Subcellular localization specified by protein acylation and phosphorylation

Radiosynthesis, Preclinical, and Clinical Positron Emission Tomography Studies of Carbon-11 Labeled Endogenous and Natural Exogenous Compounds

The presence of positron emission tomography (PET) centers at most major hospitals worldwide, along with the improvement of PET scanner sensitivity and the introduction of total body PET systems, has increased the interest in the PET tracer development using the short-lived radionuclides carbon-11. In the last few decades, methodological improvements and fully automated modules have allowed the development of carbon-11 tracers for clinical use. Radiolabeling natural compounds with carbon-11 by substituting one of the backbone carbons with the radionuclide has provided important information on the biochemistry of the authentic compounds and increased the understanding of their in vivo behavior in healthy and diseased states. The number of endogenous and natural compounds essential for human life is staggering, ranging from simple alcohols to vitamins and peptides. This review collates all the carbon-11 radiolabeled endogenous and natural exogenous compounds synthesised to date, including essential information on their radiochemistry methodologies and preclinical and clinical studies in healthy subjects.

Evolutionary Approach of Intrinsically Disordered CIP/KIP Proteins

The mammalian CIP/KIP family proteins are intrinsically disordered proteins (IDPs) that can regulate various cellular processes. However, many reports have shown that IDPs generally evolve more rapidly than ordered proteins. Here, to elucidate the functional adaptability of CIP/KIP proteins in vertebrate, we analysed the rates of evolution in relation to their structural and sequence properties and predicted the post-translational modification based on the sequence data. The results showed that CIP/KIP proteins generally could maintain their function through evolution in the vertebrate. Basically, the disordered region that acts as a flexible linker or spacer has a conserved propensity for structural disorder and a persistent, fast rate of amino acid substitution, which could result in a significantly faster rate of evolution compared to the ordered proteins. Describing the pattern of structural order-disorder evolution, this study may give an insight into the well-known characteristics of IDPs in the evolution of CIP/KIP proteins.

Overview of the potent cyanobacterial neurotoxin β-methylamino-L-alanine (BMAA) and its analytical determination

Blue-green algae are responsible for the production of different types of toxins which can be neurotoxic, hepatotoxic, cytotoxic and dermatotoxic and that can affect both aquatic and terrestrial life. Since its discovery the neurotoxin β-methylamino-L-alanine (BMAA) has been a cause for concern, being associated with the neurodegenerative disease amyotrophic lateral sclerosis/Parkinsonism-dementia complex (ALS/PDC). The initial focus was on Guam where it was observed that a high number of people were affected by the ALS/PDC complex. Subsequently, researchers were surprised to find levels of BMAA in post mortem brains from Canadian patients who also suffered from ALS/PDC. Recent research demonstrates that BMAA has been found at different levels in the aquatic food web in the brackish waters of the Baltic Sea. There is emerging evidence to suggest that sand-borne algae from Qatar can also contain BMAA. Furthermore, there is now concern because BMAA has been found not only in warmer regions of the world but also in temperate regions like Europe. The aim of this review is to focus on the methods of extraction and analysis of the neurotoxic non-protein amino acid BMAA. We also consider the neurotoxicity, aetiology, and diverse sources and routes of exposure to BMAA. In recent years, different methods have been developed for the analysis of BMAA. Some of these use HPLC-FD, UPLC-UV, UPLC-MS and LC-MS/MS using samples that have been derivatised or underivatised. To date the LC-MS/MS approach is the most widely used analytical technique as it is the most selective and sensitive method for BMAA determination.

Spatial Control of Biochemical Modification Cascades and Pathways

Information transmission in cells occurs through complex networks of proteins and genes and is relayed through cascades of biochemical modifications, which are typically studied through ordinary differential equations. However, it is becoming increasingly clear that spatial factors can strongly influence chemical information transmission in cells. In this article, we systematically disentangle the effects of space in signaling cascades. This is done by examining the effects of localization/compartmentalization and diffusion of enzymes and substrates in multiple variants of chemical modification cascades. This includes situations where the modified form of species at one stage 1) acts as an enzyme for the next stage; 2) acts as a substrate for the next stage; and 3) is involved in phosphotransfer. Our analysis reveals the multiple effects of space in signal transduction cascades. Although in some cases space plays a modulatory effect (itself of interest), in other cases, spatial regulation and control can profoundly affect the nature of information processing as a result of the subtle interplay between the patterns of localization of species, diffusion, and the nature of the modification cascades. Our results provide a platform for disentangling the role of space and spatial control in multiple cellular contexts and a basis for engineering spatial control in signaling cascades through localization/compartmentalization.

Arl3 and RP2 mediated assembly and traffic of membrane associated cilia proteins

The traffic of proteins to the outer segment of photoreceptors is a fundamentally important process, which when perturbed results in photoreceptor cell death. Recent reports have revealed a novel pathway for the traffic of lipid-modified proteins involving the small GTPase Arl3 and its effectors PDEδ and Unc119. The retinitis pigmentosa protein RP2 is a GTPase activating protein (GAP) for Arl3 and also appears to regulate the assembly and traffic of membrane associated protein complexes. We recently identified the Gβ subunit of transducin (Gβ1) as a novel RP2 interacting protein. Our data support a role for RP2 in facilitating membrane association and traffic of Gβ1, potentially prior to the formation of the obligate Gβ:Gγ heterodimer. Here, we review the recent evidence that suggests that RP2 co-operates with Arl3 and its effectors in protein complex assembly and membrane specification for lipid-modified proteins. This is exemplified by the co-ordination of cilia associated traffic for heterotrimeric G proteins and we propose a model for the role of Arl3 and RP2 in this process.

Functional nicotinic and muscarinic receptors on mesenchymal stem cells

Mesenchymal stem cells (MSCs) are under the control of a large number of signaling systems. In this study, the presence and functionality of the acetylcholine (ACh) signaling system in MSCs was examined. We detected the expression of choline acetyltransferase (ChAT), acetylcholinesterase (AChE), and the presence of ACh in MSCs. MSCs also expressed the nicotinic acetylcholine receptor subunits alpha 3, alpha 5, alpha 7, and the muscarinic acetylcholine receptor 2 (M2-receptor). The M2-receptor and the nicotinic alpha 7 receptor subunits were expressed on distinct subpopulations of cells, indicating differential regulation of cholinergic signaling between MSCs. Stimulation of MSCs with the nicotinic receptor agonist nicotine and the muscarinic receptor agonist muscarine induced immediate and transient increases in intracellular Ca(2+) concentration. Furthermore, muscarine had an inhibiting effect on the production of the intracellular signaling molecule cyclic adenosine 3',5'-monophosphate (cAMP). The AChE inhibitor chlorpyrifos, which is widely used as an agricultural insecticide, had similar effects on intracellular Ca(2+) and cAMP in MSCs. Nicotine, muscarine, and chlorpyrifos induced the phosphorylation of extracellular signal-regulated kinases 1 and 2. This study demonstrates that several components of a cholinergic signaling system are present and functional in MSCs. Environmental compounds such as nicotine and agricultural insecticides can interfere with this system and may affect cellular processes in the MSC.

A combination of multisite phosphorylation and substrate sequestration produces switchlike responses

The phosphorylation of a protein on multiple sites has been proposed to promote the switchlike regulation of protein activity. Recent theoretical work, however, indicates that multisite phosphorylation, by itself, is less effective at creating switchlike responses than had been previously thought. The phosphorylation of a protein often alters its spatial localization, or its association with other proteins, and this sequestration can alter the accessibility of the substrate to the relevant kinases and phosphatases. Sequestration thus has the potential to interact with multisite phosphorylation to modulate ultrasensitivity and threshold. Here, using simple ordinary differential equations to represent phosphorylation, dephosphorylation, and binding/sequestration, we demonstrate that the combination of multisite phosphorylation and regulated substrate sequestration can produce a response that is both a good threshold and a good switch. Several strategies are explored, including both stronger and weaker sequestration with successive phosphorylations, as well as combinations that are more elaborate. In some strategies, such as when phosphorylation and dephosphorylation are segregated, a near-optimal switch is possible, where the effective Hill number equals the number of phosphorylation sites.

Phosphorylated and nonphosphorylated serine and threonine residues evolve at different rates in mammals

Protein phosphorylation plays an important role in the regulation of protein function. Phosphorylated residues are generally assumed to be subject to functional constraint, but it has recently been suggested from a comparison of distantly related vertebrate species that most phosphorylated residues evolve at the rates consistent with the surrounding regions. To resolve the controversy, we infer the ancestral phosphoproteome of human and mouse to compare the evolutionary rates of phosphorylated and nonphosphorylated serine (S), threonine (T), and tyrosine (Y) residues. This approach enables accurate estimation of evolutionary rates as it does not assume deep conservation of phosphorylated residues. We show that phosphorylated S/T residues tend to evolve more slowly than nonphosphorylated S/T residues not only in disordered but also in ordered protein regions, indicating evolutionary conservation of phosphorylated S/T residues in mammals. Thus, phosphorylated S/T residues tend to be subject to stronger functional constraint than nonphosphorylated residues regardless of the protein regions in which they reside. In contrast, phosphorylated Y residues evolve at similar rates as nonphosphorylated ones. We also find that the human lineage has gained more phosphorylated T residues and lost fewer phosphorylated Y residues than the mouse lineage. The cause of the gain/loss imbalance remains a mystery but should be worth exploring.

Investigation of phosphorylation site responsible for CaLP (P. fucata) nucleo-cytoplasmic shuttling triggered by overexpression of p21Cip1

Calmodulin (CaM) is a highly conserved and ubiquitous Ca(2+)-binding protein regulating intracellular Ca(2+) concentration by acting as a sensor of this divalent cation in eukaryotic cells. Being such a very important signal sensor, CaM is susceptible to undergo many posttranslational modifications. One of these important modifications is its phosphorylation. Our previous investigations showed that CaM and calmodulin-like protein (CaLP) cloned from Pinctada fucata have many different characteristics in spite of their high similarity to each other. We have narrowed down that the C-terminal domains of CaM and CaLP are responsible for their discrepant subcellular localizations and shuttling of CaLP when it is co-transfected with p21(Cip1), which is commonly considered as an important cell cycle regulating protein. In this study, we first predicted the potential phosphorylation site responsible for the shuttling and confirmed by fluorescence confocal microscopy. Together with fluorescence activated cell sorter analysis, we further investigated the releasing ability of wild type and point mutated CaLP from arrested cell cycle caused by p21(Cip1) overexpression. By performing pull-down analysis and phosphorylation status of CaLP in cytoplasm fraction of transfected COS-7 cells with CaLP alone and phosphorylation status of CaLP in nuclear fraction of co-transfected COS-7 cells with CaLP and p21(Cip), we propose that the CaLP staying in the cytoplasm is in the state of phosphorylation, but when p21(Cip1) is overexpressed in mammalian cells, some signal triggers CaLP dephosphorylation and translocation into the nucleus.

Potential role of N-myristoyltransferase in cancer

Colorectal cancer is the second leading cause of malignant death, and better preventive strategies are needed. The treatment of colonic cancer remains difficult because of the lack of effective chemotherapeutic agents; therefore it is important to continue to search for cellular functions that can be disrupted by chemotherapeutic drugs resulting in the inhibition of the development and progression of cancer. The current knowledge of the modification of proteins by myristoylation involving myristoyl-CoA: protein N-myristoyltransferase (NMT) is in its infancy. This process is involved in the pathogenesis of cancer. We have reported for the first time that NMT activity and protein expression were higher in human colorectal cancer, gallbladder carcinoma and brain tumors. In addition, an increase in NMT activity appeared at an early stage in colonic carcinogenesis. It is conceivable therefore that NMT can be used as a potential marker for the early detection of cancer. These observations lead to the possibility of developing NMT specific inhibitors, which may be therapeutically useful. We proposed that HSC70 and/or enolase could be used as an anticancer therapeutic target. This review summarized the status of NMT in cancer which has been carried in our laboratory.

Phosphorylation of a novel SOCS-box regulates assembly of the HIV-1 Vif-Cul5 complex that promotes APOBEC3G degradation

HIV-1 Vif (viral infectivity factor) protein overcomes the antiviral activity of the DNA deaminase APOBEC3G by targeting it for proteasomal degradation. We report here that Vif targets APOBEC3G for degradation by forming an SCF-like E3 ubiquitin ligase containing Cullin 5 and Elongins B and C (Cul5-EloB-EloC) through a novel SOCS (suppressor of cytokine signaling)-box that binds EloC. Vif binding to EloC is negatively regulated by serine phosphorylation in the BC-box motif of the SOCS-box. Vif ubiquitination is promoted by Cul5 in vitro and in vivo, and requires an intact SOCS-box. Thus, autoubiquitination of Vif occurs within the assembled Vif-Cul5 complex, analogous to F-box proteins that are autoubiquitinated within their SCF (Skp1-Cullin-F-box) complex. These findings suggest mechanisms that regulate the assembly and activity of Cul5 E3 complexes through phosphorylation or autoubiquitination of the SOCS-box protein, and identify interactions between Vif and host cell proteins that may be therapeutic targets.

IL-8 production in human lung fibroblasts and epithelial cells activated by the Pseudomonas autoinducer N-3-oxododecanoyl homoserine lactone is transcriptionally regulated by NF-kappa B and activator protein-2

The destructive pulmonary inflammation associated with Pseudomonas aeruginosa colonization is caused, in part, by the production of the chemokine IL-8, which recruits neutrophils into the lung. The Pseudomonas autoinducer, N-3-oxododecanoyl homoserine lactone (3-O-C12-HSL), is a small lipid-soluble molecule that is essential in the regulation of many P. aeruginosa virulence factors, but little is known about how it affects eukaryotic cells. In this report we demonstrate that 3-O-C12-HSL is a potent stimulator of both IL-8 mRNA and protein from human fibroblasts and epithelial cells in vitro. The IL-8 produced from these 3-O-C12-HSL-stimulated cells was found to be functionally active by inducing the chemotaxis of neutrophils. To determine a mechanism for this IL-8 induction, deletion constructs of the IL-8 promoter were examined. It was found that the DNA region between nucleotides -1481 and -546 and the transcription factor NF-kappaB were essential for the maximal induction of IL-8 by 3-O-C12-HSL. This was confirmed by EMSAs, where 3-O-C12-HSL induced a shift with both AP-2 and NF-kappaB consensus DNA. The activation of NF-kappaB and subsequent production of IL-8 were found to be regulated by a mitogen-activated protein kinase pathway. These findings support the concept that the severe lung damage that accompanies P. aeruginosa infections is caused by an exuberant neutrophil response stimulated by 3-O-C12-HSL-induced IL-8. Understanding the mechanisms of 3-O-C12-HSL activation of lung structural cells may provide a means to help control lung damage during infections with P. aeruginosa.

Expression of N-myristoyltransferase inhibitor protein and its relationship to c-Src levels in human colon cancer cell lines

Earlier, we have reported that N-myristoyltransferase (NMT) activity is higher in colonic epithelial neoplasms than in normal appearing colonic tissue and that increase in NMT activity appears at an early stage in colonic carcinogenesis [Magnuson, B., Raju, R. V. S., Moyana, T. N., and Sharma, R. K. (1995) J. Natl. Cancer Inst. 87, 1630-1635]. In this study, we demonstrate increased NMT mRNA in well-differentiated adenocarcinomas. NMT and c-Src mRNA levels were generally elevated in a subset of human colon cancer cell lines. Western blotting analysis employing N-myristoyltransferase inhibitory protein (NIP(71)) antibody demonstrated low levels of NIP(71) in high-expressing c-Src cell lines and high levels of NIP(71) in low-expressing c-Src cell lines. Interestingly, down regulation of c-Src by antisense expression in the HT-29 cell line resulted in increased expression of NIP(71), suggesting c-Src may negatively regulate NIP(71) expression. Furthermore, this is the first study demonstrating the expression of NIP(71) in human colon cancer cell lines and a possible relationship to colon carcinogenesis.

N-myristoyltransferase

Myristoylation refers to the co-translational addition of a myristoyl group to an amino-terminal glycine residue of a protein by an ubiquitously distributed enzyme myristoyl-CoA:protein N-myristoyltransferase (NMT, EC 2.3.1.97). This review describes the basic enzymology, molecular cloning and regulation of NMT activity in various pathophysiological processes such as colon cancer and diabetes.

Recombinant bovine spleen myristoyl CoA: protein N-myristoyltransferase

Myristoyl-CoA:protein N-myristoyltransferase (NMT) is an essential eukaryotic enzyme that catalyzes the co-translational transfer of myristate to the NH2-terminal glycine residue of a number of important proteins of diverse function. Recently, we have isolated full length cDNA encoding bovine spleen NMT [27] the full length cDNA was cloned and expressed in E. coli, resulting in the expression of functionally active 50 kDa NMT. Using the combination of SP-Sepharose fast flow and Mono S fast protein liquid chromatography, the enzyme was purified 20-fold with a high yield. The spleen NMT (sNMT) fusion protein exhibited an apparent molecular weight of 53 kDa on SDS-PAGE. Upon cleavage by the Enterokinase the sNMT exhibited an apparent molecular weight of 50 kDa without loss of catalytic activity. The two synthetic peptide substrates based on the N-terminal sequence of pp60src (GSSKSKMR) and cAMP dependent protein kinase (GNAAAKKRR) have different kinetic parameters of Km values of 40 and 200 microM. Recombinant sNMT was also potently inhibited by Ni2+ (histidine binder) in a concentration dependent manner with a half maximal inhibition of 280 microM. The E. coli expressed sNMT was homogenous and showed enzyme activity.

Primary structure characterization of the photosystem II D1 and D2 subunits

Mass spectrometry techniques have been applied in a protein mapping strategy to elucidate the majority of the primary structures of the D1 and D2 proteins present in the photosystem II reaction center. Evidence verifying the post-translational processing of the initiating methionine residue and acetylation of the free amino group, similar to those reported for other higher plant species, are presented for the two subunits from pea plants (Pisum sativum L.). Further covalent modifications observed on the D1 protein include the COOH-terminal processing with a loss of nine amino acids and phosphorylation of Thr2. In addition, the studies reported in this paper provide the first definitive characterization of oxidations on specific amino acids of the D1 and D2 proteins. We believe that these oxidations, and to a much lesser extent the phosphorylations, are major contributors to the heterogeneity observed during the electrospray analysis of the intact subunits reported in the accompanying paper (Sharma, J., Panico, M., Barber, J., and Morris, H. R. (1997) J. Biol. Chem. 272, 33153-33157). Significantly, all of the regions that have been identified as those particularly susceptible to oxidation are anticipated (from current models) to be in close proximity to the redox active components of the photosystem II complex.

N-Myristoyltransferase overexpression in human colorectal adenocarcinomas

Modification of proteins by myristoylation has been proposed as a chemotherapeutic target against colon cancer because it is important in the function of various signal transduction proteins. Recently we reported that the enzyme that catalyzes this modification, N-myristoyltransferase (NMT), is elevated in colorectal adenocarcinomas [Magnuson, B. A., Raju, R. V. S., Moyana, T. N., and Sharma, R. K. (1995) J. Natl. Cancer. Inst. 87, 1630-1635]. The purpose of the present study was to investigate whether the elevated activity of NMT in colorectal adenocarcinomas is due to an increase in the production of NMT or a change in the structure of the preexisting enzyme. The expression of NMT in normal colonic mucosa and adenocarcinomas from human colorectal surgical specimens was studied by immunoblotting, and its localization was confirmed by immunohistochemistry. The molecular weight of NMT was determined by fast protein liquid chromatography. In both normal mucosa and colorectal adenocarcinomas, NMT with a molecular mass of 48.5 kDa was identified with anti-human NMT and anti-peptide antibody. However, the expression of NMT was found to be higher in the colorectal tumors. This finding was further confirmed by immunohistochemical studies which showed stronger cytoplasmic staining in the tumors. These findings represent the first description of NMT overexpression in colorectal adenocarcinomas. This has implications with regard to (i) the design of chemotherapeutic drugs and (ii) prognosis, for instance, in monitoring colorectal cancer recurrence or metastases.

Characterization of the low molecular weight photosystem II reaction center subunits and their light-induced modifications by mass spectrometry

A sensitive and simple reverse phase HPLC purification scheme was developed for the rapid separation of the small protein subunits from photosystem II reaction center preparations. The precise molecular masses of the alpha- and beta-subunits of cytochrome b559 and the psbI gene product from pea plants, found to be 4394.6 +/- 0. 6, 9283.6 +/- 0.7, and 4209.5 +/- 0.5 Da, respectively, were then successfully determined for the first time by electrospray- and fast atom bombardment-mass spectrometry. Discrepancies between the molecular weights assigned and those calculated from the respective DNA sequences were observed for alpha- and beta-subunits of cytochrome b559. Currently, the nucleotide sequence of the psbI gene product from pea plants is not available. Application of novel mapping and sequencing strategies has assured the elucidation of full primary structures of all of the purified subunits. The modifications identified here include the post-translational processing of the initiating methionine on both subunits of cytochrome b559, NH2-terminal acetylation and an mRNA editing site at residue 26 (Ser --> Phe) on the beta-subunit, and retention of the NH2-terminal formyl-Met on the psbI gene product. In addition, specific oxidation of a single amino acid residue was identified on the psbI gene product and the beta-subunit purified from light-treated reaction center preparations. Overall, these studies provide the first detailed primary structural characterization of the small subunits of the reaction center complex and their associated light-induced modifications.

In vivo and in vitro phosphorylation of rotavirus NSP5 correlates with its localization in viroplasms

NSP5 (NS26), the product of rotavirus gene 11, is a phosphoprotein whose role in the virus replication cycle is unknown. To gain further insight into its function, we obtained monoclonal antibodies against the baculovirus-expressed protein. By immunoprecipitation and immunoblotting experiments, we showed that (i) NSP5 appears in many different phosphorylated forms in rotavirus-infected cells; (ii) immunoprecipitated NSP5 from rotavirus-infected cells can be phosphorylated in vitro by incubation with ATP; (iii) NSP5, produced either by transient transfection of rotavirus gene 11 or by infection by gene 11 recombinant vaccinia virus or baculovirus, can be phosphorylated in vivo and in vitro; (iv) NSP5 expressed in Escherichia coli is phosphorylated in vitro, and thus NSP5 is a potential protein kinase; and (v) NSP5 forms dimers and interacts with NSP2. The intracellular localization of NSP5 in the course of rotavirus infection and after transient expression in COS7 cells has also been investigated. In rotavirus-infected cells, NSP5 is localized in viroplasms, but it is widespread throughout the cytoplasm of transfected COS7 cells. NSP5 produced by transfected COS7 cells did not acquire the multiphosphorylated forms observed in rotavirus-infected COS7 cells. Thus, there is a tight correlation between the localization of NSP5 in the viroplasms and its protein kinase activity in vivo or in vitro. Our results suggest that cellular or viral cofactors are indispensable to fully phosphorylate NSP5 and to reach its intracellular localization.

Double in situ hybridization reveals overlapping neuronal populations expressing the low molecular weight GTPases Rab3a and Rab3b in Rat brain

The ras-related Rab3 gene subfamily codes for small GTP-binding proteins which control a late step of exocytosis during which vesicles become docked to the plasma membrane. Rab3a and Rab3b are the most abundant Rab3 isoforms expressed in the CNS of mammals. We have shown previously that the Rab3a protein was selectively distributed and expressed in various regions of the rat brain. Here we have determined the pattern of expression of Rab3b mRNA in the brain and compared it with that of Rab3a mRNA. In addition, we examined the co-expression of these two Rab within individual neurons. In general the Rab3b transcript was detected in many regions which also express Rab3a mRNA but at a lower level than Rab3a, except in the olfactory bulb and in the pituitary where the Rab3b hybridization signal was similar and higher respectively. Double in situ hybridization revealed that Rab3a and Rab3b mRNAs were co-localized in most neurons, in all brain areas examined. However, in each of these areas, subsets of neurons appeared to preferentially express either Rab3b or Rab3a, or some neurons did not express either Rab3 homologue at detectable levels. These data support the view of a functional specialization of Rab3a and Rab3b in the control of exocytosis in neuronal and neuroendocrine cells.