Phytochrome and protein phosphorylation

Protein kinase activity in Cucumis sativus cotyledons: effect of calcium and light

Light signals received by phytochromes in plants may be transduced through protein phosphorylation. Ca(2+) as second messenger was involved in phytochrome-mediated cellular events. Our experiments with Cucumis sativus cotyledons, treated with red (R) and far-red (FR) light, showed a stimulatory effect on in vitro protein phosphorylation of histone, added as exogenous substrate to the cotyledon extracts, and also modified the phosphorylation of endogenous polypeptides. The effect of light treatments was mimicked by the addition of Ca(2+) to the phosphorylation buffer, indicating phytochrome- and Ca(2+)-dependence on activity of some protein kinases (PKs). In-gel kinase assays were performed to characterize the PKs involved at the cotyledon stage of cucumber plants. Three proteins of about 75, 57 and 47kDa with PK activity were detected between M(r) markers of 94 and 45kDa. All three were able to phosphorylate histone and undergo autophosphorylation. However, only the 75 and 57kDa proteins autophosphorylated and phosphorylated the substrate in a Ca(2+)-dependent manner, and were inhibited when calmodulin (CaM) antagonists were added to the incubation buffer. Western-blot analysis with polyclonal antibodies directed against calcium-dependent protein kinase of rice (OsCDPK11) or Arabidopsis (AtCPK2) recognised 57 and 75kDa polypeptides, respectively. These results indicate the presence in cucumber cotyledons of at least two proteins (ca. 75 and 57kDa) with activity of PKs that could be calcium-dependent protein kinases (CDPKs). Both CDPKs could be modulated by phytochromes throughout FR-HIR and VLFR responses.

Phytochrome-mediated light signals are transduced to nucleoside diphosphate kinase in Pisum sativum L. cv. Alaska

To clarify the molecular mechanism for the transduction of light signals in plants, we have established an in vitro system that uses crude membrane and soluble fractions of stem sections of etiolated Pisum sativum L. cv. Alaska after irradiation by red light, or sequential application of red and far-red light to the stem section. In a previous report (T. Hamada et al., J. Photochem. Photobiol. B: Biol. 33 (1996) 143-151) the labelling of proteins in membrane fraction by [gamma-32P] ATP at 0 degree C for 15 s and subsequent separation of proteins by two-dimensional electrophoresis allowed unambiguous identification of a heavily phosphorylated protein spot at 18 kDa (p18). In the present study we have confirmed the former results in the membrane fraction, and obtained the result that an increase in the phosphorylation of p18 by red-light irradiation is observed in the soluble fraction. Further, we have provided evidence that the p18 in the soluble fraction is purified and identified as nucleoside diphosphate (NDP) kinase by Western blotting, immuno-precipitation, amino acid sequencing and cDNA analysis. Purified p18 shows autophosphorylation activity and strong phosphorylating activity against myelin basic protein (MBP), a substrate of MAP (mitogen activated protein) kinase. The results show that phytochrome-mediated light signals are transduced to NDP kinase, which may elicit signals by providing high concentrations of, for example, GTP from GDT and ATP, by the autophosphorylation and by the protein kinase activity similar to MAP kinase.

A possible tyrosine phosphorylation of phytochrome

Red/far-red light signal transduction by the phytochrome family of photoreceptors regulates plant growth and development. We investigated the possibility that tyrosine kinases and/or phosphatases are involved in phytochrome-mediated signal transduction using crude extracts of oat seedlings that are grown in the dark. We found that a 124 kDa protein was tyrosine-phosphorylated as determined by Western blotting with a phosphotyrosine-specific monoclonal antibody. The 124 kDa protein was recognized by the anti-phosphotyrosine antibody in anti-phytochrome A immunoprecipitates. The level of anti-phosphotyrosine antibody binding to the 124 kDa protein(s) in phytochrome immunoprecipitates that had been treated with red light prior to immunoprecipitation decreased relative to dark controls. These results suggest that either phytochrome from dark-grown seedlings is tyrosine phosphorylated or that it co-immunoprecipitates with a phosphotyrosine-containing protein of the same molecular weight. The implications of these results in the regulation of (a) the putative Ser/Thr kinase activity of the photoreceptor and (b) the binding of signaling molecules, such as phospholipase C to phytochrome, are discussed.

Phosphorylation/dephosphorylation steps are key events in the phytochrome-mediated enhancement of nitrate reductase mRNA levels and enzyme activity in maize

We provide evidence to show that the increase in nitrate reductase (NR) transcript level stimulated by red light is mediated via a phosphorylation-dependent step. The light-stimulated enhancement of NR transcript level was significantly inhibited by H-7, a protein kinase inhibitor, whereas okadaic acid (OKA), a phosphatase inhibitor, had no effect. Phorbol myristate acetate (PMA), an activator of protein kinase C (PKC) enhanced the NR transcript level in dark-grown leaves. No correlation between changes in NR transcript level and NR activity (NRA) was observed. Inhibition of NRA by OKA and stimulation by H-7 indicated that NRA is increased by dephosphorylating the enzyme. We have identified a protein kinase (C type) that can phosphorylate the purified NR in vitro without the involvement of other accessory proteins. By in vivo labelling with 32P and immunoprecipitation of NR with NR antibodies it was found that in the presence of OKA most NR protein (NRP) was present in phosphorylated state, while with H-7 the reverse was seen. The red (R) and far-red (FR) light reversible experiments suggested that phytochrome (Pfr, an active form) stimulation of NRA is mediated by dephosphorylation of the enzyme, suggesting that Pfr regulates both NR transcription and NRA via phosphorylation/dephosphorylation steps controlled by separate signal transduction pathways.

Evidence for some common signal transduction events for opposite regulation of nitrate reductase and phytochrome-I gene expression by light

We have explored the possible involvement of the phosphoinositide (PI) cycle and protein kinase C (PKC) in the phytochrome (Pfr)-mediated light signal transduction pathway using nitrate reductase (NR) and phytochrome-I (PhyI) genes as model systems. We have shown earlier that phorbol myristate acetate (PMA) completely replaces the red light effect in stimulating nitrate reductase activity and transcript levels in maize. In this paper, we present detailed evidence to show that PMA mimics the red light effect and follows similar kinetics to enhance NR steady-state transcript accumulation in a nitrate-dependent manner. We also show that PMA inhibits phyI steady-state transcript accumulation in a manner similar to red light, indicating that a PKC-type enzyme(s) may be involved in mediating the light effect in both cases. Serotonin or 5-hydroxytryptamine (5-HT), a stimulator of PI turnover, was also found to mimic the red light effect in enhancing NR transcript levels and inhibiting phyI transcript accumulation, indicating the role of the PI cycle in generating second messengers for regulating the two genes. These results indicate that phytochrome-mediated light regulation of NR and phyI gene expression may involve certain common steps in the signal transduction pathway such as the PI cycle and protein phosphorylation by a PKC-type enzyme.

Phytochromes: photosensory perception and signal transduction

The phytochrome family of photoreceptors monitors the light environment and dictates patterns of gene expression that enable the plant to optimize growth and development in accordance with prevailing conditions. The enduring challenge is to define the biochemical mechanism of phytochrome action and to dissect the signaling circuitry by which the photoreceptor molecules relay sensory information to the genes they regulate. Evidence indicates that individual phytochromes have specialized photosensory functions. The amino-terminal domain of the molecule determines this photosensory specificity, whereas a short segment in the carboxyl-terminal domain is critical for signal transfer to downstream components. Heterotrimeric GTP-binding proteins, calcium-calmodulin, cyclic guanosine 5'-phosphate, and the COP-DET-FUS class of master regulators are implicated as signaling intermediates in phototransduction.

Photosensory perception and signal transduction in plants

Genetic and molecular studies are beginning to unravel the complexities of the signaling circuitry that plants use to sense and transduce information concerning the prevailing light environment. The past year has witnessed definition of discrete photosensory roles for phytochromes A and B, the cloning of a gene encoding the first apparent blue-light photoreceptor from any organism, the cloning of genes encoding additional members of the COP/DET/FUS class of light-responsive master regulators, and evidence that G proteins, Ca2+/calmodulin, and cGMP may be signaling intermediates in phototransduction.

Light induces rapid changes of the phosphorylation pattern in the cytosol of evacuolated parsley protoplasts

The fractionation of cells of a parsley suspension culture [Petroselinum crispum (Mill.) A. Hill] by protoplasting and subsequent removal of the vacuoles led to physiologically intact evacuolated protoplasts retaining light inducibility of chalcone synthase expression. Lysis of the evacuolated protoplasts permitted the isolation of a pure, highly concentrated cytosolic fraction containing major cytosolic membranes but only minor contamination by proplastids, mitochondria, and nuclei. Short-time irradiations of the cytosol with red or UV-containing white light resulted in very fast changes of the phosphorylation pattern of 18-, 40-, 48-, 55- to 70-, and 120-kDa proteins. Major differences were observed between the phosphorylation patterns obtained by red or UV-containing white light treatment, indicating a different primary action of the excited photoreceptors in vitro. Separation of the microsomal fraction from the cytosolic matrix established the localization of these proteins. Chase and photoreversibility experiments revealed that phytochrome in vitro regulates the phosphorylation of the 40-kDa protein by modifying a soluble cytosolic kinase/phosphatase system.

Biochemical evidence that phytochrome of the moss Ceratodon purpureus is a light-regulated protein kinase

The phytochrome gene of the moss Ceratodon purpureus (phyCer) codes for a novel phytochrome polypeptide with a predicted molecular mass of 145 kDa that has a COOH-terminal domain which is homologous to the catalytic domain of eukaryotic protein kinases. In this paper we report the first biochemical evidence that in fact, as predicted from the gene sequence, PhyCer represents an active, light-regulated protein kinase. In vitro phosphorylation experiments with protonemata extracts revealed the existence of a 140 kDa protein, phosphorylated in a red/far-red light dependent manner. The binding of a polyclonal antibody directed to the protein kinase catalytic domain of PhyCer enhanced the phosphorylation of a 140 kDa band when assayed in a renaturation-auto-phosphorylation experiment with nitrocellulose bound protein. These findings strongly implicate that the phyCer gene product has protein kinase activity and is capable of auto-phosphorylation. The results of the renaturation-phosphorylation experiments were essentially the same, no matter whether protein extracts from light grown or dark adapted moss protonemata were used. Thus, phyCer expression most likely is not light regulated.

Signal transduction by phytochrome: phytochromes have a module related to the transmitter modules of bacterial sensor proteins

A C-terminal section of phytochromes turned out to share sequence homologies with the full length of the transmitter modules (about 250 amino acids) of bacterial sensor proteins. Coinciding hydrophobic clusters within the homologous domains imply that the overall folding of the two different types of peptides is similar. Hence, phytochromes appear to possess the structural prerequisites to transmit signals in a way bacterial sensor proteins do. The bacterial sensor proteins are known to be environmental stimuli-regulated kinases belonging to two-component systems. After sensing a stimulus by the N-terminal part of the sensor protein, conformational alterations confer the signal to its (mostly) C-terminal transmitter module which in turn is transitionally autophosphorylated at a conserved histidine. From the histidine the phosphate is transferred to the receiver module of a system-specific regulator protein which eventually acts on transcription or enzyme activity. The histidine is not conserved in phytochromes. Instead, a conserved tyrosine is found spatially very close to the histidine position. This tyrosine might play the role of histidine, and kinase function might be associated with this part of phytochrome. In spite of this divergence, the structural similarities point to a common evolutionary origin of the phytochrome and bacterial modules.

Structural domains of phytochrome deduced from homologies in amino acid sequences

A method of semiempirical identification of structural domains is proposed. The procedure is based on the comparison of amino acid sequences in groups of homologous proteins. This approach was tested using 32 known protein sequences from different cytochrome b5, cytochrome c, lysozyme, hemoglobin, and myoglobin proteins. The method presented was able to identify all structural domains of these reference proteins. A consensus secondary structure provided information on structural content of these domains predicting correctly 21 of 23 (91%) of alpha-helices. We applied this method to six homologous phytochrome sequences from Avena, Arabadopsis, Cucurbita, Maize, Oryza, and Pisum. Some of the identified domains can be assigned to the known tertiary structure categories. For example, an alpha/beta domain is localized in the region known to stabilize the phytochrome chromophore in the red light absorbing form (Pr). One alpha-helical and one alpha/beta domains are localized in regions important for the chromophore stabilization in the far-red absorbing form (Pfr). From an analysis of noncovalent interaction patterns in another domain it is proposed that a phytochrome dimer contact involves two segments localized between residues 730 and 821 (using numbering of aligned sequences). Also, a possible antiparallel beta-sheet structure of this region has been suggested. According to this model, the long axis of the interacting structures is perpendicular to a twofold symmetry axis of the phytochrome dimer.

Bilirubin chemiluminescence induced by the attack of active oxygen species

Ultraweak chemiluminescence (CL) from bilirubin occurs in the presence of triplet oxygen and is stimulated by the addition of aldehydes. Active oxygen species also enhance bilirubin CL, in the absence of aldehydes. An inhibitory effect of active oxygen scavengers on the CL indicated that active oxygens generated from the decomposition of added hydrogen peroxide or from the xanthine-xanthine oxidase reaction contributed to the CL from bilirubin molecules. However, the contribution of singlet oxygen to the CL disappeared in the presence of formaldehyde. This suggested that the scission of tetrapyrrole bonds via a dioxetane intermediate or the production of triplet carbonyls from the oxidation of aldehydes by singlet oxygen was not involved in the CL, at least in the presence of formaldehyde. The spectrum of CL induced by the generation of active oxygen was the same as that from the aldehyde-enhanced CL reaction. We propose that the formation of a hydroperoxide (and/or hydroxide) bilirubin intermediate, but not a dioxetane, may be involved in the excitation of bilirubin molecules for CL.

Weak chemiluminescence of bilirubin and its stimulation by aldehydes

Bilirubin in an alkaline solution exhibits a weak chemiluminescence (CL) under aerobic conditions. This spontaneous CL was markedly enhanced by the addition of various aldehydes. The fluorescent emission spectrum of bilirubin, excited by weak intensity light at 350 nm, coincided with its CL emission spectrum (peak at 670 nm). CL emission from bilirubin was not quenched by active oxygen scavengers. This suggests that triplet oxygen reacts with bilirubin, and forms an oxygenated intermediate (hydroperoxide) as a primary emitter (oxidative scission of tetrapyrrole bonds in bilirubin is not involved in this CL). The Ehrlich reaction (test for monopyrroles) and hydrolsulphite reaction (test for dipyrroles) on the CL reaction mixture and unreacted bilirubin showed no differences. When the CL was initiated by singlet oxygen, rather than superoxide anion, monopyrrole, was detected in the reaction products by gel chromatography. The inhibitory effect of a scavenger of singlet oxygen on CL was eliminated in the presence of formaldehyde. Therefore, triplet carbonyl, formed by singlet oxygen through the dioxetane structure in bilirubin, is not an emitter. The reaction mechanism of bilirubin CL and the formation of a hydroperoxide intermediate is discussed in relation to the chemical structure of luciferin molecules from bioluminescent organisms.

Protein phosphorylation in isolated nuclei from etiolated Avena seedlings. Effects of red/far-red light and cholera toxin

We have studied the phosphorylation/dephosphorylation of several nuclear proteins in isolated nuclei from etiolated Avena seedlings as a function of red/far-red light. The effect of stimulatory (ADP-ribosylation by cholera toxin) or inhibitory (GDP beta S) conditions for GTP-binding proteins was also studied. Red or far-red light enhanced the phosphorylation level of 2 nuclear proteins with molecular masses of 75 and 60 kDa. The phosphorylation pattern was affected by the addition of cholera toxin or GDP beta S to the isolated nuclei. At least 2 proteins with molecular masses of 24 and 75 kDa cross-reacted by Western blot with GTP-binding protein antibodies.