Regulation of the catalytic behaviour of L-form starch phosphorylase from sweet potato roots by proteolysis

Starch phosphorylase (SP) is an enzyme used for the reversible phosphorolysis of the α-glucan in plant cells. When compared to its isoform in an animal cell, glycogen phosphorylase, a peptide containing 78 amino acids (L78) is inserted in the centre of the low-affinity type starch phosphorylase (L-SP). We found that the amino acid sequence of L78 had several interesting features including the presence of a PEST region, which serves as a signal for rapid degradation. Indeed, most L-SP molecules isolated from mature sweet potato roots were nicked in the middle of a molecule, but still retained their tertiary or quaternary structures, as well as full catalytic activity. The nicking sites on the L78 were identified by amino acid sequencing of these peptides, which also enabled us to propose a proteolytic process for L-SP. Enzyme kinetic studies of L-SP in the direction of starch synthesis indicated that the K_m decreased during the proteolytic process when starch was used as the limiting substrate, but the K_m for the other substrate (Glc-1-P) increased. On the other hand, the maximum velocities (V_max) increased for both substrates. Mobility of the nicked L-SP was retarded on a native polyacrylamide gel containing soluble starch, indicating the increased affinity for starch. Results in this study suggested that L78 and its proteolytic modifications might play a regulatory role on the catalytic behaviour of L-SP in starch biosynthesis.     

Plastidial starch phosphorylase in sweet potato roots is proteolytically modified by protein-protein interaction with the 20S proteasome

Post-translational regulation plays an important role in cellular metabolism. Earlier studies showed that the activity of plastidial starch phosphorylase (Pho1) may be regulated by proteolytic modification. During the purification of Pho1 from sweet potato roots, we observed an unknown high molecular weight complex (HX) showing Pho1 activity. The two-dimensional gel electrophoresis, mass spectrometry, and reverse immunoprecipitation analyses showed that HX is composed of Pho1 and the 20S proteasome. Incubating sweet potato roots at 45°C triggers a stepwise degradation of Pho1; however, the degradation process can be partially inhibited by specific proteasome inhibitor MG132. The proteolytically modified Pho1 displays a lower binding affinity toward glucose 1-phosphate and a reduced starch-synthesizing activity. This study suggests that the 20S proteasome interacts with Pho1 and is involved in the regulation of the catalytic activity of Pho1 in sweet potato roots under heat stress conditions.     

Primary structure of sweet potato starch phosphorylase deduced from its cDNA sequence

Sweet potato (Ipomoea batatas) starch phosphorylase cDNA clones were isolated by screening an expression library prepared from the young root poly(A)⁺ RNA successively with an antiserum, a monoclonal antibody, and a specific oligonucleotide probe. One cDNA clone had 3292 nucleotide residues in which was contained an open reading frame coding for 955 amino acids. This sequence was compared with those of potato (916 residues plus 50-residue putative transit peptide) and rabbit muscle (841 residues) phosphorylases. The sweet potato phosphorylase has an overall structural feature highly homologous to that reported for potato phosphorylase, in conformity with the finding that they belong to the same class of plant phosphorylase. High divergencies of the two enzymes are found in the about 70 residue N-termini each including a putative transit peptide, and the midchain 78 residue insert typical of type I plant phosphorylase. We consider that the very high dissimilarity found in the midchain inserts is related to the difference in proteolytic lability of the two plant phosphorylases. Some structural features of the cDNA clone were also discussed.     

A sucrose non‐fermenting‐1‐related protein kinase‐1 gene,IbSnRK1, improves starch content,composition, granule size,degree of crystallinity and gelatinization in transgenic sweet potato

Sucrose non‐fermenting‐1‐related protein kinase‐1 (SnRK1) is an essential energy‐sensing regulator and plays a key role in the global control of carbohydrate metabolism. The SnRK1 gene has been found to increase starch accumulation in several plant species. However, its roles in improving starch quality have not been reported to date. In this study, we found that the IbSnRK1 gene was highly expressed in the storage roots of sweet potato and strongly induced by exogenous sucrose. Its expression followed the circandian rhythm. Its overexpression not only increased starch content, but also decreased proportion of amylose, enlarged granule size and improved degree of crystallinity and gelatinization in transgenic sweet potato, which revealed, for the first time, the important roles of SnRK1 in improving starch quality of plants. The genes involved in starch biosynthesis pathway were systematically up‐regulated, and the content of ADP‐glucose as an important precursor for starch biosynthesis and the activities of key enzymes were significantly increased in transgenic sweet potato. These findings indicate that IbSnRK1 improves starch content and quality through systematical up‐regulation of the genes and the increase in key enzyme activities involved in starch biosynthesis pathway in transgenic sweet potato. This gene has the potential to improve starch content and quality in sweet potato and other plants.     

The phosphorylation pattern of human alphas1-casein is markedly different from the ruminant species.

Caseins are highly phosphorylated milk proteins assembled in large colloidal structures termed micelles. In the milk of ruminants, alphas1-casein has been shown to be extensively phosphorylated. In this report we have determined the phosphorylation pattern of human alphas1-casein by a combination of matrix-assisted laser desorption mass spectrometry and amino acid sequence analysis. Three phosphorylation variants were identified. A nonphosphorylated form, a variant phosphorylated at Ser18 and a variant phosphorylated at Ser18 and Ser26. Both phosphorylation sites are located in the amino acid recognition sequence of the mammary gland casein kinase. Notably, no phosphorylations were observed in the conserved region covering residues Ser70-Glu78, which is extensively phosphorylated in the ruminant alphas1-caseins.     

Co-producing lipopeptides and poly-gamma-glutamic acid by solid-state fermentation of Bacillus subtilis using soybean and sweet potato residues and its biocontrol and fertilizer synergistic effects

A Bacillus subtilis strain B6-1, previously isolated from the rhizosphere of vegetable, selectively produced antibiotics or poly-gamma-glutamic acid (gamma-PGA) in two kinds of liquid media and their co-productions were obtained when using soybean and sweet potato residues in solid-state fermentation. The antibiotics were purified and identified as fengycins. After these residue cultures were introduced, cucumber wilts were effectively suppressed. The introduction also significantly increased the dry weights of roots and shoots of cucumber seedlings, and the roots to shoots ratio, especially at lower nutrition, which indicated the fertilizer synergistic effects. So the product of soybean and sweet potato residues, cultivated with B6-1 co-producing lipopeptides and gamma-PGA, can be expected to be used as both biocontrol agents and fertilizer synergists.     

Phosphorylation of the GABAa/Benzodiazepine Receptor α Subunit by a Receptor-Associated Protein Kinase

Abstract: Partially purified preparations of GABA_a/benzodiazepine receptor from rat brain were found to contain high levels of a protein kinase activity that phosphorylated a small number of proteins in the receptor preparations, including a 50-kilodalton (kD) phosphoprotein that comigrated on two-dimensional electrophoresis with purified, immunolabeled, and photolabeled receptor α subunit. Further evidence that the comigrating 50-kD phosphoprotein was, in fact, the receptor α subunit was obtained by peptide mapping analysis: the 50-kD phosphoprotein yielded one-dimensional peptide maps identical to those obtained from iodinated, purified α subunit. Phosphoamino acid analysis revealed that the receptor α subunit is phosphorylated on serine residues by the protein kinase activity present in receptor preparations. Preliminary characterization of the receptor-associated protein kinase activity suggested that it may be a second messenger-independent protein kinase. Protein kinase activity was unaltered by cyclic AMP, cyclic GMP, calcium plus calmodulin, calcium plus phosphatidylserine, and various inhibitors of these protein kinases. Examination of the substrate specificity of the receptor-associated protein kinase indicated that the enzyme preferred basic proteins as substrates. Endogenous phosphorylation experiments indicated that the receptor α subunit may also be phosphorylated in crude membranes by a protein kinase activity present in those membranes. As with phosphorylation of the receptor in purified preparations, its phosphorylation in crude membranes also appeared to be unaffected by activators and inhibitors of second messenger-dependent protein kinases. These findings raise the possibility that the phosphorylation of the α subunit of the GABA_a/ benzodiazepine receptor by a receptor-associated protein kinase plays a role in modulating the physiological activity of the receptor in vivo.     

Protein kinase C phosphorylation regulates membrane insertion of GABAA receptor subtypes that mediate tonic inhibition

Tonic inhibition in the brain is mediated largely by specialized populations of extrasynaptic receptors, γ-aminobutyric acid receptors (GABA(A)Rs). In the dentate gyrus region of the hippocampus, tonic inhibition is mediated primarily by GABA(A)R subtypes assembled from α4β2/3 with or without the δ subunit. Although the gating of these receptors is subject to dynamic modulation by agents such as anesthetics, barbiturates, and neurosteroids, the cellular mechanisms neurons use to regulate their accumulation on the neuronal plasma membrane remain to be determined. Using immunoprecipitation coupled with metabolic labeling, we demonstrate that the α4 subunit is phosphorylated at Ser(443) by protein kinase C (PKC) in expression systems and hippocampal slices. In addition, the β3 subunit is phosphorylated on serine residues 408/409 by PKC activity, whereas the δ subunit did not appear to be a PKC substrate. We further demonstrate that the PKC-dependent increase of the cell surface expression of α4 subunit-containing GABA(A)Rs is dependent on Ser(443). Mechanistically, phosphorylation of Ser(443) acts to increase the stability of the α4 subunit within the endoplasmic reticulum, thereby increasing the rate of receptor insertion into the plasma membrane. Finally, we show that phosphorylation of Ser(443) increases the activity of α4 subunit-containing GABA(A)Rs by preventing current run-down. These results suggest that PKC-dependent phosphorylation of the α4 subunit plays a significant role in enhancing the cell surface stability and activity of GABA(A)R subtypes that mediate tonic inhibition.     

Transmodulation of epidermal growth factor receptor function by cyclic AMP-dependent protein kinase.

Binding of epidermal growth factor (EGF) to its receptor (EGFR) augments the tyrosine kinase activity of the receptor and autophosphorylation. Exposure of some tissues and cells to EGF also stimulates adenylyl cyclase activity and results in an increase in cyclic AMP (cAMP) levels. Because cAMP activates the cAMP-dependent protein kinase A (PKA), we investigated the effect of PKA on the EGFR. The purified catalytic subunit of PKA (PKAc) stoichiometrically phosphorylated the purified full-length wild type (WT) and kinase negative (K721M) forms of the EGFR. PKAc phosphorylated both WT-EGFR as well as a mutant truncated form of EGFR (Delta1022-1186) exclusively on serine residues. Moreover, PKAc also phosphorylated the cytosolic domain of the EGFR (EGFRKD). Phosphorylation of the purified WT as well as EGFRDelta1022-1186 and EGFRKD was accompanied by decreased autophosphorylation and diminished tyrosine kinase activity. Pretreatment of REF-52 cells with the nonhydrolyzable cAMP analog, 8-(4-chlorophenylthio)-cAMP, decreased EGF-induced tyrosine phosphorylation of cellular proteins as well as activation of the WT-EGFR. Similar effects were also observed in B82L cells transfected to express the Delta1022-1186 form of EGFR. Furthermore, activation of PKAc in intact cells resulted in serine phosphorylation of the EGFR. The decreased phosphorylation of cellular proteins and diminished activation of the EGFR in cells treated with the cAMP analog was not the result of altered binding of EGF to its receptors or changes in receptor internalization. Therefore, we conclude that PKA phosphorylates the EGFR on Ser residues and decreases its tyrosine kinase activity and signal transduction both in vitro and in vivo.     

Phosphorylation of human high mobility group N1 protein by protein kinase CK2

High mobility group (HMG) N1 protein, formerly known as HMG 14, is a member of the chromosomal HMG protein family. Protein kinase CK2 was previously reported to be able to phosphorylate bovine HMGN1 in vitro; Ser89 and Ser99, corresponding to Ser88 and Ser98 in human HMGN1, were shown to be major and minor recognition sites, respectively. In this report, we employed mass spectrometry and examined both the extent and the sites of phosphorylation in HMGN1 protein catalyzed by recombinant human protein kinase CK2. We found that five serine residues, i.e., Ser6, Ser7, Ser85, Ser88, and Ser98, in HMGN1 can be phosphorylated by the kinase in vitro. All five sites were previously shown to be phosphorylated in MCF-7 human breast cancer cells in vivo. Among these five sites, Ser6, Ser7, and Ser85 were new sites of phosphorylation induced by protein kinase CK2 in vitro.     

Two different protein kinases act on a different time schedule as glial filament kinases during mitosis.

Glial fibrillary acidic protein (GFAP) is a component of glial filaments specific to astroglia. We now report the spatial and temporal distributions of four phosphorylated sites in the GFAP molecule during mitosis of astroglial cells, determined by antibodies which can distinguish phosphorylated epitopes from non-phosphorylated-epitopes. Immunofluorescence microscopy showed that the Ser8 residues in the entire cytoplasmic glial filament system are initially phosphorylated when the cells enter mitosis. In cytokinesis, the phosphoSer8 residues become dephosphorylated, whereas Thr7, Ser13 and Ser34 in glial filaments at the cleavage furrow become the preferred sites of phosphorylation. The cdc2 kinase purified from mitotic cells can phosphorylate GFAP at Ser8 but not at Thr7, Ser13 or Ser34, in vitro. These results suggest that cdc2 kinase acts as a glial filament kinase only at the G2-M phase transition while other glial filament kinases are probably activated at the cleavage furrow before final separation of the daughter cells.     

Purification and characterization of wheat and pine small basic protein substrates for plant calcium-dependent protein kinase.

A wheat basic protein (WBP) was purified to homogeneity from wheat germ by a protocol involving extraction, centrifugation, batchwise elution from carboxymethylcellulose (CM-52), acidification with trifluoroacetic acid, neutralization and HPLC on a SP5PW cation exchange column. WBP is a 10 kDa protein and is phosphorylated on serine residues by wheat germ Ca(2+)-dependent protein kinase (CDPK). [32P]phosphoWBP exactly comigrates with WBP on SDS-PAGE. WBP does not inhibit either wheat germ CDPK or calmodulin-dependent myosin light chain kinase. Apart from histone H1, WBP is the best endogenous substrate yet found for wheat embryo CDPK. A 12 kDa pine basic protein (PBP) was purified to homogeneity from seeds of stone pine (Pinus pinea L.) by a simple procedure involving batchwise elution from carboxymethylcellulose and cation exchange HPLC. PBP is also a good substrate for CDPK and is phosphorylated on Ser residues. N-terminal sequencing of WBP and PBP revealed that these proteins are homologous to a family of small basic plant proteins having a phospholipid transfer function.     

Phosphorylation of P0 Glycoprotein in Peripheral Nerve Myelin

The P0 protein in mammalian PNS myelin is known to undergo several posttranslational modifications, such as glycosylation, acylation, sulfation, and phosphorylation. Phosphorylation of purified P0 protein in vitro was studied comparatively using three enzymes, i.e., calcium/phospholipid-dependent protein kinase (protein kinase C), calcium/calmodulin-dependent protein kinase II (CaM kinase II), and the catalytic subunit of cyclic AMP-dependent protein kinase (A kinase). The phosphorylation of P0 protein by CaM kinase II was the greatest, followed by that by protein kinase C; phosphorylation by A kinase, however, was much lower. In order to identify phosphorylation sites, P0 protein was phosphorylated with [32P]ATP and each kinase and then digested with lysylendopeptidase. The resulting phosphopeptides were isolated by HPLC. Subsequent amino acid sequence analysis and comparison with the known sequence of P0 protein revealed that Ser181 and Ser204 were strongly phosphorylated by both protein kinase C and CaM kinase II. In addition, Ser214 was also phosphorylated by protein kinase C, but not by CaM kinase II. Because all of these sites are located in the cytoplasmic domain of P0 protein, phosphorylation may be important for maintenance of the major dense line of PNS myelin.     

Nucleoside diphosphate kinase from Escherichia coli.

Nucleoside diphosphate (NDP) kinase from Escherichia coli was purified to homogeneity and was crystallized. Gel filtration analysis of the purified enzyme indicated that it forms a tetramer. The enzyme was phosphorylated with [gamma-32P]ATP, and the pH stability profile of the phosphoenzyme indicated that two different amino acid residues were phosphorylated. Both a histidine residue and serine residues, including Ser-119 and Ser-121, appear to be phosphorylated. A Ser119Ala/Ser121Ala double mutant (i.e., with a Ser-to-Ala double mutation at positions 119 and 121), as well as Ser119Ala and Ser121Ala mutants, was isolated. All of these retained NDP kinase activity; also, both the Ser119Ala and Ser121Ala mutants could still be autophosphorylated. In the case of the double mutant, a slight autophosphorylation activity, which was resistant to acid treatment, was still detected, indicating that an additional minor autophosphorylation site besides His-117 exists. These results are discussed in light of the recent report of N. J. MacDonald et al. on the autophosphorylation of human NDP kinase (J. Biol. Chem. 268:25780-25789, 1993).     

Identification of the major phosphorylation site of the hepatitis C virus H strain NS5A protein as serine 2321.

The hepatitis C virus (HCV) NS5A protein is phosphorylated by a cellular, serine/threonine kinase. To identify the major site(s) of NS5A phosphorylation, radiolabeled HCV-H NS5A phosphopeptides were purified and subjected to phosphoamino acid analysis and Edman degradation. These data identified the major intracellular phosphorylation site in the HCV-H NS5A protein as Ser(2321), a result verified by two additional, independent methods: (i) substitution of Ala for Ser(2321) and the concomitant disappearance of the major in vivo phosphorylated peptides and corresponding in vitro phosphorylated peptides; and (ii) comigration of the digestion products of a synthetic peptide phosphorylated on Ser(2321) with the major in vivo phosphorylated NS5A peptides. Site-directed mutagenesis of Ser(2321) suggested that phosphorylation of NS5A is dispensable for previously described interactions with NS4A and PKR, a cellular, antiviral kinase that does not appear to catalyze NS5A phosphorylation. The proline-rich nature of the amino acid sequence flanking Ser(2321) (PLPPPRS(2321) PPVPPPR) suggests that a proline-directed kinase is responsible for the majority of HCV NS5A phosphorylation, consistent with previous kinase inhibitor studies.     

HSP22, a new member of the small heat shock protein superfamily, interacts with mimic of phosphorylated HSP27 ((3D)HSP27)

Most of the members of the superfamily of mammalian small heat shock or stress proteins are abundant in muscles where they play a role in muscle function and maintenance of muscle integrity. One member of this protein superfamily, human HSP27, is rapidly phosphorylated on three serine residues (Ser(15), Ser(78), and Ser(82)) during cellular response to a number of extracellular factors. To understand better the role of HSP27, we performed a yeast two-hybrid screen of a human heart cDNA library for HSP27-interacting proteins. By using the triple aspartate mutant, a mimic of phosphorylated HSP27, as "bait" construct, a protein with a molecular mass of 21.6 kDa was identified as an HSP27-binding protein. Sequence analysis revealed that this new protein shares an overall sequence identity of 33% with human HSP27. This protein also contains the alpha-crystallin domain in its C-terminal half, a hallmark of the superfamily of small stress proteins. Thus, the new protein itself is a member of this protein superfamily, and consequently we designated it HSP22. According to the two-hybrid data, HSP22 interacts preferentially with the triple aspartate form of HSP27 as compared with wild-type HSP27. HSP22 is expressed predominantly in muscles. In vitro, HSP22 is phosphorylated by protein kinase C (at residues Ser(14) and Thr(63)) and by p44 mitogen-activated protein kinase (at residues Ser(27) and Thr(87)) but not by MAPKAPK-2.     

Role of calonyctin on free sugars in relation to starch accumulation in developing sweet potatoes

Calonyctin, a natural plant growth regulator extracted from the leaves of Calonyction aculeatum (L.) House, can promote crop growth and increase crop yield. The specific reasons for this response are unknown. This study was conducted to determine the effect of calonyctin treatment on the free sugars of sweet potato [Ipomoea batatas (L.) Lam.] as related to starch accumulation. The sweet potatoes were grown in the field in 1992, treated by foliar spray with Calonyctin concentrations of 0 (control) and 0.1 activity unit (CTSP) at 20 days after planting (DAP) at the rate of 190 liters of diluted solution/ha., and sampled periodically to determine free sugars. The response of sweet potato to calonyctin was first detected at 40 days after treatment (on 60 DAP). Data indicated that calonyctin treatment significantly increased starch synthesis in storage roots, decreased the fluctuation tendency of total sugar level during the growth period, and kept the sugar level relatively constant with a gradual rise regardless of variations in weather. The level of the reducing sugars in CTSP leaves was higher at 60 and 160 DAP and lower at 100, 120, and 140 DAP. During rainy days (100 DAP), the reducing sugars in CTSP storage roots remained at a lower level when those in controls reached high levels. The sucrose content in CTSP leaves was 40–138% greater than that in controls except at 80 and 120 DAP, and the ratio of sucrose to total nonreducing sugars remained at 100% in CTSP leaves even on rainy and cool days and above 96% in CTSP storage roots except on cool days (140 and 160 DAP), suggesting that calonyctin treatment promoted the synthesis and transfer of sucrose and supplied abundant sugar precursors for starch accumulation in storage roots.Abbreviations DAP days after planting - CTSP calonyctin-treated sweet potato with 0.1 activity unit     

Phosphorylation of HMG-I by protein kinase C attenuates its binding affinity to the promoter regions of protein kinase C gamma and neurogranin/RC3 genes

A 20-kDa DNA-binding protein that binds the AT-rich sequences within the promoters of the brain-specific protein kinase C (PKC) gamma and neurogranin/RC3 genes has been characterized as chromosomal nonhistone high-mobility-group protein (HMG)-I. This protein is a substrate of PKC alpha, beta, gamma, and delta but is poorly phosphorylated by PKC epsilon and zeta. Two major (Ser44 and Ser64) and four minor phosphorylation sites have been identified. The extents of phosphorylation of Ser44 and Ser64 were 1:1, whereas those of the four minor sites all together were <30% of the major one. These PKC phosphorylation sites are distinct from those phosphorylated by cdc2 kinase, which phosphorylates Thr53 and Thr78. Phosphorylation of HMG-I by PKC resulted in a reduction of DNA-binding affinity by 28-fold as compared with 12-fold caused by the phosphorylation with cdc2 kinase. HMG-I could be additively phosphorylated by cdc2 kinase and PKC, and the resulting doubly phosphorylated protein exhibited a >100-fold reduction in binding affinity. The two cdc2 kinase phosphorylation sites of HMG-I are adjacent to the N terminus of two of the three predicted DNA-binding domains. In comparison, one of the major PKC phosphorylation sites, Ser64, is adjacent to the C terminus of the second DNA-binding domain, whereas Ser44 is located within the spanning region between the first and second DNA-binding domains. The current results suggest that phosphorylation of the mammalian HMG-I by PKC alone or in combination with cdc2 kinase provides an effective mechanism for the regulation of HMG-I function.