit. Depletion of LAS1L-associated proteins results in a p53-dependent G1 arrest and leads to defects in processing of the pre-rRNA internal transcribed spacer 2 region. We further show that the nucleolar localization of this complex requires active RNA polymerase I transcription and the small ubiquitin-like modifier–specific protease SENP3. Taken together, our data identify a novel mammalian complex required for 60S ribosomal subunit synthesis, providing further insight into the intricate, yet poorly described, process of ribosome biogenesis in higher eukaryotes.
Cell communication is essential for eukaryotic development, but our knowledge of molecules and mechanisms required for intercellular communication is fragmentary. In particular, the connection between signal sensing and regulation of cell polarity is poorly understood. In the filamentous ascomycete Neurospora crassa, germinating spores mutually attract each other and subsequently fuse. During these tropic interactions, the two communicating cells rapidly alternate between two different physiological states, probably associated with signal delivery and response. The MAK2 MAP kinase cascade mediates cell–cell signaling. Here, we show that the conserved scaffolding protein HYM1/MO25 controls the cell shape-regulating NDR kinase module as well as the signal-receiving MAP kinase cascade. HYM1 functions as an integral part of the COT1 NDR kinase complex to regulate the interaction with its upstream kinase POD6 and thereby COT1 activity. In addition, HYM1 interacts with NRC1, MEK2, and MAK2, the three kinases of the MAK2 MAP kinase cascade, and co-localizes with MAK2 at the apex of growing cells. During cell fusion, the three kinases of the MAP kinase module as well as HYM1 are recruited to the point of cell–cell contact. hym-1 mutants phenocopy all defects observed for MAK2 pathway mutants by abolishing MAK2 activity. An NRC1-MEK2 fusion protein reconstitutes MAK2 signaling in hym-1, while constitutive activation of NRC1 and MEK2 does not. These data identify HYM1 as a novel regulator of the NRC1-MEK2-MAK2 pathway, which may coordinate NDR and MAP kinase signaling during cell polarity and intercellular communication.
Intercellular communication and cellular morphogenesis are essential for eukaryotic development. Our knowledge of molecules and mechanisms associated with these processes is, however, fragmentary. In particular, the molecular connection between signal sensing and regulation of cell polarity is poorly understood. Fungal hyphae share with neurons and pollen tubes the distinction of being amongst the most highly polarized cells in biology. The robust genetic tractability of filamentous fungi provides an unparalleled opportunity to determine common principles that underlie polarized growth and its regulation through cell communication. In Neurospora crassa, germinating spores mutually attract each other, establish physical contact through polarized tropic growth, and fuse. During this process, the cells rapidly alternate between two different physiological states, probably associated with signal delivery and response. Here, we show that the conserved scaffolding protein HYM1/MO25 interacts with the polarity and cell shape-regulating NDR kinase complex as well as a MAP kinase module, which is essential for cell communication during the tropic interaction. We propose that this dual use of a common regulator in both molecular complexes may represent an intriguing mechanism of linking the perception of external cues with the polarization machinery to coordinate communication and tropic growth of interacting cells.
Mitogen-activated protein kinase (MAPK) pathways are crucial signaling instruments in eukaryotes. Most ascomycetes possess three MAPK modules that are involved in key developmental processes like sexual propagation or pathogenesis. However, the regulation of these modules by adapters or scaffolds is largely unknown. Here, we studied the function of the cell wall integrity (CWI) MAPK module in the model fungus Sordaria macrospora. Using a forward genetic approach, we found that sterile mutant pro30 has a mutated mik1 gene that encodes the MAPK kinase kinase (MAPKKK) of the proposed CWI pathway. We generated single deletion mutants lacking MAPKKK MIK1, MAPK kinase (MAPKK) MEK1, or MAPK MAK1 and found them all to be sterile, cell fusion-deficient and highly impaired in vegetative growth and cell wall stress response. By searching for MEK1 interaction partners via tandem affinity purification and mass spectrometry, we identified previously characterized developmental protein PRO40 as a MEK1 interaction partner. Although fungal PRO40 homologs have been implicated in diverse developmental processes, their molecular function is currently unknown. Extensive affinity purification, mass spectrometry, and yeast two-hybrid experiments showed that PRO40 is able to bind MIK1, MEK1, and the upstream activator protein kinase C (PKC1). We further found that the PRO40 N-terminal disordered region and the central region encompassing a WW interaction domain are sufficient to govern interaction with MEK1. Most importantly, time- and stress-dependent phosphorylation studies showed that PRO40 is required for MAK1 activity. The sum of our results implies that PRO40 is a scaffold protein for the CWI pathway, linking the MAPK module to the upstream activator PKC1. Our data provide important insights into the mechanistic role of a protein that has been implicated in sexual and asexual development, cell fusion, symbiosis, and pathogenicity in different fungal systems.
The specific response to environmental cues is crucial for cell differentiation and is often mediated by highly conserved eukaryotic MAP kinase (MAPK) pathways. How these pathways react specifically to huge numbers of different cues is still unclear, and current literature about adapter and scaffolding proteins remains scarce. However, gaining fundamental insight into molecular signaling determinants is pivotal for combating diseases with impaired signal transduction processes, such as Alzheimer's disease or cancer. Importantly, signal transduction can easily be studied in lower eukaryotes like filamentous fungi that are readily genetically tractable. The fungus Sordaria macrospora has a long history as an ideal model system for cell differentiation, and we show here that the proposed cell wall integrity (CWI) MAPK module of this fungus controls differentiation of sexual fruiting bodies, cell fusion, polar growth and cell wall stress response. We further discovered that developmental protein PRO40 binds the MAPK kinase kinase (MAPKKK), the MAPK kinase (MAPKK) and upstream activator protein kinase C (PKC1) of the CWI pathway and is required for MAK1 activity, thereby providing evidence that PRO40 is a scaffold protein. Collectively, our findings reveal a molecular role for a protein implicated in development, cell fusion, symbiosis, and pathogenicity in different fungi.
Programmed −1 ribosomal frameshifting is utilized by a number of RNA viruses as a means of ensuring the correct ratio of viral structural to enzymatic proteins available for viral particle assembly. Altering frameshifting efficiencies upsets this ratio, interfering with virus propagation. We have previously demonstrated that compounds that alter the kinetics of the peptidyl-transfer reaction affect programmed −1 ribosomal frameshift efficiencies and interfere with viral propagation in yeast. Here, the use of a genetic approach lends further support to the hypothesis that alterations affecting the ribosome’s peptidyltransferase activity lead to changes in frameshifting efficiency and virus loss. Mutations in the RPL3 gene, which encodes a ribosomal protein located at the peptidyltransferase center, promote approximately three- to fourfold increases in programmed −1 ribosomal frameshift efficiencies and loss of the M1 killer virus of yeast. The mak8-1 allele of RPL3 contains two adjacent missense mutations which are predicted to structurally alter the Mak8-1p. Furthermore, a second allele that encodes the N-terminal 100 amino acids of L3 (called L3Δ) exerts a trans-dominant effect on programmed −1 ribosomal frameshifting and killer virus maintenance. Taken together, these results support the hypothesis that alterations in the peptidyltransferase center affect programmed −1 ribosomal frameshifting.
We isolated a novel gene designated mak (male germ cell-associated kinase) by using weak cross-hybridization with a tyrosine kinase gene (v-ros). Sequence analysis of the cDNA corresponding to the 2.6-kilobase transcript revealed that the predicted product of rat mak consisted of 622 amino acids and contained protein kinase consensus motifs in its amino-terminal region. Comparison of the deduced amino acid sequence of mak in the kinase domain with those of other protein kinase genes demonstrated that mak was approximately 40% identical to the cdc2-CDC28 gene family in Schizosaccharomyces pombe, Saccharomyces cerevisiae, and humans but less identical to most other protein kinase gene products. Expression of mak was highly tissue specific, and its transcripts were detected almost exclusively in testicular cells entering and after meiosis but hardly detectable in ovarian cells including oocytes, after the dictyotene stage. These results suggest that the mak gene plays an important role in spermatogenesis.
A large number of cell wall proteins are encoded in the Neurospora crassa genome. Strains carrying gene deletions of 65 predicted cell wall proteins were characterized. Deletion mutations in two of these genes (wsc-1 and ham-7) have easily identified morphological and inhibitor-based defects. Their phenotypic characterization indicates that HAM-7 and WSC-1 function during cell-to-cell hyphal fusion and in cell wall integrity maintenance, respectively. wsc-1 encodes a transmembrane protein with extensive homology to the yeast Wsc family of sensor proteins. In N. crassa, WSC-1 (and its homolog WSC-2) activates the cell wall integrity MAK-1 MAP kinase pathway. The GPI-anchored cell wall protein HAM-7 is required for cell-to-cell fusion and the sexual stages of the N. crassa life cycle. Like WSC-1, HAM-7 is required for activating MAK-1. A Δwsc-1;Δham-7 double mutant fully phenocopies mutants lacking components of the MAK-1 MAP kinase cascade. The data identify WSC-1 and HAM-7 as the major cell wall sensors that regulate two distinct MAK-1-dependent cellular activities, cell wall integrity and hyphal anastomosis, respectively.
Many analyses have examined subnucleolar structures in eukaryotic cells, but the relationship between morphological structures, pre-rRNA processing, and ribosomal particle assembly has remained unclear. Using a visual assay for export of the 60S ribosomal subunit, we isolated a ts-lethal mutation, rix9-1, which causes nucleolar accumulation of an Rpl25p-eGFP reporter construct. The mutation results in a single amino acid substitution (F176S) in Rlp7p, an essential nucleolar protein related to ribosomal protein Rpl7p. The rix9-1 (rlp7-1) mutation blocks the late pre-RNA cleavage at site C2 in ITS2, which separates the precursors to the 5.8S and 25S rRNAs. Consistent with this, synthesis of the mature 5.8S and 25S rRNAs was blocked in the rlp7-1 strain at nonpermissive temperature, whereas 18S rRNA synthesis continued. Moreover, pre-rRNA containing ITS2 accumulates in the nucleolus of rix9-1 cells as revealed by in situ hybridization. Finally, tagged Rlp7p was shown to associate with a pre-60S particle, and fluorescence microscopy and immuno-EM localized Rlp7p to a subregion of the nucleolus, which could be the granular component (GC). All together, these data suggest that pre-rRNA cleavage at site C2 specifically requires Rlp7p and occurs within pre-60S particles located in the GC region of the nucleolus.
rRNA processing; ribosome biogenesis; ribosome export; nucleolus; preribosome
Titin-like giant polypeptides of muscle have protein kinase domains near their C-termini. These kinases are autoinhibited by portions of their own sequences. A putative activator for Caenorhabditis elegans twitchin kinase, MAK-1 (MAPKAP kinase 2), is expressed in nematode striated muscle, partially colocalizes with twitchin in sarcomeres, and binds to and phosphorylates twitchin kinase in vitro.
In Caenorhabditis elegans, twitchin is a giant polypeptide located in muscle A-bands. The protein kinase of twitchin is autoinhibited by 45 residues upstream (NL) and 60 residues downstream (CRD) of the kinase catalytic core. Molecular dynamics simulation on a twitchin fragment revealed that the NL is released by pulling force. However, it is unclear how the CRD is removed. To identify proteins that may remove the CRD, we performed a yeast two-hybrid screen using twitchin kinase as bait. One interactor is MAK-1, C. elegans orthologue of MAPKAP kinase 2. MAPKAP kinase 2 is phosphorylated and activated by p38 MAP kinase. We demonstrate that the CRD of twitchin is important for binding to MAK-1. mak-1 is expressed in nematode body wall muscle, and antibodies to MAK-1 localize between and around Z-disk analogues and to the edge of A-bands. Whereas unc-22 mutants are completely resistant, mak-1 mutants are partially resistant to nicotine. MAK-1 can phosphorylate twitchin NL-Kin-CRD in vitro. Genetic data suggest the involvement of two other mak-1 paralogues and two orthologues of p38 MAP kinase. These results suggest that MAK-1 is an activator of twitchin kinase and that the p38 MAP kinase pathway may be involved in the regulation of twitchin.
The REIL1 and REIL2 proteins of Arabidopsis thaliana are evolutionarily conserved homologs of the cytosolic 60S ribosomal maturation factors Rei1 and its paralog Reh1 of Saccharomyces cerevisiae. We previously demonstrated that the REIL proteins like the yeast homologs are required for the growth of both organisms at suboptimal temperatures. In addition, the cold sensitivity of the yeast Δrei1 mutant was almost fully rescued by heterologous expression of the REIL1 protein. These phenomena and conservation of co-expressed genes linked the function of REIL proteins to the maturation of the eukaryotic ribosome in A. thaliana. Here we demonstrate that REIL proteins interact in yeast-2-hybrid assays with A. thaliana homologs of the yeast proteins, Rlp24, Rpl24A, Rlp24B, Arx1, and Jjj1. These proteins take part in the cytosolic 60S ribosomal maturation process within yeast and physically interact with Rei1. Our study does not provide proof but is consistent with a conserved role of the A. thaliana REIL proteins in ribosomal maturation and demonstrates the potential of future investigations that aim to unravel the protein interactions of REIL proteins in planta.
Cold sensitivity; REIL proteins; cytosolic maturation of the eukaryotic ribosome; leaf growth; protein–protein interaction; yeast-2-hybrid assay
ICK/MRK (intestinal cell kinase/MAK-related kinase), MAK (male germ cell-associated kinase), and MOK (MAPK/MAK/MRK-overlapping kinase) are closely related serine/threonine protein kinases in the protein kinome. The biological functions and regulatory mechanisms of the ICK/MAK/MOK family are still largely elusive. Despite significant similarities in their catalytic domains, they diverge markedly in the sequence and structural organization of their C-terminal non-catalytic domains, raising the question as to whether they have distinct, overlapping, or redundant biological functions. In order to gain insights into their biological activities and lay a fundamental groundwork for functional studies, we investigated the spatio-temporal distribution patterns and the expression dynamics of ICK/MAK/MOK protein kinases in the intestine. We found that ICK/MAK/MOK proteins display divergent expression patterns along the duodenum-to-colon axis and during postnatal murine development. Furthermore, they are differentially partitioned between intestinal epithelium and mesenchyme. A significant increase in the protein level of ICK, but not MAK, was induced in human primary colon cancer specimens. ICK protein level was up-regulated whereas MOK protein level was down-regulated in mouse intestinal adenomas as compared with their adjacent normal intestinal mucosa. These data suggest distinct roles for ICK/MAK/MOK protein kinases in the regulation of intestinal neoplasia. Taken together, our findings demonstrate that the expressions of ICK/MAK/MOK proteins in the intestinal tract can be differentially and dynamically regulated, implicating a significant functional diversity within this group of protein kinases.
MAK-V/Hunk is a scantily characterized AMPK-like protein kinase. Recent findings identified MAK-V as a pro-survival and anti-apoptotic protein and revealed its role in embryonic development as well as in tumorigenesis and metastasis. However molecular mechanisms of MAK-V action and regulation of its activity remain largely unknown. We identified Nedd4 as an interaction partner for MAK-V protein kinase. However, this HECT-type E3 ubiquitin ligase is not involved in the control of MAK-V degradation by the ubiquitin-proteasome system that regulates MAK-V abundance in cells. However, Nedd4 in an ubiquitin ligase-independent manner rescued developmental defects in Xenopus embryos induced by MAK-V overexpression, suggesting physiological relevance of interaction between MAK-V and Nedd4. This identifies Nedd4 as the first known regulator of MAK-V function.
A member of the mitogen-activated protein kinase superfamily, MAK, has been proposed to have an important role in spermatogenesis, since Mak gene expression is highly restricted to testicular germ cells. To assess the biological function of MAK, we have established MAK-deficient (Mak−/−) mice. Mak−/− mice developed normally, and no gross abnormalities were observed. Spermatogenesis of the Mak−/− mice was also intact, and most of the mice were fertile. However, Mak−/− male-derived litter sizes and their sperm motility in vitro were mildly reduced. These data show that function of MAK is not essential for spermatogenesis and male fertility.
A systems-level understanding of biological processes and information flow requires the mapping of cellular component interactions, among which protein–protein interactions are particularly important. Fission yeast (Schizosaccharomyces pombe) is a valuable model organism for which no systematic protein-interaction data are available. We exploited gene and protein properties, global genome regulation datasets, and conservation of interactions between budding and fission yeast to predict fission yeast protein interactions in silico. We have extensively tested our method in three ways: first, by predicting with 70–80% accuracy a selected high-confidence test set; second, by recapitulating interactions between members of the well-characterized SAGA co-activator complex; and third, by verifying predicted interactions of the Cbf11 transcription factor using mass spectrometry of TAP-purified protein complexes. Given the importance of the pathway in cell physiology and human disease, we explore the predicted sub-networks centered on the Tor1/2 kinases. Moreover, we predict the histidine kinases Mak1/2/3 to be vital hubs in the fission yeast stress response network, and we suggest interactors of argonaute 1, the principal component of the siRNA-mediated gene silencing pathway, lost in budding yeast but preserved in S. pombe. Of the new high-quality interactions that were discovered after we started this work, 73% were found in our predictions. Even though any predicted interactome is imperfect, the protein network presented here can provide a valuable basis to explore biological processes and to guide wet-lab experiments in fission yeast and beyond. Our predicted protein interactions are freely available through PInt, an online resource on our website (www.bahlerlab.info/PInt).
Cbf11; TOR; Mak1/2/3; support vector machine; random forest
Killer strains of Saccharomyces cerevisiae are those carrying a 1.5 x 10(6)-dalton double-stranded (ds) ribonucleic acid (RNA) (M) in virus-like particles and secreting a protein toxin. Most yeast (koller or not) also carry a 3 x 10(6)-dalton dsRNA (L). We have mapped mutations in eight of the chromosomal genes needed for maintaining M (mak genes). The mak genes are widely distributed on the yeast map, with no multigene complexes. We show that mutants defective in these and other mak genes lose M dsRNA, but not L dsRNA. The mak3-1 mutation results in markedly decreased cellular levels of L dsRNA, but mak3-1 stains do not lose L dsRNA completely. Mutation of mak16 results in temperature-sensitive growth, whereas mutations in mak13, mak15, mak17, mak20, mak22, and mak27 result in slow growth at any temperature. No effect of mak mutations on mating, meiosis, sporulation, germination, homothallism, or ultraviolet sensitivity has been found. The specificity of mak mutations is discussed.
Ribosome biogenesis requires >300 assembly factors in Saccharomyces cerevisiae. Ribosome assembly factors Imp3, Mrt4, Rlp7 and Rlp24 have sequence similarity to ribosomal proteins S9, P0, L7 and L24, suggesting that these pre-ribosomal factors could be placeholders that prevent premature assembly of the corresponding ribosomal proteins to nascent ribosomes. However, we found L7 to be a highly specific component of Rlp7-associated complexes, revealing that the two proteins can bind simultaneously to pre-ribosomal particles. Cross-linking and cDNA analysis experiments showed that Rlp7 binds to the ITS2 region of 27S pre-rRNAs, at two sites, in helix III and in a region adjacent to the pre-rRNA processing sites C1 and E. However, L7 binds to mature 25S and 5S rRNAs and cross-linked predominantly to helix ES7Lb within 25S rRNA. Thus, despite their predicted structural similarity, our data show that Rlp7 and L7 clearly bind at different positions on the same pre-60S particles. Our results also suggest that Rlp7 facilitates the formation of the hairpin structure of ITS2 during 60S ribosomal subunit maturation.