Light signals are transduced to the phosphorylation of 15 kDa proteins in Neurospora crassa

Epigenetic and Posttranslational Modifications in Light Signal Transduction and the Circadian Clock in Neurospora crassa

Blue light, a key abiotic signal, regulates a wide variety of physiological processes in many organisms. One of these phenomena is the circadian rhythm presents in organisms sensitive to the phase-setting effects of blue light and under control of the daily alternation of light and dark. Circadian clocks consist of autoregulatory alternating negative and positive feedback loops intimately connected with the cellular metabolism and biochemical processes. Neurospora crassa provides an excellent model for studying the molecular mechanisms involved in these phenomena. The White Collar Complex (WCC), a blue-light receptor and transcription factor of the circadian oscillator, and Frequency (FRQ), the circadian clock pacemaker, are at the core of the Neurospora circadian system. The eukaryotic circadian clock relies on transcriptional/translational feedback loops: some proteins rhythmically repress their own synthesis by inhibiting the activity of their transcriptional factors, generating self-sustained oscillations over a period of about 24 h. One of the basic mechanisms that perpetuate self-sustained oscillations is post translation modification (PTM). The acronym PTM generically indicates the addition of acetyl, methyl, sumoyl, or phosphoric groups to various types of proteins. The protein can be regulatory or enzymatic or a component of the chromatin. PTMs influence protein stability, interaction, localization, activity, and chromatin packaging. Chromatin modification and PTMs have been implicated in regulating circadian clock function in Neurospora. Research into the epigenetic control of transcription factors such as WCC has yielded new insights into the temporal modulation of light-dependent gene transcription. Here we report on epigenetic and protein PTMs in the regulation of the Neurospora crassa circadian clock. We also present a model that illustrates the molecular mechanisms at the basis of the blue light control of the circadian clock.

Global warming, plant paraquat resistance, and light signal transduction through nucleoside diphosphate kinase as a paradigm for increasing food supply

Light signal transduction was studied in extracts of mycelia of the fungus Neurospora crassa, and the third internodes of dark-grown Pisum sativum cv Alaska. Both processes increased the phosphorylation of nucleoside diphosphate kinase (NDPK). NDPK may function as a carrier of reduction equivalents, as it binds NADH, thereby providing electrons to transform singlet oxygen to superoxide by catalases (CAT). As the C-termini of NDPK interact with CAT which receive singlet oxygen, emitted from photoreceptors post light perception (which is transmitted to ambient triplet oxygen), we hypothesize that this may increase phospho-NDPK. Singlet oxygen, emitted from the photoreceptor, also reacts with unsaturated fatty acids in membranes thereby forming malonedialdehyde, which in turn could release ions from, e.g., the thylacoid membrane thereby reducing the rate of photosynthesis. A mutant of Alaska pea, which exhibited two mutations in chloroplast NDPK-2 and one mutation in mitochondrial localized NDPK-3, was resistant to reactive oxygen species including singlet oxygen and showed an increase in the production of carotenoids, anthocyanine, and thereby could reduce the concentration of singlet oxygen. The reduction of the concentration of singlet oxygen is predicted to increase the yield of crop plants, such as Alaska pea, soybean, rice, wheat, barley, and sugarcane. This approach to increase the yield of crop plants may contribute not only to enhance food supply, but also to reduce the concentration of CO(2) in the atmosphere.

Interaction of nucleoside diphosphate kinase and catalases for stress and light responses in Neurospora crassa

Nucleoside diphosphate kinase (NDK) is an ubiquitous enzyme with the function of a signal transducer. In Neurospora crassa, an ndk-1(P72H) mutant carrying the point mutation Pro72His was isolated. We found that ndk-1(P72H) showed hypersensitivity to oxidative and heat stress and a decrease in the levels of catalase (Cat)-1 and -3 induced by oxidative, heat stress and illumination compared with wild type (Wt). We found, by conducting a yeast two-hybrid assay, that Cat-1 interacted with NDK-1. NDK-1 was suggested to control Cat-1 and Cat-3 at the post-transcriptional level in response to heat, oxidative stress and light.

Photomorphogenetic characteristics are severely affected in nucleoside diphosphate kinase-1 (ndk-1)-disrupted mutants in Neurospora crassa

We previously demonstrated that the NDK-1 (Nucleoside Diphosphate Kinase-1) point mutant, ndk-1(P72H), displays a defective phenotype in light-induced perithecial polarity in Neurospora crassa. To investigate the biological function of NDK-1 in detail, we isolated two ndk-1 mutants, ndk-1(RIP-1) and ndk-1(RIP-2), using the RIPing (repeat induced point mutation) method. Notably, we detected no accumulation of ndk-1(RIP-1) mRNA and truncated NDK-1(RIP-2) protein. The ndk-1(RIP) mutants exhibited altered morphogenesis; (1) aerial hypha was not formed with no conidium formation, (2) the mutants exhibited colonial, and very slow mycelial growth on a solid medium and by shaking culture in a liquid medium, (3) light-induced carotenoid accumulation in mutant mycelia is reduced to less than half that by wild type, (4) the mutants exhibited spiral growth of mycelia, and (5) female sterility with defective protoperithecium formation. The morphogenetic processes of 1, 3, and 5 are light induced in the wild type. Moreover, despite only 10-20% of total nucleoside diphosphate kinase activity, the accumulation of relevant transcripts in the ndk-1(RIP) mutants, such as al-1 and al-2, was similar to that of wild type.

Protein kinase C modulates light responses in Neurospora by regulating the blue light photoreceptor WC-1

The Neurospora protein kinase C (NPKC) is a regulator of light responsive genes. We have studied the function of NPKC in light response by investigating its biochemical and functional interaction with the blue light photoreceptor white-collar 1 (WC-1), showing that activation of NPKC leads to a significant decrease in WC-1 protein levels. Furthermore, we show that WC-1 and NPKC interact in a light-regulated manner in vivo, and that protein kinase C (PKC) phosphorylates WC-1 in vitro. We designed dominant negative and constitutively active forms of PKC which are able to induce either a large increase of WC-1 protein level or a strong reduction respectively. Moreover, these changes in PKC activity result in an altered light response. As WC-1 is a key component of Neurospora circadian clock and regulates the clock oscillator component FRQ we investigated the effect of NPKC-mutated forms on FRQ levels. We show that changes in PKC activity affect FRQ levels and the robustness of the circadian clock. Together these data identify NPKC as a novel component of the Neurospora light signal transduction pathway that modulates the circadian clock.

Reactive oxygen species affect photomorphogenesis in Neurospora crassa

In Neurospora crassa, several biological phenomena such as the synthesis of carotenoids in the mycelia and polarity of perithecia are regulated by light. We found that a sod-1 mutant, with a defective Cu,Zn-type superoxide dismutase (SOD), showed accelerated light-dependent induction of carotenoid accumulation in the mycelia compared with the wild type. The initial rate of light-induced carotenoid accumulation in the sod-1 mutant was faster than that in the vvd mutant known to accumulate high concentrations. This acceleration was suppressed by treatment with antioxidant reagents. Light-induced transcription of genes involved in carotenoid synthesis, al-1, -2, and -3, was sustained in the sod-1 mutant, whereas it was transient in the wild type. Moreover sod-1 was defective in terms of light-induced polarity of perithecia. By genetic analysis, the enhancement in light-inducible carotenoid synthesis in sod-1 was dependent on the wild type alleles of wc-1 and wc-2. However, the sod-1;vvd double mutant showed additive effects on the carotenoid accumulation in the mycelia. These results suggested that intracellular reactive oxygen species regulated by SOD-1 could affect the light-signal transduction pathway via WC proteins.

Putative functions of nucleoside diphosphate kinase in plants and fungi

The putative functions of NDP (nucleoside diaphosphate) kinases from various organisms focusing to fungi and plants are described. The biochemical reactions catalyzed by NDP kinase are as follows. (i) Phosphotransferring activity from mainly ATP to cognate NDPs generating nucleoside triphosphates (NTPs). (ii) Autophosphorylation activity from ATP and GTP. (iii) Protein kinase (phosphotransferring) activity phosphorylating such as myelin basic protein. NDP kinase could function to provide NTPs as a housekeeping enzyme. However, recent works proved possible functions of the NDP kinases in the processes of signal transduction in various organisms, as described below. 1) By use of the extracts of the mycelia of a filamentous fungus Neurospora crassa blue-light irradiation could increase the phosphorylation of a 15-kDa protein, which was purified and identified to be NDP kinase (NDK-1). By use of the etiolated seedlings of Pisum sativum cv Alaska and Oryza sativa red-light irradiation of intact plants increased the phosphorylation of NDP kinase. However, successive irradiation by red-far-red reversed the reaction, indicating that phytochrome-mediated light signals are transduced to the phosphorylation of NDP kinase. 2) NDP kinase localizing in mitochondria is encoded by nuclear genome and different from those localized in cytoplasm. NDP kinase in mitochondria formed a complex with succinyl CoA synthetase. 3) In Spinicia oleraceae two different NDP kinases were detected in the chloroplast, and in Pisum sativum two forms of NDP kinase originated from single species of mRNA could be detected in the choloroplast. However, the function of NDP kinases in the choloroplast is not yet known. 4) In Neurospora crassa a Pro72His mutation in NDP kinase (ndk-1Pro72His) deficient in the autophosphorylation and protein kinase activity resulted in lacking the light-induced polarity of perithecia. In wild-type directional light irradiation parallel to the solid medium resulted in the formation of the perithecial beak at the top of perithecia, which was designated as "light-induced polarity of perithecia." In wild-type in darkness the beak was formed at random places on perithecia, and in ndkPro72His mutant the perithecial beak was formed at random places even under directional light illumination. The introduction of genomic DNA and cDNA for ndk-1 demonstrated that the wild-type DNAs suppressed the mutant phenotype. With all these results except for the demonstration in Neurospora, most of the phenomena are elusive and should be solved in the molecular levels concerning with NDP kinases.

A point mutation in nucleoside diphosphate kinase results in a deficient light response for perithecial polarity in Neurospora crassa

In Neurospora crassa, the phosphorylation of nucleoside diphosphate kinase (NDK)-1 is rapidly enhanced after blue light irradiation. We have investigated the function of NDK-1 in the blue light signal transduction pathway. A mutant called psp (phosphorylation of small protein) shows undetectable phosphorylation of NDK-1 and is defective in light-responsive regulation of perithecial polarity. Sequencing analysis of ndk-1 cDNA by reverse transcription-polymerase chain reaction revealed that proline 72 of ndk-1 was replaced with histidine in psp. The mutation ndk-1(P72H) resulted in accumulation of normal levels of mRNA and of about 25% of NDK-1(P72H) protein compared with that of wild type as determined by Western blot analysis. The ectopic expression of cDNA and introduction of genomic DNA of wild type ndk-1 in psp (ndk-1(P72H)) suppressed the reduction in accumulation and phosphorylation of NDK-1 and the light-insensitive phenotype. These findings demonstrated that the phenotype of psp was caused by the ndk-1(P72H) mutation. Biochemical analysis using recombinant NDK-1 and NDK-1(P72H) indicated that the P72H substitution in NDK-1 was responsible for the decrease in phosphotransfer activities, 5% of autophosphorylation activity, and 2% of V(max) for protein kinase activity phosphorylating myelin basic protein, compared with those of wild type NDK-1, respectively.

Isolation and characterization of Neurospora crassa nucleoside diphosphate kinase NDK-1

We have previously reported that phosphorylation of a 15-kDa protein increased after blue-light irradiation in Neurospora crassa. In this study, the 15-kDa protein was purified using four columns; DEAE-cellulose, Blue-Sepharose, SP-Sepharose and Mono Q. The 15-kDa protein was shown to be homologous with nucleoside diphosphate kinase by amino acid sequencing and was also shown to possess nucleoside diphosphate kinase activity. A gene encoding N. crassa nucleoside diphosphate kinase, ndk-1, was isolated from the mycelial cDNA and genomic libraries. The deduced amino acid sequence of NDK-1 was identical to that of the 15-kDa protein. Northern blot analysis suggested that WC-1 and WC-2, the key factors of blue-light signal transduction in N. crassa, did not regulate NDK-1 at the transcriptional level. NDK-1 also showed rapid autophosphorylation activity and protein kinase activity against myelin basic protein with a Km value of 0.36 mM. These results suggest that NDK-1 acts as a signal transducer by phosphorylating proteins.

Blue light signaling chains in Phycomyces: phototransduction of carotenogenesis and morphogenesis involves distinct protein kinase/phosphatase elements

Carotenogenesis and morphogenesis represent two of the several responses sensitive to blue light which characterize the lower eukaryote Phycomyces blakesleeanus. Speculating that reversible phosphorylation may be an intracellular event beyond the photoperception step, we resorted to the use of first-choice inhibitors of protein phosphatases and protein kinases. The mycelial beta-carotene content of dark-grown cultures was induced by all agents administered, while the morphogenic output showed the typical trend effected by light only with one of the protein kinase inhibitors. Our data provide convincing evidence that protein phosphorylation plays a regulatory role in photocarotenogenesis and photomorphogenesis of Phycomyces. According to the model we propose, the putative signaling elements involved are anticipated to have a repressive function in the dark so that the responses are maintained in the "off" mode until the moment photon information has to flow through the regulatory circuit.

Post-translational modification of proteins by reversible phosphorylation in prokaryotes

Microorganisms have developed three different systems for catalyzing protein phosphorylation and using this reversible modificaiton to regulate their cellular activities. The first 'classical' system utilizes nucleoside-triphosphates as phosphoryl donors and leads to the modification of protein substrates at serine/threonine or tyrosine residues. The second system, called 'two-component system', requires first a sensor kinase which autophosphorylates at a histidine residue at the expense of adenosine-triphosphate, then a response regulator which is modified in turn at an aspartate residue and thereafter induces a metabolic change within the cell. The third system, called 'PTS system', makes use of phosphoenol pyruvate to generate a phosphoryl group which is passed down a chain of several proteins and finally transferred to a sugar. There is increasing evidence that, contrary to an early concept, these systems and the corresponding enzymes (protein kinases and phosphoprotein phosphatases) share a number of structural and functional similarities with the phosphorylation-dephosphorylation machineries found in eukaryotes. Therefore one can expect that microorganisms will serve, once again, as a basic model for exploring and understanding a key regulatory mechanism, reversible protein phosphorylation, which concerns all organisms.

Blue light regulation in Neurospora crassa

The fungus Neurospora crassa has been shown to be a paradigm for photobiological, biochemical, and genetic studies of blue light perception and signal transduction. Several different developmental and morphological processes of Neurospora are regulated by blue light and can be divided into early and late blue light responses. The characterization of two central regulator proteins of blue light signal transduction in Neurospora crassa, WC1 and WC2, and the isolation of light-regulated genes, indicate transcriptional control as a central step in blue light signalling.

Diversity and specificity of protein-phosphorylating systems in bacteria

Bacteria harbor three different protein-phosphorylating systems which regulate distinct physiological processes: first, the nucleotide-dependent system which modifies hydroxyl groups of amino acids in protein substrates; second, the two-component system which involves both sensor kinase and response regulator; third, the phosphoenolpyruvate-dependent phosphotransferase system. These systems share a number of structural and functional similarities with the protein-phosphorylating systems of eukaryotes.

Pyrophosphate is a source of phosphoryl groups for Escherichia coli protein phosphorylation

Pyrophosphate can serve as a source of phosphoryl groups for the phosphorylation of E. coli proteins. The main target of such phosphorylation is a 49-kDa protein which is covalently modified at serine. The same phosphorylation pattern is obtained in the presence of ATP, which suggests that pyrophosphate can substitute for ATP for bacterial protein phosphorylation.

Blue light induced ADP ribosylation of 38 and 56 kDa proteins in the soluble fraction of mycelia of Neurospora crassa

Soluble fractions prepared from the mycelia of wild type (74-OR23-1A) and band (bd) exhibited an increase in the rate of the ADP ribosylation of a 38 kDa protein from nicotinamide adenine [32P]dinucleotide ([32P]NAD) in the presence of 10(-7) M riboflavin caused by blue light irradiation in vitro. The soluble fraction was mixed with a reaction mixture containing 5 microCi [32P]NAD at 0 degree C for 20 s and then it was irradiated with blue light (420 nm, 42 mumol m-2 s-1) for 12.5, 25, 50, 100, 200 or 400 s at 0 degree C or for 100 s with photon irradiance of 0.42, 4.2, 6.4 or 42 mumol m-2 s-1. Immediately after irradiation, the reaction was stopped and analysed by sodium dodecyl sulphate-polyacrylamide gel electrophoresis. An increase in the ADP ribosylation of the 38 kDa protein could be detected within 100 s of irradiation, and the enhancement in the rate of ADP ribosylation of the 38 kDa protein was proportional to the increase in the photon irradiance. By the irradiation with blue light for 200 or 400 s, the ADP ribosylation of a 56 kDa protein could also be detected. Analysis by two-dimensional gel electrophoresis of proteins after ADP ribosylation of them revealed that the 38 kDa proteins displayed at least four radioactive protein spots and the 56 kDa protein a single radioactive protein spot. Soluble fractions of mycelia prepared from blind mutants wc-1, wc-2, delta ps15-1, lis-1, lis-2 and lis-3 exhibited also the enhancement of the ADP ribosylation of the 38 kDa protein by blue light irradiation, and at least wc-1, delta ps15-1, lis-1 and lis-2 displayed a similar blue light response in the 56 kDa protein.

Histidine and tyrosine phosphorylation in pea mitochondria: evidence for protein phosphorylation in respiratory redox signalling

A 37 kDa protein in pea mitochondria was found to contain phosphorylated residues. Phosphorylation was acid-labile but stable in alkali solution, a unique property of phosphorylation on histidine, indicating that a signal transduction pathway with homology to bacterial two-component systems might exist in plant mitochondria. We also describe the first example of tyrosine phosphorylation in plant organelles and the first indication of protein phosphorylation as part of a redox signalling mechanism in mitochondria. Labelling of three proteins (28, 27 and 12 kDa) was found to be dependent on the redox state of the reaction medium. Their phospho-groups were resistant to alkali as well as acid treatment and labelling was inhibited by the tyrosine kinase inhibitor genistein.

Histidine and aspartate phosphorylation: two-component systems and the limits of homology

Autophosphorylating histidine kinase and response-regulator domains constitute the building blocks of two-component signaling systems. These systems use a unique phosphotransfer chemistry to regulate many aspects of bacterial physiology. Homologous systems are now being found in eukaryotes. Despite their common mechanism of phosphotransfer, the two-component systems display an extensive diversity in the arrangement of their domains, and flexibility in their roles in different signal transduction circuits.