An unsuspected ecdysteroid/steroid phosphatase activity in the key T-cell regulator, Sts-1: surprising relationship to insect ecdysteroid phosphate phosphatase

Phylogenomics of the Ecdysteroid Kinase-like (EcKL) Gene Family in Insects Highlights Roles in Both Steroid Hormone Metabolism and Detoxification

The evolutionary dynamics of large gene families can offer important insights into the functions of their individual members. While the ecdysteroid kinase-like (EcKL) gene family has previously been linked to the metabolism of both steroid molting hormones and xenobiotic toxins, the functions of nearly all EcKL genes are unknown, and there is little information on their evolution across all insects. Here, we perform comprehensive phylogenetic analyses on a manually annotated set of EcKL genes from 140 insect genomes, revealing the gene family is comprised of at least 13 subfamilies that differ in retention and stability. Our results show the only two genes known to encode ecdysteroid kinases belong to different subfamilies and therefore ecdysteroid metabolism functions must be spread throughout the EcKL family. We provide comparative phylogenomic evidence that EcKLs are involved in detoxification across insects, with positive associations between family size and dietary chemical complexity, and we also find similar evidence for the cytochrome P450 and glutathione S-transferase gene families. Unexpectedly, we find that the size of the clade containing a known ecdysteroid kinase is positively associated with host plant taxonomic diversity in Lepidoptera, possibly suggesting multiple functional shifts between hormone and xenobiotic metabolism. Our evolutionary analyses provide hypotheses of function and a robust framework for future experimental studies of the EcKL gene family. They also open promising new avenues for exploring the genomic basis of dietary adaptation in insects, including the classically studied coevolution of butterflies with their host plants.

TULA Proteins in Men, Mice, Hens, and Lice: Welcome to the Family

The two members of the UBASH3/STS/TULA protein family have been shown to critically regulate key biological functions, including immunity and hemostasis, in mammalian biological systems. Negative regulation of signaling through immune receptor tyrosine-based activation motif (ITAM)- and hemITAM-bearing receptors mediated by Syk-family protein tyrosine kinases appears to be a major molecular mechanism of the down-regulatory effect of TULA-family proteins, which possess protein tyrosine phosphatase (PTP) activity. However, these proteins are likely to carry out some PTP-independent functions as well. Whereas the effects of TULA-family proteins overlap, their characteristics and their individual contributions to cellular regulation also demonstrate clearly distinct features. Protein structure, enzymatic activity, molecular mechanisms of regulation, and biological functions of TULA-family proteins are discussed in this review. In particular, the usefulness of the comparative analysis of TULA proteins in various metazoan taxa, for identifying potential roles of TULA-family proteins outside of their functions already established in mammalian systems, is examined.

The Sts Proteins: Modulators of Host Immunity

The suppressor of TCR signaling (Sts) proteins, Sts-1 and Sts-2, are a pair of closely related signaling molecules that belong to the histidine phosphatase (HP) family of enzymes by virtue of an evolutionarily conserved C-terminal phosphatase domain. HPs derive their name from a conserved histidine that is important for catalytic activity and the current evidence indicates that the Sts HP domain plays a critical functional role. Sts-1HP has been shown to possess a readily measurable protein tyrosine phosphatase activity that regulates a number of important tyrosine-kinase-mediated signaling pathways. The in vitro catalytic activity of Sts-2HP is significantly lower than that of Sts-1HP, and its signaling role is less characterized. The highly conserved unique structure of the Sts proteins, in which additional domains, including one that exhibits a novel phosphodiesterase activity, are juxtaposed together with the phosphatase domain, suggesting that Sts-1 and -2 occupy a specialized intracellular signaling niche. To date, the analysis of Sts function has centered predominately around the role of Sts-1 and -2 in regulating host immunity and other responses associated with cells of hematopoietic origin. This includes their negative regulatory role in T cells, platelets, mast cells and other cell types, as well as their less defined roles in regulating host responses to microbial infection. Regarding the latter, the use of a mouse model lacking Sts expression has been used to demonstrate that Sts contributes non-redundantly to the regulation of host immunity toward a fungal pathogen (C. albicans) and a Gram-negative bacterial pathogen (F. tularensis). In particular, Sts-/- animals demonstrate significant resistance to lethal infections of both pathogens, a phenotype that is correlated with some heightened anti-microbial responses of phagocytes derived from mutant mice. Altogether, the past several years have seen steady progress in our understanding of Sts biology.

Protein Phosphohistidine Phosphatases of the HP Superfamily

Histidine phosphorylation of proteins is increasingly recognised as an important regulatory posttranslational modification in eukaryotes as well as prokaryotes. The HP (Histidine Phosphatase) superfamily, named for a key catalytic His residue, harbors two known groups of protein phosphohistidine phosphatases (PPHPs). The bacterial SixA protein acts as a regulator of His-Asp phosphorelays with two substrates characterized in vitro and/or in vivo. The recently characterized eukaryotic PHPP PGAM5 only has one currently known substrate, NDPK-B, through which it helps regulate T-cell signaling. SixA and PGAM5 appear to share no particular sequence or structural features relating to their PPHP activity suggesting that PHPP activity has arisen independently in different lineages of the HP superfamily. Further members of the HP superfamily may thus harbor (additional) unsuspected PHPP activity.

Diverse radiofrequency sensitivity and radiofrequency effects of mobile or cordless phone near fields exposure in Drosophila melanogaster

Introduction

The impact of electromagnetic fields on health is of increasing scientific interest. The aim of this study was to examine how the Drosophila melanogaster animal model is affected when exposed to portable or mobile phone fields.

Methods/Results

Two experiments have been designed and performed in the same laboratory conditions. Insect cultures were exposed to the near field of a 2G mobile phone (the GSM 2G networks support and complement in parallel the 3G wide band or in other words the transmission of information via voice signals is served by the 2G technology in both mobile phones generations) and a 1880 MHz cordless phone both digitally modulated by human voice. Comparison with advanced statistics of the egg laying of the second generation exposed and non-exposed cultures showed limited statistical significance for the cordless phone exposed culture and statistical significance for the 900 MHz exposed insects. We calculated by physics, simulated and illustrated in three dimensional figures the calculated near fields of radiation inside the experimenting vials and their difference. Comparison of the power of the two fields showed that the difference between them becomes null when the experimental cylinder radius and the height of the antenna increase.

Conclusions/Significance

Our results suggest a possible radiofrequency sensitivity difference in insects which may be due to the distance from the antenna or to unexplored intimate factors. Comparing the near fields of the two frequencies bands, we see similar not identical geometry in length and height from the antenna and that lower frequencies tend to drive to increased radiofrequency effects.

Mechanisms of sterol uptake and transport in yeast

Sterols are essential lipid components of eukaryotic membranes. Here we summarize recent advances in understanding how sterols are transported between different membranes. Baker's yeast is a particularly attractive organism to dissect this lipid transport pathway, because cells can synthesize their own major sterol, ergosterol, in the membrane of the endoplasmic reticulum from where it is then transported to the plasma membrane. However, Saccharomyces cerevisiae is also a facultative anaerobic organism, which becomes sterol auxotroph in the absence of oxygen. Under these conditions, cells take up sterol from the environment and transport the lipid back into the membrane of the endoplasmic reticulum, where the free sterol becomes esterified and is then stored in lipid droplets. Steryl ester formation is thus a reliable readout to assess the back-transport of exogenously provided sterols from the plasma membrane to the endoplasmic reticulum. Structure/function analysis has revealed that the bulk membrane function of the fungal ergosterol can be provided by structurally related sterols, including the mammalian cholesterol. Foreign sterols, however, are subject to a lipid quality control cycle in which the sterol is reversibly acetylated. Because acetylated sterols are efficiently excreted from cells, the substrate specificity of the deacetylating enzymes determines which sterols are retained. Membrane-bound acetylated sterols are excreted by the secretory pathway, more soluble acetylated sterol derivatives such as the steroid precursor pregnenolone, on the other hand, are excreted by a pathway that is independent of vesicle formation and fusion. Further analysis of this lipid quality control cycle is likely to reveal novel insight into the mechanisms that ensure sterol homeostasis in eukaryotic cells. Article from a special issue on Steroids and Microorganisms.

A novel histidine tyrosine phosphatase, TULA-2, associates with Syk and negatively regulates GPVI signaling in platelets

T-cell ubiquitin ligand-2 (TULA-2) is a recently discovered histidine tyrosine phosphatase thought to be ubiquitously expressed. In this work, we have investigated whether TULA-2 has a key role in platelet glycoprotein VI (GPVI) signaling. This study indicates that TULA-2 is expressed in human and murine platelets and is able to associate with Syk and dephosphorylate it. Ablation of TULA-2 resulted in hyperphosphorylation of Syk and its downstream effector phospholipase C-γ2 as well as enhanced GPVI-mediated platelet functional responses. In addition, shorter bleeding times and a prothrombotic phenotype were observed in mice lacking TULA-2. We therefore propose that TULA-2 is the primary tyrosine phosphatase mediating the dephosphorylation of Syk and thus functions as a negative regulator of GPVI signaling in platelets.

Structural and functional characterization of the 2H-phosphatase domain of Sts-2 reveals an acid-dependent phosphatase activity

The suppressors of T cell receptor (TCR) signaling 1 and 2 (Sts-1 and -2, respectively) are multidomain proteins that negatively regulate the signaling of membrane-bound receptors, including TCR and the epidermal growth factor receptor (EGFR). Sts-1 was recently shown to be a new type of protein tyrosine phosphatase (PTP), with the phosphatase activity located within its C-terminal phosphoglycerate mutase (PGM) homology domain and key for the regulation of TCR signaling in T cells. The activity of the related Sts-2 enzyme is significantly less than that of Sts-1. Here we investigate the phosphatase activity of the PGM domain of Sts-2, Sts-2(PGM). The crystal structure of Sts-2(PGM) is remarkably similar to Sts-1(PGM), including conservation of all catalytic residues. Insight into mechanistic details is provided by the structures of the apo, tungstate-bound, and phosphate-bound enzyme. The active site shows stringent specificity, with the k(cat) optimum at pH 5.0 suggesting that Sts-2 might function as an acid-dependent phosphatase. Mutation of active site residues Gln372, Ala446, Glu481, Ser552, and Ser582 to their equivalents in Sts-1 increases the phosphatase activity of Sts-2(PGM) toward model substrates. Overall, our data demonstrate that Sts-2(PGM) adopts the conformation of an active phosphatase whose activity is fundamentally different from that of Sts-1 despite the strong structural homology. They also demonstrate that nonconserved active site residues are responsible for the difference in activity between the two isoforms. These differences reflect possible distinct physiological substrates.

Identification of nucleases and phosphatases by direct biochemical screen of the Saccharomyces cerevisiae proteome

The availability of yeast strain collections expressing individually tagged proteins to facilitate one-step purification provides a powerful approach to identify proteins with particular biochemical activities. To identify novel exo- and endo-nucleases that might function in DNA repair, we undertook a proteomic screen making use of the movable ORF (MORF) library of yeast expression plasmids. This library consists of 5,854 yeast strains each expressing a unique yeast ORF fused to a tripartite tag consisting of His₆, an HA epitope, a protease 3C cleavage site, and the IgG-binding domain (ZZ) from protein A, under the control of the GAL1 promoter for inducible expression. Pools of proteins were partially purified on IgG sepharose and tested for nuclease activity using three different radiolabeled DNA substrates. Several known nucleases and phosphatases were identified, as well as two new members of the histidine phosphatase superfamily, which includes phosphoglycerate mutases and phosphatases. Subsequent characterization revealed YDR051c/Det1 to be an acid phosphatase with broad substrate specificity, whereas YOR283w has a broad pH range and hydrolyzes hydrophilic phosphorylated substrates. Although no new nuclease activities were identified from this screen, we did find phosphatase activity associated with a protein of unknown function, YOR283w, and with the recently characterized protein Det1. This knowledge should guide further genetic and biochemical characterization of these proteins.

TULA-family proteins: an odd couple

Two members of the TULA family (TULA/STS-2/UBASH3A and TULA-2/STS-1/UBASH3B) recently emerged as novel regulators of several cellular functions. The degree of structural similarity between the TULA-family proteins is typical for proteins that belong to the same family. Furthermore, the experiments with knockout mice lacking these proteins may be interpreted as suggesting that functions of TULA-family proteins in T lymphocytes overlap. At the same time, TULA and TULA-2 exhibit clear functional dissimilarities, starting with the finding that a conserved phosphatase domain present in both proteins exhibits remarkable differences in enzymatic activity; TULA-2 is an active phosphatase capable of dephosphorylating multiple tyrosine-phosphorylated proteins, whereas the phosphatase activity of TULA is extremely low. In contrast, TULA, but not TULA-2, facilitates growth factor withdrawal-induced apoptosis in T cells. In spite of their apparent importance, the functional role of TULA-family proteins is not well understood. In particular, the role of functional dissimilarities between them remains unclear.

Phosphoconjugation and dephosphorylation reactions of steroid hormone in insects

In insects, the major products of phase II metabolism of ecdysteroids, which include the molting hormone, are phosphate esters. The phosphoconjugation pathway is a reversible process, comprising two enzyme systems: ecdysteroid 22-kinase (EcKinase) and ecdysteroid-phosphate phosphatase (EPPase). We report here that: (1) the biochemical characteristics of EcKinase and EPPase, (2) the physiological significance of the reciprocal conversion of ecdysteroids and ecdysteroid phosphates in the ovary-egg system in insects, (3) the biochemical mechanism by which ecdysteroid phosphates are synthesized in the ovary, transferred to eggs, and finally dephosphorylated in eggs, and (4) the possible catalytic steps of EcKinase and EPPase on the basis of the data obtained by an in silico study. From these studies, it is obvious that ecdysteroid phosphates as well as steroid sulfates, which are major products of phase II metabolism in mammals, function as precursors for the formation of biologically active hormones.

Structural and functional characterization of the c-terminal domain of the ecdysteroid phosphate phosphatase from bombyx mori reveals a new enzymatic activity

Here, we present the crystal structure of the ecdysone phosphate phosphatase (EPPase) phosphoglycerate mutase (PGM) homology domain, the first structure of a steroid phosphate phosphatase. The structure reveals an alpha/beta-fold common to members of the two histidine (2H)-phosphatase superfamily with strong homology to the Suppressor of T-cell receptor signaling-1 (Sts-1 PGM) protein. The putative EPPase PGM active site contains signature residues shared by 2H-phosphatase enzymes, including a conserved histidine (His80) that acts as a nucleophile during catalysis. The physiological substrate ecdysone 22-phosphate was modeled in a hydrophobic cavity close to the phosphate-binding site. EPPase PGM shows limited substrate specificity with an ability to hydrolyze steroid phosphates, the phospho-tyrosine (pTyr) substrate analogue para-nitrophenylphosphate ( pNPP) and pTyr-containing peptides and proteins. Altogether, our data demonstrate a new protein tyrosine phosphatase (PTP) activity for EPPase. They suggest that EPPase and its closest homologues can be grouped into a distinct subfamily in the large 2H-phosphatase superfamily of proteins.

TULA proteins regulate activity of the protein tyrosine kinase Syk

TULA belongs to a two-member family: TULA (STS-2) is a lymphoid protein, whereas STS-1/TULA-2 is expressed ubiquitously. TULA proteins were implicated in the regulation of signaling mediated by protein tyrosine kinases (PTKs). The initial experiments did not fully reveal the molecular mechanism of these effects, but suggested that both TULA proteins act in a similar fashion. It was shown recently that STS-1/TULA-2 dephosphorylates PTKs. In this study, we analyzed the effects of TULA proteins on Syk, a PTK playing an important role in lymphoid signaling. First, we have shown that TULA-2 decreases tyrosine phosphorylation of Syk in vivo and in vitro and that the intact phosphatase domain of TULA-2 is essential for this effect. We have also shown that TULA-2 exhibits a certain degree of substrate specificity. Our results also indicate that inactivated TULA-2 increases tyrosine phosphorylation of Syk in cells co-transfected to overexpress these proteins, thus acting as a dominant-negative form that suppresses dephosphorylation of Syk caused by endogenous TULA-2. Furthermore, we have demonstrated that phosphatase activity of TULA is negligible as compared to that of TULA-2 and that this finding correlates with an increase in Syk tyrosine phosphorylation in cells overexpressing TULA. This result is consistent with the dominant-negative effect of inactivated TULA-2, arguing that TULA acts in this system as a negative regulator of TULA-2-dependent dephosphorylation. To summarize, our findings indicate that TULA proteins may exert opposite effects on PTK-mediated signaling and suggest that a regulatory mechanism based on this feature may exist.

Multidomain STS/TULA proteins are novel cellular regulators

Proteins of the STS/TULA family recently emerged as important regulators of cellular functions. They exhibit a unique domain architecture, featuring at least three interactive/functional domains. Despite a significant degree of homology between the two members of this family, there are considerable functional differences between them. Thus, one of them is ubiquitously expressed in mammalian tissues and exhibits high phosphatase activity, whereas the other one is expressed in lymphocytes only and exhibits very low phosphatase activity, but is capable of promoting apoptosis, an activity unique for this family member. Among several functions reported for STS/TULA proteins, the most characterized one is the regulation of protein tyrosine kinase-mediated signaling. Interestingly, gene deletion of neither family member results in a discernible phenotype, whereas simultaneous deletion of both members causes hyperreactivity of T cells. Despite their apparent importance, the physiological role and the molecular basis of the effects of STS/TULA proteins remain poorly understood. This brief review summarizes what is currently known about the STS/TULA family and outlines the unresolved questions in this area.

PGAM5 tethers a ternary complex containing Keap1 and Nrf2 to mitochondria

Eukaryote cells balance production of reactive oxygen species (ROS) with levels of anti-oxidant enzyme activity to maintain cellular redox homeostasis. Mitochondria are a major source of ROS, while many anti-oxidant genes are regulated by the Nrf2 transcription factor. Keap1, a redox-regulated substrate adaptor for a cullin-based ubiquitin ligase, targets Nrf2 for proteosome-mediated degradation and represses Nrf2-dependent gene expression. We have previously identified a member of the phosphoglycerate mutase family, PGAM5, as a Keap1-binding protein. In this report, we demonstrate that PGAM5 is targeted to the outer membrane of mitochondria by an N-terminal mitochondrial-localization sequence. Furthermore, we provide evidence that PGAM5 forms a ternary complex containing both Keap1 and Nrf2, in which the dimeric Keap1 protein simultaneously binds both PGAM5 and Nrf2 through their conserved E(S/T)GE motifs. Knockdown of either Keap1 or PGAM5 activates Nrf2-dependent gene expression. We suggest that this ternary complex provides a molecular framework for understanding how nuclear anti-oxidant gene expression is regulated in response to changes in mitochondrial function(s).

The histidine phosphatase superfamily: structure and function

The histidine phosphatase superfamily is a large functionally diverse group of proteins. They share a conserved catalytic core centred on a histidine which becomes phosphorylated during the course of the reaction. Although the superfamily is overwhelmingly composed of phosphatases, the earliest known and arguably best-studied member is dPGM (cofactor-dependent phosphoglycerate mutase). The superfamily contains two branches sharing very limited sequence similarity: the first containing dPGM, fructose-2,6-bisphosphatase, PhoE, SixA, TIGAR [TP53 (tumour protein 53)-induced glycolysis and apoptosis regulator], Sts-1 and many other activities, and the second, smaller, branch composed mainly of acid phosphatases and phytases. Human representatives of both branches are of considerable medical interest, and various parasites contain superfamily members whose inhibition might have therapeutic value. Additionally, several phosphatases, notably the phytases, have current or potential applications in agriculture. The present review aims to draw together what is known about structure and function in the superfamily. With the benefit of an expanding set of histidine phosphatase superfamily structures, a clearer picture of the conserved elements is obtained, along with, conversely, a view of the sometimes surprising variation in substrate-binding and proton donor residues across the superfamily. This analysis should contribute to correcting a history of over- and mis-annotation in the superfamily, but also suggests that structural knowledge, from models or experimental structures, in conjunction with experimental assays, will prove vital for the future description of function in the superfamily.