Phosphorylation of the yeast phospholipid synthesis regulatory protein Opi1p by protein kinase C

Opi1p is a negative regulator of expression of phospholipid-synthesizing enzymes in the yeast Saccharomyces cerevisiae. In this work, we examined the phosphorylation of Opi1p by protein kinase C. Using a purified maltose-binding protein-Opi1p fusion protein as a substrate, protein kinase C activity was time- and dose-dependent, and dependent on the concentrations of Opi1p and ATP. Protein kinase C phosphorylated Opi1p on a serine residue. The Opi1p synthetic peptide GVLKQSCRQK, which contained a protein kinase C sequence motif at Ser(26), was a substrate for protein kinase C. Phosphorylation of a purified S26A mutant maltose-binding protein-Opi1p fusion protein by the kinase was reduced when compared with the wild-type protein. A major phosphopeptide present in purified wild-type Opi1p was absent from the purified S26A mutant protein. In vivo labeling experiments showed that the phosphorylation of Opi1p was physiologically relevant, and that the extent of phosphorylation of the S26A mutant protein was reduced by 50% when compared with the wild-type protein. The physiological consequence of the phosphorylation of Opi1p at Ser(26) was examined by measuring the effect of the S26A mutation on the expression of the phospholipid synthesis gene INO1. The beta-galactosidase activity driven by an INO1-CYC-lacI'Z reporter gene in opi1Delta mutant cells expressing the S26A mutant Opi1p was about 50% lower than that of cells expressing the wild-type Opi1p protein. These data supported the conclusion that phosphorylation of Opi1p at Ser(26) mediated the attenuation of the negative regulatory function of Opi1p on the expression of the INO1 gene.     

Phosphorylation of Saccharomyces cerevisiae CTP synthetase at Ser424 by protein kinases A and C regulates phosphatidylcholine synthesis by the CDP-choline pathway

The Saccharomyces cerevisiae URA7-encoded CTP synthetase is phosphorylated and stimulated by protein kinases A and C. Previous studies have revealed that Ser424 is the target site for protein kinase A. Using a purified S424A mutant CTP synthetase enzyme, we examined the effect of Ser424 phosphorylation on protein kinase C phosphorylation. The S424A mutation in CTP synthetase caused a 50% decrease in the phosphorylation of the enzyme by protein kinase C and an 80% decrease in the stimulatory effect on CTP synthetase activity by protein kinase C. The S424A mutation caused increases in the apparent Km values of CTP synthetase and ATP of 20-and 2-fold, respectively, in the protein kinase C reaction. The effect of the S424A mutation on the phosphorylation reaction was dependent on time and protein kinase C concentration. A CTP synthetase synthetic peptide (SLGRKDSHSA) containing Ser424 was a substrate for protein kinase C. Comparison of phosphopeptide maps of the wild type and S424A mutant CTP synthetase enzymes phosphorylated by protein kinases A and C indicated that Ser424 was also a target site for protein kinase C. Phosphorylation of Ser424 accounted for 10% of the total phosphorylation of CTP synthetase by protein kinase C. The incorporation of [methyl-3H]choline into phosphocholine, CDP-choline, and phosphatidylcholine in cells carrying the S424A mutant CTP synthetase enzyme was reduced by 48, 32, and 46%, respectively, when compared with control cells. These data indicated that phosphorylation of Ser424 by protein kinase A or by protein kinase C was required for maximum phosphorylation and stimulation of CTP synthetase and that the phosphorylation of this site played a role in the regulation of phosphatidylcholine synthesis by the CDP-choline pathway.     

Characterization of protein kinase A-mediated phosphorylation of ezrin in gastric parietal cell activation

Gastric ezrin was initially identified as a phosphoprotein associated with parietal cell activation. To explore the nature of ezrin phosphorylation, proteins from resting and secreting gastric glands were subjected to two-dimensional SDS-PAGE. Histamine triggers acid secretion and a series of acidic isoforms of ezrin on two-dimensional SDS-PAGE. Mass spectrometric analysis of these acidic ezrin spots induced by stimulation suggests that Ser66 is phosphorylated. To determine whether Ser66 is a substrate of protein kinase A (PKA), recombinant proteins of ezrin, both wild type and S66A mutant, were incubated with the catalytic subunit of PKA and [32P]ATP. Incorporation of 32P into wild type but not the mutant ezrin verified that Ser66 is a substrate of PKA. In addition, expression of S66A mutant ezrin in cultured parietal cells attenuates the dilation of apical vacuolar membrane associated with stimulation by histamine, indicating that PKA-mediated phosphorylation of ezrin is necessary for acid secretion. In fact, expression of phosphorylation-like S66D mutant in parietal cells mimics histamine-stimulated apical vacuole remodeling. Further examination of H,K-ATPase distribution revealed a blockade of stimulation-induced proton pump mobilization in S66A but not S66D ezrin-expressing parietal cells. These data suggest that PKA-mediated phosphorylation of ezrin plays an important role in mediating the remodeling of the apical membrane cytoskeleton associated with acid secretion in parietal cells.     

Phosphorylation at serine 10, a major phosphorylation site of p27(Kip1), increases its protein stability

The association of the p27(Kip1) protein with cyclin and cyclin-dependent kinase complexes inhibits their kinase activities and contributes to the control of cell proliferation. The p27(Kip1) protein has now been shown to be phosphorylated in vivo, and this phosphorylation reduces the electrophoretic mobility of the protein. Substitution of Ser(10) with Ala (S10A) markedly reduced the extent of p27(Kip1) phosphorylation and prevented the shift in electrophoretic mobility. Phosphopeptide mapping and phosphoamino acid analysis revealed that phosphorylation at Ser(10) accounted for approximately 70% of the total phosphorylation of p27(Kip1), and the extent of phosphorylation at this site was approximately 25- and 75-fold greater than that at Ser(178) and Thr(187), respectively. The phosphorylation of p27(Kip1) was markedly reduced when the positions of Ser(10) and Pro(11) were reversed, suggesting that a proline-directed kinase is responsible for the phosphorylation of Ser(10). The extent of Ser(10) phosphorylation was markedly increased in cells in the G(0)-G(1) phase of the cell cycle compared with that apparent for cells in S or M phase. The p27(Kip1) protein phosphorylated at Ser(10) was significantly more stable than the unphosphorylated form. Furthermore, a mutant p27(Kip1) in which Ser(10) was replaced with glutamic acid in order to mimic the effect of Ser(10) phosphorylation exhibited a marked increase in stability both in vivo and in vitro compared with the wild-type or S10A mutant proteins. These results suggest that Ser(10) is the major site of phosphorylation of p27(Kip1) and that phosphorylation at this site, like that at Thr(187), contributes to regulation of p27(Kip1) stability.     

Phosphorylation of the yeast choline kinase by protein kinase C. Identification of Ser25 and Ser30 as major sites of phosphorylation

The Saccharomyces cerevisiae CKI1-encoded choline kinase catalyzes the committed step in phosphatidylcholine synthesis via the Kennedy pathway. The enzyme is phosphorylated on multiple serine residues, and some of this phosphorylation is mediated by protein kinase A. In this work we examined the hypothesis that choline kinase is also phosphorylated by protein kinase C. Using choline kinase as a substrate, protein kinase C activity was dose- and time-dependent and dependent on the concentrations of choline kinase (K(m) = 27 microg/ml) and ATP (K(m) = 15 microM). This phosphorylation, which occurred on a serine residue, was accompanied by a 1.6-fold stimulation of choline kinase activity. The synthetic peptide SRSSSQRRHS (V5max/K(m) = 17.5 mm(-1) micromol min(-1) mg(-1)) that contains the protein kinase C motif for Ser25 was a substrate for protein kinase C. A Ser25 to Ala (S25A) mutation in choline kinase resulted in a 60% decrease in protein kinase C phosphorylation of the enzyme. Phosphopeptide mapping analysis of the S25A mutant enzyme confirmed that Ser25 was a protein kinase C target site. In vivo the S25A mutation correlated with a decrease (55%) in phosphatidylcholine synthesis via the Kennedy pathway, whereas an S25D phosphorylation site mimic correlated with an increase (44%) in phosphatidylcholine synthesis. Although the S25A (protein kinase C site) mutation did not affect the phosphorylation of choline kinase by protein kinase A, the S30A (protein kinase A site) mutation caused a 46% reduction in enzyme phosphorylation by protein kinase C. A choline kinase synthetic peptide (SQRRHSLTRQ) containing Ser30 was a substrate (V(max)/K(m) = 3.0 mm(-1) micromol min(-1) mg(-1)) for protein kinase C. Comparison of phosphopeptide maps of the wild type and S30A mutant choline kinase enzymes phosphorylated by protein kinase C confirmed that Ser30 was also a target site for protein kinase C.     

PKA-mediated protein phosphorylation protects ezrin from calpain I cleavage

Ezrin is localized to the apical membrane of parietal cells and couples the cAMP-dependent protein kinase (PKA) activation cascade to the regulated HCl secretion in gastric parietal cells. Our recent studies demonstrate the functional relevance of PKA-mediated phosphorylation of ezrin in parietal cell secretion [R. Zhou, X. Cao, C. Watson, Y. Miao, Z. Guo, J.G. Forte, X. Yao, Characterization of protein kinase A-mediated phosphorylation of ezrin in gastric parietal cell activation, J. Biol. Chem. 278 (2003) 35651]. Here we show that activation of PKA protects ezrin from calpain I-mediated proteolysis without alteration of calpain I activation and fodrin breakdown. To determine whether phosphorylation of Ser66 by PKA affects the insensitivity to the calpain I-mediated cleavage, recombinant proteins of ezrin, both wild type and S66A/D mutants, were incubated with the purified calpain I. Indeed, phosphorylation-like S66D mutant ezrin is resistant to calpain I-mediated proteolysis while wild type and S66A mutant were sensitive. In fact, expression of phosphorylation-like S66D, but not S66A, mutant in parietal cells confers its resistance to calpain I-mediated proteolysis. Taken together, these results indicate that phosphorylation of ezrin by PKA modulates its sensitivity to calpain I cleavage.     

Regulation of the yeast INO1 gene. The products of the INO2, INO4 and OPI1 regulatory genes are not required for repression in response to inositol

The ino2Delta, ino4Delta, opi1Delta, and sin3Delta mutations all affect expression of INO1, a structural gene for inositol-1-phosphate synthase. These same mutations affect other genes of phospholipid biosynthesis that, like INO1, contain the repeated element UAS(INO) (consensus 5' CATGTGAAAT 3'). In this study, we evaluated the effects of these four mutations, singly and in all possible combinations, on growth and expression of INO1. All strains carrying an ino2Delta or ino4Delta mutation, or both, failed to grow in medium lacking inositol. However, when grown in liquid culture in medium containing limiting amounts of inositol, the opi1Delta ino4Delta strain exhibited a level of INO1 expression comparable to, or higher than, the wild-type strain growing under the same conditions. Furthermore, INO1 expression in the opi1Delta ino4Delta strain was repressed in cells grown in medium fully supplemented with both inositol and choline. Similar results were obtained using the opi1Delta ino2Delta ino4Delta strain. Regulation of INO1 was also observed in the absence of the SIN3 gene product. Therefore, while Opi1p, Sin3p, and the Ino2p/Ino4p complex all affect the overall level of INO1 expression in an antagonistic manner, they do not appear to be responsible for transmitting the signal that leads to repression of INO1 in response to inositol. Various models for Opi1p function were tested and no evidence for binding of Opi1p to UAS(INO), or to Ino2p or Ino4p, was obtained.     

Activation of Akt/protein kinase B in epithelial cells by the Salmonella typhimurium effector sigD

The serine-threonine kinase Akt is a protooncogene involved in the regulation of cell proliferation and survival. Activation of Akt is initiated by binding to the phospholipid products of phosphoinositide 3-kinase at the inner leaflet of the plasma membranes followed by phosphorylation at Ser(473) and Thr(308). We have found that Akt is activated by Salmonella enterica serovar Typhimurium in epithelial cells. A bacterial effector protein, SigD, which is translocated into host cells via the specialized type III secretion system, is essential for Akt activation. In HeLa cells, wild type S. typhimurium induced translocation of Akt to membrane ruffles and phosphorylation at residues Thr(308) and Ser(473) and increased kinase activity. In contrast, infection with a SigD deletion mutant did not induce phosphorylation or activity although Akt was translocated to membrane ruffles. Complementation of the SigD deletion strain with a mutant containing a single Cys to Ser mutation (C462S), did not restore the Akt activation phenotype. This residue has previously been shown to be essential for inositol phosphatase activity of the SigD homologue, SopB. Our data indicate a novel mechanism of Akt activation in which the endogenous cellular pathway does not convert membrane-associated Akt into its active form. SigD is also the first bacterial effector to be identified as an activator of Akt.     

Phosphorylation of the pro-apoptotic protein BAD on serine 155, a novel site, contributes to cell survival

Phosphorylation of BAD, a pro-apoptotic member of the Bcl-2 protein family, on either Ser112 or Ser136 is thought to be necessary and sufficient for growth factors to promote cell survival. Here we report that Ser155, a site phosphorylated by protein kinase A (PKA), also contributes to cell survival. Ser112 is thought to be the critical PKA target, but we found that BAD fusion proteins containing Ala at Ser112 (S112A) or Ser136 (S136A) or at both positions (S112/136A) were still heavily phosphorylated by PKA in an in vitro kinase assay. BAD became insensitive to phosphorylation by PKA only when both Ser112 and Ser136, or all three serines (S112/136/155) were mutated to alanine. In HEK293 cells, BAD fusion proteins mutated at Ser155 were refractory to phosphorylation induced by elevation of cyclic AMP(cAMP) levels. Phosphorylation of the S112/136A mutant was >90% inhibited by H89, a PKA inhibitor. The S155A mutant induced more apoptosis than the wild-type protein in serum-maintained CHO-K1 cells, and apoptosis induced by the S112/136A mutant was potentiated by serum withdrawal. These data suggest that Ser155 is a major site of phosphorylation by PKA and serum-induced kinases. Like Ser112 and Ser136, phosphorylation of Ser155 contributes to the cancellation of the pro-apoptotic function of BAD.     

Phosphorylation of Saccharomyces cerevisiae choline kinase on Ser30 and Ser85 by protein kinase A regulates phosphatidylcholine synthesis by the CDP-choline pathway

The Saccharomyces cerevisiae CKI-encoded choline kinase is phosphorylated on a serine residue and stimulated by protein kinase A. We examined the hypothesis that amino acids Ser(30) and Ser(85) contained in a protein kinase A sequence motif in choline kinase are target sites for protein kinase A. The synthetic peptides SQRRHSLTRQ (V(max)/K(m) = 10.8 microm(-1) nmol min(-1) mg(-1)) and GPRRASATDV (V(max)/K(m) = 0.15 microm(-1) nmol min(-1) mg(-1)) containing the protein kinase A motif for Ser(30) and Ser(85), respectively, within the choline kinase protein were substrates for protein kinase A. Choline kinase with Ser(30) to Ala (S30A) and Ser(85) to Ala (S85A) mutations were constructed alone and in combination by site-directed mutagenesis and expressed in a cki1Delta eki1Delta double mutant that lacks choline kinase activity. The mutant enzymes were expressed normally, but the specific activity of choline kinase in cells expressing the S30A, S85A, and S30A,S85A mutant enzymes was reduced by 44, 8, and 60%, respectively, when compared with the control. In vivo labeling experiments showed that the extent of phosphorylation of the S30A, S85A, and S30A,S85A mutant enzymes was reduced by 70, 17, and 83%, respectively. Phosphorylation of the S30A, S85A, and S30A,S85A mutant enzymes by protein kinase A in vitro was reduced by 60, 7, and 96%, respectively, and peptide mapping analysis of the mutant enzymes confirmed the phosphorylation sites in the enzyme. The incorporation of (3)H-labeled choline into phosphocholine and phosphatidylcholine in cells bearing the S30A, S85A, and S30A,S85A mutant enzymes was reduced by 56, 27, and 81%, respectively, and by 58, 33, and 84%, respectively, when compared with control cells. These data supported the conclusion that phosphorylation of choline kinase on Ser(30) and Ser(85) by protein kinase A regulates PC synthesis by the CDP-choline pathway.     

Serine 64 phosphorylation enhances the antiapoptotic function of Mcl-1

Mcl-1 is an antiapoptotic Bcl-2 family member that is highly regulated and when dysregulated contributes to cancer. The Mcl-1 protein is phosphorylated at multiple sites in response to different signaling events. Phosphorylations at Thr163 (by ERK) and Ser159 (by glycogen-synthase kinase 3beta) have recently been shown to slow and enhance, respectively, Mcl-1 protein turnover. Phosphorylation is also known to be stimulated at other, as-yet uncharacterized sites in the G2/M phase of the cell cycle. Using an S peptide-tagged Mcl-1 T163A mutant, Ser64 was identified as a novel Mcl-1 phosphorylation site by mass spectrometry. Immunoblotting demonstrated that phosphorylation at this site was maximal in cells in G2/M phase, was enhanced by tumor necrosis factor-alpha-related apoptosis-inducing ligand (TRAIL) treatment, was blocked by inhibitors of CDK (but not ERK or glycogen-synthase kinase 3beta), and was stimulated in vitro by CDK 1, CDK2, and JNK1. The half-life of a nonphosphorylatable S64A Mcl-1 mutant was indistinguishable from that of the wild type polypeptide. In contrast, this mutant failed to protect cells from TRAIL-mediated apoptosis, whereas reconstitution with the phosphomimetic S64E Mcl-1 mutant rendered cells TRAIL-resistant. This anti-apoptotic phenotype of the S64E Mcl-1 mutant was also associated with enhanced binding to the proapoptotic proteins Bim, Noxa, and Bak. A pharmacological CDK inhibitor that reduced Ser64 phosphorylation also sensitized cells to TRAIL cytotoxicity. Collectively, these observations not only identify G2/M-associated phosphorylation at Ser64 as a critical determinant of the antiapoptotic activity of Mcl-1 but also elucidate a novel mechanism by which CDK1/2 inhibitors can enhance the effectiveness of the cytotoxic cytokine TRAIL.     

Identification of phosphorylation sites on the yeast ribonucleotide reductase inhibitor Sml1

Sml1 is a small protein in Saccharomyces cerevisiae which inhibits the activity of ribonucleotide reductase (RNR). RNR catalyzes the rate-limiting step of de novo dNTP synthesis. Sml1 is a downstream effector of the Mec1/Rad53 cell cycle checkpoint pathway. The phosphorylation by Dun1 kinase during S phase or in response to DNA damage leads to diminished levels of Sml1. Removal of Sml1 increases the population of active RNR, which raises cellular dNTP levels. In this study using mass spectrometry and site-directed mutagenesis, we have identified the region of Sml1 phosphorylation to be between residues 52 and 64 containing the sequence GSSASASASSLEM. This is the first identification of a phosphorylation sequence of a Dun1 biological substrate. This sequence is quite different from the consensus Dun1 phosphorylation sequence reported previously from peptide library studies. The specific phosphoserines were identified to be Ser(56), Ser(58), and Ser(60) by chemical modification of these residues to S-ethylcysteines followed by collision activated dissociation. To investigate further Sml1 phosphorylation, we constructed the single mutants S56A, S58A, S60A, and the triple mutant S56A/S58A/S60A and compared their degrees of phosphorylation with that of wild type Sml1. We observed a 90% decrease in the relative phosphorylation of S60A compared with that of wild type, a 25% decrease in S58A, and little or no decrease in the S56A mutant. There was no observed phosphate incorporation in the triple mutant, suggesting that Ser(56), Ser(58), and Ser(60) in Sml1 are the sites of phosphorylation. Further mutagenesis studies reveal that Dun1 kinase requires an acidic residue at the +3 position, and there is cooperativity between the phosphorylation sites. These results show that Dun1 has a unique phosphorylation motif.     

Regulation of cloned cardiac L-type calcium channels by cGMP-dependent protein kinase

We have studied the effect of 8-bromo-cyclic GMP (8-Br-cGMP) on cloned cardiac L-type calcium channel currents to determine the site and mechanism of action underlying the functional effect. Rabbit cardiac alpha(1C) subunit, in the presence or absence of beta(1) subunit (rabbit skeletal muscle) or beta(2) subunit (rat cardiac/brain), was expressed in Xenopus oocytes, and two-electrode voltage-clamp recordings were made 2 or 3 days later. Application of 8-Br-cGMP caused decreases in calcium channel currents in cells expressing the alpha(1C) subunit, whether or not a beta subunit was co-expressed. No inhibition of currents by 8-Br-cGMP was observed in the presence of the protein kinase G inhibitor KT5823. Substitutions of serine residues by alanine were made at residues Ser(533) and Ser(1371) on the alpha(1C) subunit. As for wild type, the mutant S1371A exhibited inhibition of calcium channel currents by 8-Br-cGMP, whereas no effect of 8-Br-cGMP was observed for mutant S533A. Inhibition of calcium currents by 8-Br-cGMP was also observed in the additional presence of the alpha(2)delta subunit for wild type channels but not for the mutant S533A. These results indicate that cGMP causes inhibition of L-type calcium channel currents by phosphorylation of the alpha(1C) subunit at position Ser(533) via the action of protein kinase G.