Analysis of steady-state protein phosphorylation in mitochondria using a novel fluorescent phosphosensor dye

The phosphorylation of mitochondrial proteins is pivotal to the regulation of respiratory activity in the cell and to signaling pathways leading to apoptosis, as well as for other vital mitochondrial processes. A number of protein kinases have been identified in mitochondria but the physiological substrates for many of these remain unknown or poorly understood. By necessity, most studies of mitochondrial phosphoproteins to date have been conducted using in vitro incorporation of 32P. However, proteins that are highly phosphorylated from in situ reactions are not necessarily detected by this approach. In this study, a new small molecule fluorophore has been employed to characterize steady-state levels of mitochondrial phosphoproteins. The dye is capable of sensitive detection of phosphorylated amino acid residues in proteins separated by gel electrophoresis. When the fluorescent dye is combined with a total protein stain in a sequential gel staining procedure, the phosphorylated proteins can be visualized in the same gel as the total proteins. To optimize resolution of the proteins in mitochondria, a previously described sucrose gradient fractionation method was employed prior to gel electrophoresis. Phosphorylated proteins, as defined by the fluorescence of the phosphosensor, were excised from the gels and identified by peptide mass fingerprinting. One novel and prominent phosphoprotein identified in this manner was determined to be the 42-kDa subunit of mitochondrial complex I.     

Phosphoproteins and the phosphoenolpyruvate:sugar phosphotransferase system of Streptococcus salivarius. Detection of two different ATP-dependent phosphorylations of the phosphocarrier protein HPr

Phosphoproteins which arise from incubation of Streptococcus salivarius ATCC25975 crude extracts with [32P]phosphoenolpyruvate and [gamma-32P]ATP, were separated and detected by sodium dodecyl sulphate-polyacrylamide gel electrophoresis and autoradiography. These procedures were carried out using the methodology that has been developed to allow for the detection of phosphoproteins containing 1-P-histidinyl and 3-P-histidinyl residues, and also to distinguish between these and phosphoproteins containing acid-stable phosphoamino acids such as phosphoserine, phosphothreonine, and phosphotyrosine. Extracts of cells which had been grown with various sugars as carbon sources were investigated to determine both constitutive and inducible phosphoproteins. No evidence was found for phosphoproteins specifically induced by a sugar, and in particular no evidence was found for any IIIsugar phosphocarrier protein of the phosphoenolpyruvate:sugar phosphotransferase system (PTS). Incubation with [gamma-32P]ATP showed that histidine-containing phosphocarrier protein (HPr) of the PTS could be phosphorylated to give both acid-stable and acid-labile phosphoamino acid residues. The acid-labile ATP-dependent phosphorylation activity was activated by glucose-6-P and appeared to produce a 3-P-histidinyl residue in HPr.     

Global quantitative phosphoprotein analysis using Multiplexed Proteomics technology

Systematic parallel analysis of the phosphorylation status of networks of interacting proteins involved in the regulatory circuitry of cells and tissues is certain to drive research in the post-genomics era for many years to come. Reversible protein phosphorylation plays a critical regulatory role in a multitude of cellular processes, including alterations in signal transduction pathways related to oncogene and tumor suppressor gene products in cancer. While fluorescence detection methods are likely to offer the best solution to global protein quantitation in proteomics, to date, there has been no satisfactory method for the specific and reversible fluorescent detection of gel-separated phosphoproteins from complex samples. The newly developed Pro-Q Diamond phosphoprotein dye technology is suitable for the fluorescent detection of phosphoserine-, phosphothreonine-, and phosphotyrosine-containing proteins directly in sodium dodecyl sulfate (SDS)-polyacrylamide gels and two-dimensional (2-D) gels. Additionally, the technology is appropriate for the determination of protein kinase and phosphatase substrate preference. Other macromolecules, such as DNA, RNA, and sulfated glycans, fail to be detected with Pro-Q Diamond dye. The staining procedure is rapid, simple to perform, readily reversible and fully compatible with modern microchemical analysis procedures, such as matrix-assisted laser desorption/ionization-time of flight (MALDI-TOF) mass spectrometry. Pro-Q Diamond dye technology can detect as little as 1-2 ng of beta-casein, a pentaphosphorylated protein, and 8 ng of pepsin, a monophosphorylated protein. Fluorescence signal intensity correlates with the number of phosphorylated residues on the protein. Through combination of Pro-Q Diamond phosphoprotein stain with SYPRO(R) Ruby protein gel stain, Multiplexed Proteomics technology permits quantitative, dichromatic fluorescence detection of proteins in 2-D gels. This evolving discovery platform allows the parallel determination of protein expression level changes and altered post-translational modification patterns within a single 2-D gel experiment. The linear responses of the fluorescence dyes utilized, allow rigorous quantitation of changes over an unprecedented 500-1000-fold concentration range.     

Dynamics of protein phosphorylation on Ser/Thr/Tyr in Bacillus subtilis

The Ser/Thr/Tyr phosphoproteome of Bacillus subtilis was analyzed by a 2-D gel-based approach combining Pro-Q Diamond staining and [(33)P]-labeling. In exponentially growing B. subtilis cells 27 proteins could be identified after staining with Pro-Q Diamond and/or [(33)P]-labeling and one additional protein was labeled solely by [(33)P] resulting in a total of 28 potentially phosphorylated proteins. These proteins are mainly involved in enzymatic reactions of basic carbon metabolism and the regulation of the alternative sigma factor sigma(B). We also found significant changes of the phosphoproteome including increased phosphorylation and dephosphorylation rates of some proteins as well as the detection of four newly phosphorylated proteins in response to stress or starvation. For nine proteins, phosphorylation sites at serine or threonine residues were determined by MS. These include the known phosphorylation sites of Crh, PtsH, and RsbV. Additionally, we were able to identify novel phosphorylation sites of AroA, Pyk, and YbbT. Interestingly, the phosphorylation of RsbRA, B, C, and D, four proteins of a multicomponent protein complex involved in environmental stress signaling, was found during exponential growth. For RsbRA, B, and D, phosphorylation of one of the conserved threonine residues in their C-termini were verified by MS (T171, T186, T181, respectively).     

Analysis of connexin phosphorylation sites

Most connexins, the proteins that form gap junction channels, are phosphoproteins. Connexin phosphorylation has been thought to regulate gap junctional protein trafficking, gap junction assembly, channel gating, and turnover. Connexin phosphorylation has been investigated in a variety of ways. Some connexins show mobility shifts in sodium dodecyl sulfate-polyacrylamide gel electrophoresis on phosphorylation. Kinase modulators can change the level of connexin phosphorylation and affect gap junctional communication levels. Metabolic labeling of cultured cells has allowed both phosphoamino acid identification and generation of phosphotryptic peptide maps. However, identification of the location of phosphorylated residues within the connexin sequence has required either targeted peptide synthesis, in vitro phosphorylation of known sites, and two-dimensional comigration studies or liquid chromatographic separation and N-terminal sequencing of peptides. In addition to these conventional methods, we discuss new applications of mass spectrometry to the identification of phosphorylated peptides and the specific residues phosphorylated within the connexin-derived peptide.     

Lysosomal enzyme binding receptor protein from monkey brain and its phosphorylation.

The lysosomal enzyme binding receptor protein isolated from monkey brain by phosphomannan-Sepharose affinity chromatography was phosphorylated by [gamma-32P] ATP by protein kinases tightly associated with the receptor protein. A greater than 200 kDa protein was phosphorylated on both serine and tyrosine residues and a approximately 45 kDa protein was phosphorylated on only serine residues as evidenced by SDS-gel electrophoresis, autoradiography and phosphoamino acid analysis [(Panneerselvam, Ramamoorthy & Balasubramanian (1987) Biochem Biophys Res Commun, 147, 927-935)]. 125I-labelled lysosomal enzymes could be cross-linked to the receptor protein in the presence of disuccinimidyl suberate. Phosphorylation of the receptor on both serine and tyrosine residues was inhibited by quercetin, polylysine and polymyxin B. Catalytic subunit of cyclic AMP-dependent protein kinase preferentially phosphorylated the approximately 45 kDa protein. In the presence of Triton X-100, phosphorylation of a few additional protein bands on non-tyrosine residues was observed. There was a marked reduction in the efficiency of binding lysosomal enzymes by the phosphorylated receptor protein in comparison to the unphosphorylated receptor protein.     

Insulin-induced stimulation of protein phosphorylation in Neurospora crassa cells.

1) Insulin stimulated the phosphorylation of at least 14 discrete proteins in Neurospora crassa cells. Specific proteins were phosphorylated at serine, threonine, and tyrosine residues, as determined by phosphoamino acid analysis of discrete spots on two-dimensional gels. 2) Insulin stimulated the phosphorylation by [gamma-32P]ATP of at least six discrete proteins in solubilized N. crassa membrane preparations at serine and tyrosine residues. 3) A phosphotyrosine-containing protein of 38 kDa, pI 7.0-7.2, reacted by both immunoblotting and immunoprecipitation with antiserum to P2, a peptide from the human insulin receptor that contains an autophosphorylated tyrosine residue. In N. crassa cells, therefore, as in mammalian cells, insulin induces a variety of protein phosphorylations, some of which may be part of an evolutionarily conserved signal transduction pathway.     

Fibronectin phosphorylation by ecto-protein kinase

The presence of membrane-associated, extracellular protein kinase (ecto-protein kinase) and its substrate proteins was examined with serum-free cultures of Swiss 3T3 fibroblast. When cells were incubated with [gamma-32]ATP for 10 min at 37 degrees C, four proteins with apparent molecular weights between 150 and 220 kDa were prominently phosphorylated. These proteins were also radiolabeled by lactoperoxidase catalyzed iodination and were sensitive to mild tryptic digestion, suggesting that they localized on the cell surface or in the extracellular matrix. Phosphorylation of extracellular proteins with [gamma-32P]ATP in intact cell culture is consistent with the existence of ecto-protein kinase. Anti-fibronectin antibody immunoprecipitated one of the phosphoproteins which comigrated with a monomer and a dimer form of fibronectin under reducing and nonreducing conditions of electrophoresis, respectively. The protein had affinity for gelatin as demonstrated by retention with gelatin-conjugated agarose. This protein substrate of ecto-protein kinase was thus concluded to be fibronectin. The sites of phosphorylation by ecto-protein kinase were compared with those of intracellularly phosphorylated fibronectin by the analysis of radiolabeled amino acids and peptides. Ecto-protein kinase phosphorylated fibronectin at serine and threonine residues which were distinct from the sites of intracellular fibronectin phosphorylation.     

A sensitive method for determining the phosphorylation status of natriuretic peptide receptors: cGK-Ialpha does not regulate NPR-A

Natriuretic peptide receptor A (NPR-A) and natriuretic peptide receptor B (NPR-B) are transmembrane guanylyl cyclases that catalyze the synthesis of cGMP in response to natriuretic peptides. Phosphorylation and dephosphorylation regulate these receptors and have been traditionally studied by (32)PO(4) labeling of transfected cells. However, this approach cannot be used to determine the phosphorylation state of receptors isolated from unlabeled sources. Here, we use Pro-Q Diamond and SYPRO Ruby dyes to quantify the phosphorylation status and protein levels, respectively, of natriuretic peptide receptors from tissues and cells. Strong Pro-Q Diamond signals for NPR-A and NPR-B were obtained when receptors were isolated from lung tissue, liver tissue and overexpressing cells. The level of NPR-A Pro-Q staining was also high in kidney but was much lower in heart tissue. In contrast, the SYPRO Ruby protein signal was weaker and more variable. In a direct comparison, Pro-Q Diamond staining was as sensitive as but more specific than the (32)PO(4) labeling method. The two approaches were highly correlated (R(2) = 0.98). We exploited these techniques to measure the effect of cGMP-dependent protein kinase Ialpha on the phosphate content and guanylyl cyclase activity of NPR-A. Neither value was significantly affected in cells overexpressing cGK-Ialpha or in tissues from mice lacking cGK-I. We conclude that cGK-I does not regulate the cyclase activity or phosphorylation state of NPR-A. Furthermore, we find that Pro-Q Diamond staining is a sensitive method for measuring the phosphate levels of natriuretic peptide receptors, but protein levels are best detected by Western blot analysis, not SYPRO Ruby staining.     

Analysis of the Arabidopsis cell suspension phosphoproteome in response to short-term low temperature and abscisic acid treatment

To shed light on the early protein phosphorylation events involved in plant cell signaling in response to environmental stresses, we studied changes in the phosphorylation status of the Arabidopsis cell suspension proteome after short-term low temperature and abscisic acid (ABA) treatment. We used radioactive pulse-labeling of Arabidopsis cell suspension cultures and two-dimensional (2-D) gel electrophoresis to identify proteins that are differentially phosphorylated in response to these treatments. Changes in the phosphorylation levels of several proteins were detected in response to short-term (5 min or less) cold (4°C) and chilling (12°C) stress and ABA treatment, and we observed that some of these changes were common between these treatments. In addition, we used Pro-Q Diamond phosphoprotein gel stain to study the steady-state protein phosphorylation status under the same treatments. We demonstrated that Pro-Q Diamond effectively stained phosphorylated proteins, however, the overall Pro-Q Diamond 2-D gel staining pattern of proteins extracted from low-temperature and ABA-treated cells was not consistent with the gel patterns obtained by in vivo radioactive labeling of phosphoproteins. These in vivo pulsed-labeling experiments demonstrate that the Arabidopsis phosphoproteome is dynamic in response to short-term low temperature and ABA treatment, and thus represents a strategy for the identification of signaling proteins that could be utilized in the production of chilling or freeze tolerant crop varieties.     

Cell adhesion-dependent differences in endogenous protein phosphorylation on the surface of various cell lines

Endogenous phosphorylation of intact cells was studied with four mouse, hamster and human cell lines using [gamma-32P]ATP and [gamma-32P]GTP as exogenous substrates. With all four cell lines distinct differences in the phosphoprotein patterns could be demonstrated for cells grown in suspension culture compared to cells grown in monolayers. Two major, apparently ubiquitous phosphoproteins with molecular weights of 135 000 (128 000 in HeLa cells) and 105 000, representing up to 60% of total phosphorylation, were phosphorylated only in cells grown in suspension. These phosphoproteins and the kinase(s) were located on the surface of the suspension cells. Evidence showed that phosphorylation was apparently not a true endogenous reaction, that rather it occurred by cell-cell collision, showing exponentially increasing 32P incorporation with increasing cell population density. Phosphorylation of pp135 and pp105 was established with ATP as well as with GTP and was not dependent on cyclic nucleotides cyclic AMP, cyclic GMP and cyclic CMP. The substrate-attached cells of all four cell lines have protein kinases on the cell surface. The lack of pp135 and pp105 phosphorylation may be due to the fact that these phosphoproteins are not expressed at all on the surface of substrate-attached cells or that these phosphoproteins are already fully phosphorylated.     

Metaphase protein phosphorylation in Xenopus laevis eggs.

Cytoplasmic extracts of metaphase (M-phase)-arrested Xenopus laevis eggs support nuclear envelope breakdown and chromosome condensation in vitro. Induction of nuclear breakdown is inhibited by AMPP(NH)P, a nonhydrolyzable ATP analog, but not by ATP or gamma-S-ATP, a hydrolyzable ATP analog, suggesting that protein phosphorylation may be required for M-phase nuclear events in vitro. By addition of [gamma-32P]ATP, we have identified in cytoplasmic extracts and in intact eggs at least six phosphoproteins that are present during M-phase but absent in G1/S-phase. These phosphoproteins also appear in response to partially purified preparations of maturation-promoting factor. A subset of these proteins are thiophosphorylated by gamma-S-ATP under conditions that promote nuclear envelope breakdown and chromosome condensation. Each of these proteins is phosphorylated on serine and threonine, and one, a 42-kilodalton protein, is also phosphorylated on tyrosine both in extracts and in intact eggs. These results indicate that activation of protein kinases accounts for at least part of the increased phosphorylation in M-phase and that both protein-serine-threonine kinases and protein-tyrosine kinases may play a role in controlling M-phase nuclear behavior.     

Mass spectrometric identification of a novel phosphorylation site in subunit NDUFA10 of bovine mitochondrial complex I

Mitochondrial Complex I (NADH:ubiquinone oxidoreductase) consists of at least 46 subunits. Phosphorylation of the 42-kDa subunit NDUFA10 was recently reported using a novel phosphoprotein stain [Schulenberg et al. (2003) Analysis of steady-state protein phosphorylation in mitochondria using a novel fluorescent phosphosensor dye. J. Biol. Chem. 278, 27251]. Two smaller Complex I phosphoproteins, ESSS and MWFE, and their sites of modification, have since been determined [Chen et al. (2004) The phosphorylation of subunits of complex I from bovine heart mitochondria. J. Biol. Chem. 279, 26036]. Here we identify the site of phosphorylation in NDUFA10 from bovine heart mitochondria by tandem mass spectrometry. A single phosphopeptide spanning residues 47-60 was identified and confirmed by synthesis to be (47)LITVDGNICSGKpSK(60), establishing serine-59 as the site of phosphorylation.     

Extracellular phosphorylation in the parasite, Leishmania major

Intact promastigotes or cell-free extracts of the parasite Leishmania major were labelled with adenosine 5'[gamma-32P]-triphosphate (ATP). This resulted in the identification of eleven phosphoproteins. [gamma-32P]ATP incorporation into endogenous and exogenous substrates was insensitive to most of the commonly used protein kinase inhibitors and activators indicating that the leishmanial enzyme(s) may represent a new class of kinase(s). In addition, exogenous substrate specificity was inconsistent with the preferences of second messenger-dependent protein kinases. Cyclic AMP had differential effects on phosphorylation in intact cells and lysates. The majority of kinase activity could be attributed to an externally oriented membrane-associated protein kinase(s), as no specific cytosolic phosphoproteins were found and intact cells phosphorylated exogenous substrates. Labelled ATP did not cross the membrane and [alpha-32P]ATP was an unsuitable substrate for the phosphorylation activity. The ectokinase activity on live Leishmania exhibited a different substrate preference when compared to the protein kinase activity in the particulate fraction, suggesting that more than one protein kinase may be present in L. major. Three serine-labelled phosphoproteins were specifically released into the medium. The presence of an ecto-kinase and these released phosphoproteins may play a significant role in host-parasite interactions.     

Proteome of the bacterium <Emphasis Type="Italic">Mycoplasma gallisepticum</Emphasis>

Using modern proteomic assays, we have identified the products of gene expression and posttranslational modifications of proteins of the bacterium Mycoplasma gallisepticum S6. Combinations of different technologies of protein separation by electrophoresis and mass-spectrometric analysis gave us a total of 446 proteins, i.e. 61% of the annotated proteins of this microorganism. The Pro-Q Diamond and Pro-Q Emerald dye technology was used for fluorescent detection of ten phosphoproteins and two glycoproteins. The acylation of proteins was studied by electrophoresis after in vivo labeling with different ¹⁴C-labeled fatty acids, followed by autoradiography. Sixteen acylated proteins were identified, with a quarter of them involved in plasma membrane construction and another quarter involved in cell energy metabolism.     

Fat cell protein phosphorylation. Identification of phosphoprotein-2 as ATP-citrate lyase.

We have purified to apparent homogeneity a phosphoprotein from rat adipose tissue which is rapidly phosphorylated in vitro by ATP. The native phosphoprotein has an approximate sedimentation coefficient of 14.8 S. On sodium dodecyl sulfate-polyacrylamide slab gel electrophoresis, the protein dissociated into identical subunits of Mr = 128,000. A phosphoprotein with similar properties was also isolated from liver. Purified phosphoproteins from fat cells and liver had ATP-citrate lyase activity and co-migrated on sodium dodecyl sulfate gels with fat cell phosphoprotein-2, the phosphorylation of which is increased by incubating fat cells with insulin. The phosphoamino acid residue of the cells with insulin. The phosphoamino acid residue of the phosphoprotein was identified as tau-phosphohistidine. These evidences suggest that fat cell phosphoprotein-2 is ATP-citrate lyase.     

The new platinum-based anticancer agent LA-12 induces retinol binding protein 4 in vivo

Background

High temperature is a critical abiotic stress that reduces crop yield and quality. Rice (Oryza sativa L.) plants remodel their proteomes in response to high temperature stress. Moreover, phosphorylation is the most common form of protein post-translational modification (PTM). However, the differential expression of phosphoproteins induced by heat in rice remains unexplored.

Methods

Phosphoprotein in the leaves of rice under heat stress were displayed using two-dimensional electrophoresis (2-DE) and Pro-Q Diamond dye. Differentially expressed phosphoproteins were identified by MALDI-TOF-TOF-MS/MS and confirmed by Western blotting.

Results

Ten heat-phosphoproteins were identified from twelve protein spots, including ribulose bisphos-phate carboxylase large chain, 2-Cys peroxiredoxin BAS1, putative mRNA binding protein, Os01g0791600 protein, OSJNBa0076N16.12 protein, putative H(+)-transporting ATP synthase, ATP synthase subunit beta and three putative uncharacterized proteins. The identification of ATP synthase subunit beta was further validated by Western-blotting. Four phosphorylation site predictors were also used to predict the phosphorylation sites and the specific kinases for these 10 phosphoproteins.

Conclusion

Heat stress induced the dephosphorylation of RuBisCo and the phosphorylation of ATP-β, which decreased the activities of RuBisCo and ATP synthase. The observed dephosphorylation of the mRNA binding protein and 2-Cys peroxiredoxin may be involved in the transduction of heat-stress signaling, but the functional importance of other phosphoproteins, such as H⁺-ATPase, remains unknown.     

Characterization of specific differences in protein phosphorylation of the plasma membrane and the endoplasmic reticulum of mouse fibroblasts.

Endogenous phosphorylation was studied with highly purified fractions of the plasma membrane and the endoplasmic reticulum of SV40-transformed mouse fibroblasts using [gamma-32P]ATP and [gamma-32P]GTP as precursors. With ATP maximum overall incorporation of 32P into both membrane fractions occurred at pH 7.8 in the presence of 10 mM MgCl2 after incubation for 1 min. GTP could be utilized only by the plasma membrane fraction showing maximum incorporation of 32P at pH 7.8 and 10 mM MgCl2 after incubation for 3 min. The pattern of phosphoproteins of the plasma membrane is represented by more than 15 proteins whereas the endoplasmic reticulum essentially contained only one phosphorylated component of 35 000 molecular weight. The comparison of ATP- and GTP-specific phosphorylation of the plasma membrane revealed GTP to be a less efficient precursor yielding a similar phosphoprotein pattern with one significant difference: the GTP-specific main component exhibited a molecular weight of about 100 000 and the ATP-specific main component a molecular weight of 110 000. The relative distribution of individual phosphoproteins in the pattern of the plasma membrane was dependent on pH but not on MgCl2 concentration or time of incubation. Increasing concentrations of plasma membrane protein altered the patterns of phosphoproteins dramatically: At high protein concentrations the ATP-specific main component (Mr = 110 000) was no more phosphorylated whereas with GTP the main component Mr = 100 000 was essentially the sole phosphorylated protein.     

Tyrosine phosphorylation of a membrane protein from Pseudomonas solanacearum.

We have investigated a tyrosine kinase activity from Pseudomonas solanacearum, an economically important plant pathogen. In vitro incubation of membrane fractions with [gamma-32P]ATP and subsequent sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed an 85-kDa phosphoprotein. Phosphorylation of this protein on tyrosine residues was demonstrated by phosphoamino acid analysis of base hydrolysis products and by immunoanalysis of Western blots (immunoblots) with antiphosphotyrosine monoclonal antibody. In vitro incubation of membranes with ATP was not required for recognition by the antibody, indicating that the 85-kDa protein is phosphorylated in vivo. These results demonstrate that membranes from P. solanacearum exhibit a tyrosine kinase activity toward an endogenous membrane protein. This bacterium provides an opportunity to study the structure and function of a prokaryotic tyrosine kinase.     

Characterization of the pp70 protein phosphorylation in the extract of rice young panicle

The endogenous phosphorylation pattern of the extract prepared from rice young panicle has been examined. The extract was phosphorylated in vitro by incubating with [gamma-32p] ATP, and then analyzed by SDS-PAGE and autoradiography. At least eight major phosphoproteins with apparent molecular weights of 70, 60, 52, 40, 33, 30, 20, 16 and 15 kd were detected in the crude extract of rice young panicle. The pp70 which represents the major phosphoprotein in the crude extract of young panicle of rice is a cytosolic protein. Photoaffinity labeling with 8-azido-ATP revealed that a major protein with molecular weight 42,000 dalton but not pp70 can be specifically bound ATP. This suggests that pp70 is not a protein kinase itself, but a substrate of protein kinase. The in vitro phosphorylation of the pp70 occurs at a serine or threonine residue and is time and temperature dependent. The phosphorylation of pp70 does not depend by exogenous Ca++ or cAMP, suggesting that pp70 is a major substrate of an Ca++ or cAMP independent protein kinase in rice young panicle.