Patt1, a novel protein acetyltransferase that is highly expressed in liver and downregulated in hepatocellular carcinoma,enhances apoptosis of hepatoma cells

Protein acetylation is increasingly recognized as an important post-translational modification. Although a lot of protein acetyltransferases have been identified, a few putative acetyltransferases are yet to be studied. In this study, we identified a novel protein acetyltransferase, Patt1, which belongs to GNAT family. Patt1 exhibited histone acetyltransferase activity and auto-acetylation activity. Deletion and mutation analysis of the predicted acetyltransferase domain in Patt1 showed that the conserved Glu139 was an important residue for its protein acetyltransferase activity. Furthermore, we found that Patt1 was highly expressed in liver and significantly downregulated in hepatocellular carcinoma tissues. In addition, we showed that overexpression of Patt1 enhanced the apoptosis of hepatoma cells dependent on its acetyltransferase activity, whereas knockdown of Patt1 significantly protected Chang liver cells from apoptosis. These data suggest that Patt1 might be involved in the development of hepatocellular carcinoma, and could be served as a potential therapy target for hepatocellular carcinoma.     

Myb induced myeloid protein 1 (Mim-1) is an acetyltransferase

We have screened protein extracts from chicken blood cells for acetyltransferases. An in gel acetyltransferase assay revealed that a 32 kDa protein, which is more prevalent in whole blood when compared with erythrocyte cells, possessed an auto-acetylation activity. This protein was purified by a series of chromatographic steps, sequenced by Edman degradation and subsequently identified as Myb induced myeloid protein (Mim-1). Mim-1 has similarities to the conserved acetyltransferase motifs found in the GNAT superfamily of proteins and also contains three minimal GK acetylation motifs. These data identify Mim-1 as an acetyltransferase.     

Phosphorylation of 69-kDa choline acetyltransferase at threonine 456 in response to amyloid-beta peptide 1-42

Choline acetyltransferase synthesizes acetylcholine in cholinergic neurons. In the brain, these neurons are especially vulnerable to effects of beta-amyloid (A beta) peptides. Choline acetyltransferase is a substrate for several protein kinases. In the present study, we demonstrate that short term exposure of IMR32 neuroblastoma cells expressing human choline acetyltransferase to A beta-(1-42) changes phosphorylation of the enzyme, resulting in increased activity and alterations in its interaction with other cellular proteins. Using mass spectrometry, we identified threonine 456 as a new phosphorylation site in choline acetyltransferase from A beta-(1-42)-treated cells and in purified recombinant ChAT phosphorylated in vitro by calcium/calmodulin-dependent protein kinase II (CaM kinase II). Whereas phosphorylation of choline acetyltransferase by protein kinase C alone caused a 2-fold increase in enzyme activity, phosphorylation by CaM kinase II alone did not alter enzyme activity. A 3-fold increase in choline acetyltransferase activity was found with coordinate phosphorylation of threonine 456 by CaM kinase II and phosphorylation of serine 440 by protein kinase C. This phosphorylation combination was observed in choline acetyltransferase from A beta-(1-42)-treated cells. Treatment of cells with A beta-(1-42) resulted in two phases of activation of choline acetyltransferase, the first within 30 min and associated with phosphorylation by protein kinase C and the second by 10 h and associated with phosphorylation by both CaM kinase II and protein kinase C. We also show that choline acetyltransferase from A beta-(1-42)-treated cells co-immunoprecipitates with valosin-containing protein, and mutation of threonine 456 to alanine abolished the A beta-(1-42)-induced effects. These studies demonstrate that A beta-(1-42) can acutely regulate the function of choline acetyltransferase, thus potentially altering cholinergic neurotransmission.     

Identification of a novel nuclear localization signal common to 69- and 82-kDa human choline acetyltransferase

We demonstrated previously that 69- and 82-kDa human choline acetyltransferase are localized predominantly to the cytoplasm and the nucleus, respectively. We have now identified a nuclear localization signal common to both forms of enzyme using confocal microscopy to study the subcellular compartmentalization of choline acetyltransferase tagged with green fluorescent protein in living HEK 293 cells. To identify functional nuclear localization and export signals, portions of full-length 69-kDa choline acetyltransferase were cloned into the vector peGFP-N1 and the cellular distribution patterns of the fusion proteins observed. Of the nine constructs studied, one yielded a protein with nuclear localization and another produced a protein with cytoplasmic localization. Mutation of the critical amino acids in this novel putative nuclear localization signal in the 69- and 82-kDa enzymes demonstrated that it is functional in both proteins. Moreover, 69-kDa choline acetyltransferase but not the 82-kDa enzyme is transported out of the nucleus by the leptomycin B-sensitive Crm-1 export pathway. By using bikaryon cells expressing both 82-kDa choline acetyltransferase and the nuclear protein heterogeneous nuclear ribonucleoprotein with green and red fluorescent tags, respectively, we found that the 82-kDa enzyme does not shuttle out of the nucleus in measurable amounts. These data suggest that 69-kDa choline acetyltransferase is a nucleocytoplasmic shuttling protein with a predominantly cytoplasmic localization determined by a functional nuclear localization signal and unidentified putative nuclear export signal. For 82-kDa choline acetyltransferase, the presence of the unique amino-terminal nuclear localization signal plus the newly identified nuclear localization signal may be involved in a process leading to predominantly nuclear accumulation of this enzyme, or alternatively, the two nuclear localization signals may be sufficient to overcome the force(s) driving nuclear export.     

Allosteric Regulation of a Protein Acetyltransferase in Micromonospora aurantiaca by the Amino Acids Cysteine and Arginine

ACT domains (amino acid-binding domains) are linked to a wide range of metabolic enzymes that are regulated by amino acid concentration. Seventy proteins with ACT-GCN5-related N-acetyltransferase (GNAT) domain organization were found in actinomycetales. In this study, we investigate the ACT-containing GNAT acetyltransferase, Micau_1670 (MaKat), from Micromonospora aurantiaca ATCC 27029. Arginine and cysteine were identified as ligands by monitoring the conformational changes that occur upon amino acids binding to the ACT domain in the MaKat protein using FRET assay. It was found that MaKat is an amino acid-regulated protein acetyltransferase, whereas arginine and cysteine stimulated the activity of MaKat with regard to acetylation of acetyl-CoA synthetase (Micau_0428). Our research reveals the biochemical characterization of a protein acetyltransferase that contains a fusion of a GNAT domain with an ACT domain and provides a novel signaling pathway for regulating cellular protein acetylation. These findings indicate that acetylation of proteins and acetyltransferase activity may be tightly linked to cellular concentrations of some amino acids in actinomycetales.     

Host cell factor and an uncharacterized SANT domain protein are stable components of ATAC, a novel dAda2A/dGcn5-containing histone acetyltransferase complex in Drosophila

Gcn5 is a conserved histone acetyltransferase (HAT) found in a number of multisubunit complexes from Saccharomyces cerevisiae, mammals, and flies. We previously identified Drosophila melanogaster homologues of the yeast proteins Ada2, Ada3, Spt3, and Tra1 and showed that they associate with dGcn5 to form at least two distinct HAT complexes. There are two different Ada2 homologues in Drosophila named dAda2A and dAda2B. dAda2B functions within the Drosophila version of the SAGA complex (dSAGA). To gain insight into dAda2A function, we sought to identify novel components of the complex containing this protein, ATAC (Ada two A containing) complex. Affinity purification and mass spectrometry revealed that, in addition to dAda3 and dGcn5, host cell factor (dHCF) and a novel SANT domain protein, named Atac1 (ATAC component 1), copurify with this complex. Coimmunoprecipitation experiments confirmed that these proteins associate with dGcn5 and dAda2A, but not with dSAGA-specific components such as dAda2B and dSpt3. Biochemical fractionation revealed that ATAC has an apparent molecular mass of 700 kDa and contains dAda2A, dGcn5, dAda3, dHCF, and Atac1 as stable subunits. Thus, ATAC represents a novel histone acetyltransferase complex that is distinct from previously purified Gcn5/Pcaf-containing complexes from yeast and mammalian cells.     

Acetyl-CoA:1-O-alkyl-2-lyso-sn-glycero-3-phosphocholine acetyltransferase is directly activated by p38 kinase

Acetyl-CoA:1-O-alkyl-2-lyso-sn-glycero-3-phosphocholine acetyltransferase, along with phospholipase A2, is a key regulator of platelet-activating factor biosynthesis via the remodeling pathway. We have now obtained evidence in human neutrophils indicating that this enzyme is regulated by a specific member of the mitogen-activated protein kinases, namely the p38 kinase. We earlier demonstrated that tumor necrosis factor-alpha (TNF-alpha) as well as N-formyl-methionyl-leucyl-phenylalanine treatment leads to increased phosphorylation and activation of p38 kinase in human neutrophils. Strikingly, in the present study these stimuli increased the catalytic activity of acetyltransferase up to 3-fold, whereas 4-phorbol 12-myristate 13-acetate, which activates the extracellular-regulated kinases (ERKs) but not p38 kinase, had no effect. Furthermore, a selective inhibitor of p38 kinase, SB 203580, was able to abolish the TNF-alpha- and N-formyl-methionyl-leucyl-phenylalanine-induced activation of acetyltransferase. The same effect was not observed in the presence of an inhibitor that blocked ERK activation (PD 98059). Complementing the findings in intact cells, we have shown that recombinant, activated p38 kinase added to microsomes in the presence of Mg2+ and ATP increased acetyltransferase activity to the same degree as in microsomes obtained from TNF-alpha-stimulated cells. No activation of acetyltransferase occurred upon treatment of microsomes with either recombinant, activated ERK-1 or ERK-2. Finally, the increases in acetyltransferase activity induced by TNF-alpha could be ablated by treating the microsomes with alkaline phosphatase. Thus acetyltransferase appears to be a downstream target for p38 kinase but not ERKs. These data from whole cells as well as cell-free systems fit a model wherein stimulus-induced acetyltransferase activation is mediated by a phosphorylation event catalyzed directly by p38 kinase.     

Modulation of acetyl-CoA:1-alkyl-2-lyso-sn-glycero-3-phosphocholine (lyso-PAF) acetyltransferase in human polymorphonuclears. The role of cyclic AMP-dependent and phospholipid sensitive, calcium-dependent protein kinases

In order to characterize the mechanism of activation of the enzyme 1-O-alkyl-2-lyso-sn-glycero-3-phosphocholine:acetyl-CoA acetyltransferase (EC 2.3.1.67) which is the limiting step in the regulation of the synthesis of the potent inflammatory mediator 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine; homogenates from human polymorphonuclear leukocytes were incubated in the presence of the catalytic subunit of cyclic AMP-dependent protein kinase and in the presence of a partially purified phospholipid sensitive, calcium-dependent protein kinase (PrKC). The first kinase was found to enhance up to 3-fold acetyltransferase activity in a dose- and time-dependent manner. In homogenates from PMN previously stimulated with complement-coated zymosan particles, the decay of acetyltransferase activity was partially prevented by the addition of soybean trypsin inhibitor and almost completely inhibited when the homogenates were supplemented with inhibitors of alkaline phosphatase such as 50 mM KF and 100 microM paranitrophenylphosphate. Under these conditions it was possible to initiate the decay of acetyltransferase activity by adding an excess of alkaline phosphatase. Preincubation of PMN with 12-O-tetradecanoylphorbol-13-acetate previous or simultaneously to the addition of ionophore A23187 reduced the increase in acetyltransferase produced by ionophore A23187, whereas the generation of superoxide anions was enhanced. Addition of partially purified PrKC to homogenates from ionophore A23187-stimulated PMN, reduced acetyltransferase activity by 63%, whereas only a 16% inhibition was observed on homogenates from resting PMN. These data indicate the modulation of acetyltransferase activity in human polymorphonuclear leukocytes by a phosphorylation-dephosphorylation mechanism linked to cyclic AMP-dependent protein kinase. Phospholipid sensitive, calcium-dependent protein kinase seems not to be involved in the mechanism of activation, but, most probably, in the generation of negative activation signals.     

BRPF3-HUWE1-mediated regulation of MYST2 is required for differentiation and cell-cycle progression in embryonic stem cells

Brpf-histone acetyltransferase (HAT) complexes have important roles in embryonic development and regulating differentiation in ESCs. Among Brpf family, Brpf3 is a scaffold protein of Myst2 histone acetyltransferase complex that plays crucial roles in gene regulation, DNA replication, development as well as maintaining pluripotency in embryonic stem cells (ESCs). However, its biological functions in ESCs are not elucidated. In this study, we find out that Brpf3 protein level is critical for Myst2 stability and E3 ligase Huwe1 functions as a novel negative regulator of Myst2 via ubiquitin-mediated degradation. Importantly, Brpf3 plays an antagonistic role in Huwe1-mediated degradation of Myst2, suggesting that protein–protein interaction between Brpf3 and Myst2 is required for retaining Myst2 stability. Further, Brpf3 overexpression causes the aberrant upregulation of Myst2 protein levels which in turn induces the dysregulated cell-cycle progression and also delay of early embryonic development processes such as embryoid-body formation and lineage commitment of mouse ESCs. The Brpf3 overexpression-induced phenotypes can be reverted by Huwe1 overexpression. Together, these results may provide novel insights into understanding the functions of Brpf3 in proper differentiation as well as cell-cycle progression of ESCs via regulation of Myst2 stability by obstructing Huwe1-mediated ubiquitination. In addition, we suggest that this is a useful report which sheds light on the function of an unknown gene in ESC field.Subject terms: Epigenetics, Ubiquitin ligases     

Cyclic AMP-induced conformational changes in mycobacterial protein acetyltransferases

The activities of a number of proteins are regulated by the binding of cAMP and cGMP to cyclic nucleotide binding (CNB) domains that are found associated with one or more effector domains with diverse functions. Although the conserved architecture of CNB domains has been extensively studied by x-ray crystallography, the key to unraveling the mechanisms of cAMP action has been protein dynamics analyses. Recently, we have identified a novel cAMP-binding protein from mycobacteria, where cAMP regulates the activity of an associated protein acetyltransferase domain. In the current study, we have monitored the conformational changes that occur upon cAMP binding to the CNB domain in these proteins, using a combination of bioluminescence resonance energy transfer and amide hydrogen/deuterium exchange mass spectrometry. Coupled with mutational analyses, our studies reveal the critical role of the linker region (positioned between the CNB domain and the acetyltransferase domain) in allosteric coupling of cAMP binding to activation of acetyltransferase catalysis. Importantly, major differences in conformational change upon cAMP binding were accompanied by stabilization of the CNB and linker domain alone. This is in contrast to other cAMP-binding proteins, where cyclic nucleotide binding has been shown to involve intricate and parallel allosteric relays. Finally, this powerful convergence of results from bioluminescence resonance energy transfer and hydrogen/deuterium exchange mass spectrometry reaffirms the power of solution biophysical tools in unraveling mechanistic bases of regulation of proteins in the absence of high resolution structural information.     

Induction of choline acetyltransferase in the neuroblastoma x glioma cell line NG108-15

The mechanism of the induction of choline acetyltransferase activity in the hybrid cell line NG108-15 was studied. Induction by cyclic AMP analogs, forskolin, and prostaglandin E₁ + theophylline was found to be rapid with an increase in choline acetyltransferase specific activity detectable within 8 hrs and maximal after 24 hrs. Immunoblot analysis was used to demonstrate that the increase in choline acetyltransferase specific activity induced by prostaglandin E₁ + theophylline was due to an increase in enzyme protein. Cycloheximide effectively blocked the induction of choline acetyltransferase by prostaglandin E₁ + theophylline. These results demonstrate that the induction of choline acetyltransferase activity involves the synthesis of new enzyme protein. Attempts to measure choline acetyltransferase turnover by blocking its synthesis with cycloheximide indicated that this enzyme is a relatively stable protein with a half-life of greater than 24 hrs.     

Accumulation of a fusion protein containing 2S albumin induces novel vesicles in vegetative cells of Arabidopsis.

We have previously reported that precursor-accumulating (PAC) vesicles found exclusively in developing seeds are involved in a transport of seed storage proteins, such as 2S albumin, from the endoplasmic reticulum to protein-storage vacuoles. Here, we constructed chimeric genes that encode fusion proteins consisting of both various lengths of polypeptides derived from pumpkin 2S albumin and a selectable marker enzyme, phosphinothricin acetyltransferase. The chimeric genes were expressed in transgenic Arabidopsis in order to investigate the mechanism of the PAC vesicle formation. A fusion protein expressed by one of the chimeric genes is accumulated as a proprotein-precursor form, and localized in novel vesicles of vegetative cells. The vesicles show distinct features that well much to the PAC vesicles. Despite of the accumulation of the fusion protein, the transgenic Arabidopsis is still sensitive to phosphinothricin. Phosphinothricin acetyltransferase contained in the fusion protein is obviously compartmentalized in the PAC-like vesicles that do not permit the detoxification of this herbicide. These results indicate that the PAC-like vesicle can be induced in vegetative cells by the ectopic expression of the protein that is destined to be compartmentalized into the PAC vesicles.     

A single enzyme catalyzes both platelet-activating factor production and membrane biogenesis of inflammatory cells. Cloning and characterization of acetyl-CoA:LYSO-PAF acetyltransferase

Platelet-activating factor (PAF) is a potent proinflammatory lipid mediator eliciting a variety of cellular functions. Lipid mediators, including PAF are produced from membrane phospholipids by enzymatic cascades. Although a G protein-coupled PAF receptor and degradation enzymes have been cloned and characterized, the PAF biosynthetic enzyme, aceyl-CoA:lyso-PAF acetyltransferase, has not been identified. Here, we cloned lyso-PAF acetyltransferase, which is critical in stimulus-dependent formation of PAF. The enzyme is a 60-kDa microsomal protein with three putative membrane-spanning domains. The enzyme was induced by bacterial endotoxin (lipopolysaccharide), which was suppressed by dexamethasone treatment. Surprisingly, the enzyme catalyzed not only biosynthesis of PAF from lyso-PAF but also incorporation of arachidonoyl-CoA to produce PAF precursor membrane glycerophospholipids (lysophosphatidylcholine acyltransferase activity). Under resting conditions, the enzyme prefers arachidonoyl-CoA and contributes to membrane biogenesis. Upon acute inflammatory stimulation with lipopolysaccharide, the activated enzyme utilizes acetyl-CoA more efficiently and produces PAF. Thus, our findings provide a novel concept that a single enzyme catalyzes membrane biogenesis of inflammatory cells while producing a prophlogistic mediator in response to external stimuli.