Suppression of a phosphatidylinositol 3-kinase signal by a specific spliced variant of Drosophila PTEN

Drosophila PTEN (dPTEN) plays indispensable roles in the development of Drosophila melanogaster by controlling cell size and number. Although three potential spliced forms of dPTEN have been isolated, functional distinction among these forms remains elusive. In this study, we demonstrate that all spliced forms of dPTEN dephosphorylate phosphatidylinositol 3,4,5-trisphosphate (PI(3,4,5)P(3)); however, PI(3,4,5)P(3)-dependent activation of Drosophila Akt is suppressed specifically by one of three spliced forms, dPTEN3. Further, dPTEN3 dramatically changes its expression during the Drosophila development, while the other forms are expressed throughout the development. Our results suggest that dPTEN3 is the predominant spliced form that participates in PI(3,4,5)P(3)-mediated signaling pathways.

The crystal structure of Pseudomonas avirulence protein AvrPphB: a papain-like fold with a distinct substrate-binding site

AvrPphB is an avirulence (Avr) protein from the plant pathogen Pseudomonas syringae that can trigger a disease-resistance response in a number of host plants including Arabidopsis. AvrPphB belongs to a novel family of cysteine proteases with the charter member of this family being the Yersinia effector protein YopT. AvrPphB has a very stringent substrate specificity, catalyzing a single proteolytic cleavage in the Arabidopsis serine/threonine kinase PBS1. We have determined the crystal structure of AvrPphB by x-ray crystallography at 1.35-A resolution. The structure is composed of a central antiparallel beta-sheet, with alpha-helices packing on both sides of the sheet to form a two-lobe structure. The core of this structure resembles the papain-like cysteine proteases. The similarity includes the AvrPphB active site catalytic triad of Cys-98, His-212, and Asp-227 and the oxyanion hole residue Asn-93. Based on analogy with inhibitor complexes of the papain-like proteases, we propose a model for the substrate-binding mechanism of AvrPphB. A deep and positively charged pocket (S2) and a neighboring shallow surface (S3) likely bind to aspartic acid and glycine residues in the substrate located two (P2) and three (P3) residues N terminal to the cleavage site, respectively. Further implications about the specificity of plant pathogen recognition are also discussed.

Crystal Structure of a Phosphoinositide Phosphatase, MTMR2

Myotubularin-related proteins are a large subfamily of protein tyrosine phosphatases (PTPs) that dephosphorylate D3-phosphorylated inositol lipids. Mutations in members of the myotubularin family cause the human neuromuscular disorders myotubular myopathy and type 4B Charcot-Marie-Tooth syndrome. The crystal structure of a representative member of this family, MTMR2, reveals a phosphatase domain that is structurally unique among PTPs. A series of mutants are described that exhibit altered enzymatic activity and provide insight into the specificity of myotubularin phosphatases toward phosphoinositide substrates. The structure also reveals that the GRAM domain, found in myotubularin family phosphatases and predicted to occur in approximately 180 proteins, is part of a larger motif with a pleckstrin homology (PH) domain fold. Finally, the MTMR2 structure will serve as a model for other members of the myotubularin family and provide a framework for understanding the mechanism whereby mutations in these proteins lead to disease.

A PTEN-related 5-phosphatidylinositol phosphatase localized in the Golgi

Phosphoinositides play important roles as signaling molecules in different cell compartments by regulating the localization and activity of proteins through their interaction with specific domains. The activity of these lipids depends on which sites on the inositol ring are phosphorylated. Signaling pathways dependent on phosphoinositides phosphorylated at the D3 position of this ring (3-phosphoinositides) are negatively regulated by 3-phosphoinositide-specific phosphatases that include PTEN and myotubularin. Using the conserved PTEN catalytic core motif, we have identified a new protein in the Dictyostelium genome called phospholipid-inositol phosphatase (PLIP), which defines a new subfamily of phosphoinositide phosphatases clearly distinct from PTEN or other closely related proteins. We show that PLIP is able to dephosphorylate a broad spectrum of phosphoinositides, including 3-phosphoinositides. In contrast to previously characterized phosphoinositide phosphatases, PLIP has a preference for phosphatidylinositol 5-phosphate, a newly discovered phosphoinositide. We found that PLIP is localized in the Golgi, with its phosphatase domain facing the cytoplasmic compartment. PLIP null cells created via homologous recombination are unable to effectively aggregate to form multicellular organisms at low cell densities. The presence of PLIP in the Golgi suggests that it may be involved in membrane trafficking.

Cleavage of Arabidopsis PBS1 by a bacterial type III effector

Plant disease-resistance (R) proteins are thought to function as receptors for ligands produced directly or indirectly by pathogen avirulence (Avr) proteins. The biochemical functions of most Avr proteins are unknown, and the mechanisms by which they activate R proteins have not been determined. In Arabidopsis, resistance to Pseudomonas syringae strains expressing AvrPphB requires RPS5, a member of the class of R proteins that have a predicted nucleotide-binding site and leucine-rich repeats, and PBS1, a protein kinase. AvrPphB was found to proteolytically cleave PBS1, and this cleavage was required for RPS5-mediated resistance, which indicates that AvrPphB is detected indirectly via its enzymatic activity.

Membrane-binding and activation mechanism of PTEN

PTEN is a tumor suppressor that reverses the action of phosphoinositide 3-kinase by catalyzing the removal of the 3' phosphate of phosphoinositides. Despite the critical role of PTEN in cell signaling and regulation, the mechanisms of its membrane recruitment and activation is still poorly understood. PTEN is composed of an N-terminal phosphatase domain, a C2 domain, and a C-terminal tail region that contains the PSD-95/Dlg/ZO-1 homology (PDZ) domain-binding sequence and multiple phosphorylation sites. Our in vitro surface plasmon resonance measurements using immobilized vesicles showed that both the phosphatase domain and the C2 domain, but not the C-terminal tail, are involved in electrostatic membrane binding of PTEN. Furthermore, the phosphorylation-mimicking mutation on the C-terminal tail of PTEN caused an approximately 80-fold reduction in its membrane affinity, mainly by slowing the membrane-association step. Subcellular localization studies of PTEN transfected into HEK293T and HeLa cells indicated that targeting of PTEN to the plasma membrane is coupled with rapid degradation and that the phosphatase domain and the C2 domain are both necessary and sufficient for its membrane recruitment. Results also indicated that the phosphorylation regulates the targeting of PTEN to the plasma membrane not by blocking the PDZ domain-binding site but by interfering with electrostatic membrane binding of PTEN. On the basis of these results, we propose a membrane-binding and activation mechanism for PTEN, in which the phosphorylation/dephosphorylation of the C-terminal region serves as an electrostatic switch that controls the membrane translocation of the protein.

The yersinia virulence factor YopM forms a novel protein complex with two cellular kinases

Pathogenic Yersinia contain a virulence plasmid that encodes genes for intracellular effectors, which neutralize the host immune response. One effector, YopM, is necessary for Yersinia virulence, but its function in host cells is unknown. To identify potential cellular pathways affected by YopM, proteins that co-immunoprecipitate with YopM in mammalian cells were isolated and identified by mass spectrometry. Results demonstrate that two kinases, protein kinase C-like 2 (PRK2) and ribosomal S6 protein kinase 1 (RSK1), interact directly with YopM. These two kinases associate only when YopM is present, and expression of YopM in cells stimulates the activity of both kinases. RSK1 is activated directly by interaction with YopM, and RSK1 kinase activity is required for YopM-stimulated PRK2 activity. YopM activation of RSK1 occurs independently of the actions of YopJ on the MAPK pathway. YopM is also required for Yersinia-induced changes in RSK1 mobility in infected macrophage cells. These results identify the first intracellular targets of YopM and suggest YopM acts to stimulate the activity of PRK2 and RSK1.

Regulation of myotubularin-related (MTMR)2 phosphatidylinositol phosphatase by MTMR5, a catalytically inactive phosphatase

The myotubularin (MTM) family constitutes one of the most highly conserved protein-tyrosine phosphatase subfamilies in eukaryotes. MTM1, the archetypal member of this family, is mutated in X-linked myotubular myopathy, whereas mutations in the MTM-related (MTMR)2 gene cause the type 4B1 Charcot-Marie-Tooth disease, a severe hereditary motor and sensory neuropathy. In this study, we identified a protein that specifically interacts with MTMR2 but not MTM1. The interacting protein was shown by mass spectrometry to be MTMR5, a catalytically inactive member of the MTM family. We also demonstrate that MTMR2 interacts with MTMR5 via its coiled-coil domain and that mutations in the coiled-coil domain of either MTMR2 or MTMR5 abrogate this interaction. Through this interaction, MTMR5 increases the enzymatic activity of MTMR2 and dictates its subcellular localization. This article demonstrates an active MTM member being regulated by an inactive family member.

Biochemical characterization of the Yersinia YopT protease: cleavage site and recognition elements in Rho GTPases

The Gram-negative bacterial pathogen Yersinia delivers six effector proteins into the host cells to thwart the host innate immune response. One of the effectors, YopT, causes the disruption of the actin cytoskeleton and contributes to the inhibition of phagocytosis of the pathogen. YopT functions as a cysteine protease to cleave Rho family GTPases. We have analyzed the YopT cleavage products of Rho GTPases by TLC and determined their chemical structure by MS. Amino acid labeling experiments were performed to locate the exact site in RhoA where the YopT cleavage occurs. Our data unambiguously demonstrate that YopT cleaves N-terminal to the prenylated cysteine in RhoA, Rac, and Cdc42 and that the cleavage product of the GTPases is geranylgeranyl cysteine methyl ester. YopT cleaves GTP- and GDP-bound forms of RhoA equally, suggesting that the cleavage does not depend upon the conformation status of the GTPases. YopT also cleaves both farnesylated and geranylgeranylated forms of RhoA. The polybasic sequence in the C terminus of RhoA is essential for YopT substrate recognition and cleavage.

Overview of the Alliance for Cellular Signaling

The Alliance for Cellular Signaling is a large-scale collaboration designed to answer global questions about signalling networks. Pathways will be studied intensively in two cells--B lymphocytes (the cells of the immune system) and cardiac myocytes--to facilitate quantitative modelling. One goal is to catalyse complementary research in individual laboratories; to facilitate this, all alliance data are freely available for use by the entire research community.

PTEN and myotubularin phosphatases: from 3-phosphoinositide dephosphorylation to disease

The phosphatase and tensin homolog deleted on chromosome ten (PTEN) and myotubularin (MTM1) represent subfamilies of protein tyrosine phosphatases whose principal physiological substrates are D3-phosphorylated inositol phospholipids. As lipid phosphatases, PTEN- and MTM1-related (MTMR) proteins dephosphorylate the products of phosphoinositide 3-kinases and antagonize downstream effectors that utilize 3-phosphoinositides as ligands for protein targeting domains or allosteric activation. Here, we describe the cellular mechanisms of PTEN and MTMR function and their role in the etiology of cancer and other human diseases.

Sorting out the cellular functions of sorting nexins

Sorting nexins are a diverse group of cellular trafficking proteins that are unified by the presence of a phospholipid-binding motif - the PX domain. The ability of these proteins to bind specific phospholipids, as well as their propensity to form protein-protein complexes, points to a role for these proteins in membrane trafficking and protein sorting. It will be interesting to determine whether the various sorting nexins have specialized or generalized roles in protein trafficking.

Use of double-stranded RNA-mediated interference to determine the substrates of protein tyrosine kinases and phosphatases

Despite the wealth of information generated by genome-sequencing projects, the identification of in vivo substrates of specific protein kinases and phosphatases is hampered by the large number of candidate enzymes, overlapping enzyme specificity and sequence similarity. In the present study, we demonstrate the power of RNA interference (RNAi) to dissect signal transduction cascades involving specific kinases and phosphatases. RNAi is used to identify the cellular tyrosine kinases upstream of the phosphorylation of Down-Syndrome cell-adhesion molecule (Dscam), a novel cell-surface molecule of the immunoglobulin-fibronectin super family, which has been shown to be important for axonal path-finding in Drosophila. Tyrosine phosphorylation of Dscam recruits the Src homology 2 domain of the adaptor protein Dock to the receptor. Dock, the ortho- logue of mammalian Nck, is also essential for correct axonal path-finding in Drosophila. We further determined that Dock is tyrosine-phosphorylated in vivo and identified DPTP61F as the protein tyrosine phosphatase responsible for maintaining Dock in its non-phosphorylated state. The present study illustrates the versatility of RNAi in the identification of the physiological substrates for protein kinases and phosphatases.

A Yersinia effector and a Pseudomonas avirulence protein define a family of cysteine proteases functioning in bacterial pathogenesis

A Yersinia effector known as YopT and a Pseudomonas avirulence protein known as AvrPphB define a family of 19 proteins involved in bacterial pathogenesis. We show that both YopT and AvrPphB are cysteine proteases, and their proteolytic activities are dependent upon the invariant C/H/D residues conserved in the entire YopT family. YopT cleaves the posttranslationally modified Rho GTPases near their carboxyl termini, releasing them from the membrane. This leads to the disruption of actin cytoskeleton in host cells. The proteolytic activity of AvrPphB is essential for autoproteolytic cleavage of an AvrPphB precursor as well as for eliciting the hypersensitive response in plants. These findings provide new insights into mechanisms of animal and plant pathogenesis.

Role of Yersinia murine toxin in survival of Yersinia pestis in the midgut of the flea vector

Transmission by flea bite is a relatively recent adaptation that distinguishes Yersinia pestis, the plague bacillus, from closely related enteric bacteria. Here, a plasmid-encoded phospholipase D (PLD), previously characterized as Yersinia murine toxin (Ymt), was shown to be required for survival of Y. pestis in the midgut of its principal vector, the rat flea Xenopsylla cheopis. Intracellular PLD activity appeared to protect Y. pestis from a cytotoxic digestion product of blood plasma in the flea gut. By enabling colonization of the flea midgut, acquisition of this PLD may have precipitated the transition of Y. pestis to obligate arthropod-borne transmission.

Cloning, expression, and characterization of the Galalpha 1,3Gal high affinity lectin from the mushroom Marasmius oreades

The purification and unique carbohydrate binding properties, including blood group B-specific agglutination and preferential binding to Galalpha1,3Gal-containing sugar epitopes, of the Marasmius oreades agglutinin (MOA) are reported in an accompanying paper (Winter, H. C., Mostafapour, K., and Goldstein, I. J. (2002) J. Biol. Chem. 277, 14996-15001). Here we describe the cloning, characterization, and expression of MOA. MOA was digested with trypsin and endoproteinase Asp-N, and the peptide fragments were purified by high performance liquid chromatography. Amino acid sequence data were obtained for eight peptides. Using oligonucleotides deduced from the peptide sequences for a reverse transcriptase-PCR, a 41-base pair cDNA was obtained. The 41-base pair fragment allowed the generation a full-length cDNA using 5' and 3' rapid amplification of cDNA ends. MOA cDNA encodes a protein of 293 amino acids that contains a ricin domain. These carbohydrate binding domains were first described in subunits of bacterial toxins and are also commonly found in polysaccharide-degrading enzymes. Whereas these proteins are known to display a variety of sugar binding specificities, none to date are known to share MOA's high affinity for Galalpha1,3Gal and Galalpha1,3Galbeta1,4GlcNAc. Recombinantly expressed and purified MOA retains the specificity and affinity observed with the native protein. This study provides the basis for analyzing the underlying cause for the unusual binding specificity of MOA.

Yersinia effectors target mammalian signalling pathways

Animals have an immune system to fight off challenges from both viruses and bacteria. The first line of defence is innate immunity, which is composed of cells that engulf pathogens as well as cells that release potent signalling molecules to activate an inflammatory response and the adaptive immune system. Pathogenic bacteria have evolved a set of weapons, or effectors, to ensure survival in the host. Yersinia spp. use a type III secretion system to translocate these effector proteins, called Yops, into the host. This report outlines how Yops thwart the signalling machinery of the host immune system.

PTEN and myotubularin: novel phosphoinositide phosphatases

Protein tyrosine phosphatases (PTPs) are a diverse group of enzymes that contain a highly conserved active site motif, Cys-x5-Arg (Cx5R). The PTP superfamily enzymes, which include tyrosine-specific, dual specificity, low-molecular-weight, and Cdc25 phosphatases, are key mediators of a wide variety of cellular processes, including growth, metabolism, differentiation, motility, and programmed cell death. The PTEN/MMAC1/TEP1 gene was originally identified as a candidate tumor suppressor gene located on human chromosome 10q23; it encodes a protein with sequence similarity to PTPs and tensin. Recent studies have demonstrated that PTEN plays an essential role in regulating signaling pathways involved in cell growth and apoptosis, and mutations in the PTEN gene are now known to cause tumorigenesis in a number of human tissues. In addition, germ line mutations in the PTEN gene also play a major role in the development of Cowden and Bannayan-Zonana syndromes, in which patients often suffer from increased risk of breast and thyroid cancers. Biochemical studies of the PTEN phosphatase have revealed a molecular mechanism by which tumorigenesis may be caused in individuals with PTEN mutations. Unlike most members of the PTP superfamily, PTEN utilizes the phosphoinositide second messenger, phosphatidylinositol 3,4,5-trisphosphate (PIP3), as its physiologic substrate. This inositol lipid is an important regulator of cell growth and survival signaling through the Ser/Thr protein kinases PDK1 and Akt. By specifically dephosphorylating the D3 position of PIP3, the PTEN tumor suppressor functions as a negative regulator of signaling processes downstream of this lipid second messenger. Mutations that impair PTEN function result in a marked increase in cellular levels of PIP3 and constitutive activation of Akt survival signaling pathways, leading to inhibition of apoptosis, hyperplasia, and tumor formation. Certain structural features of PTEN contribute to its specificity for PIP3, as well as its role(s) in regulating cellular proliferation and apoptosis. Recently, myotubularin, a second PTP superfamily enzyme associated with human disease, has also been shown to utilize a phosphoinositide as its physiologic substrate.

Drosophila Ack targets its substrate, the sorting nexin DSH3PX1, to a protein complex involved in axonal guidance

Dock, the Drosophila orthologue of Nck, is an adaptor protein that is known to function in axonal guidance paradigms in the fly including proper development of neuronal connections in photoreceptor cells and axonal tracking in Bolwig's organ. To develop a better understanding of axonal guidance at the molecular level, we purified proteins in a complex with the SH2 domain of Dock from fly Schneider 2 cells. A protein designated p145 was identified and shown to be a tyrosine kinase with sequence similarity to mammalian Cdc-42-associated tyrosine kinases. We demonstrate that Drosophila Ack (DAck) can be co-immunoprecipitated with Dock and DSH3PX1 from fly cell extracts. The domains responsible for the in vitro interaction between Drosophila Ack and Dock were identified, and direct protein-protein interactions between complex members were established. We conclude that DSH3PX1 is a substrate for DAck in vivo and in vitro and define one of the major in vitro sites of DSH3PX1 phosphorylation to be Tyr-56. Tyr-56 is located within the SH3 domain of DSH3PX1, placing it in an important position for regulating the binding of proline-rich targets. We demonstrate that Tyr-56 phosphorylation by DAck diminishes the DSH3PX1 SH3 domain interaction with the Wiskott-Aldrich Syndrome protein while enabling DSH3PX1 to associate with Dock. Furthermore, when Tyr-56 is mutated to aspartate or glutamate, the binding to Wiskott-Aldrich Syndrome protein is abrogated. These results suggest that the phosphorylation of DSH3PX1 by DAck targets this sorting nexin to a protein complex that includes Dock, an adaptor protein important for axonal guidance.

The archetype STYX/dead-phosphatase complexes with a spermatid mRNA-binding protein and is essential for normal sperm production

Differentiation of spermatids into spermatozoa is regulated via phosphorylated RNA-binding proteins that modulate the expression of stage-specific mRNAs. We demonstrate that the phosphoserine, -threonine or -tyrosine, interaction protein, Styx, complexes with a testicular RNA-binding protein and is essential for normal spermiogenesis. Ablation of Styx expression in mouse disrupts round and elongating spermatid development, resulting in a >1,000-fold decrease in spermatozoa production. Moreover, Styx(-/-) males are infertile because of structural head abnormalities in residual epididymal sperm. Immunoprecipitation of Styx with Crhsp-24, a phosphorylated RNA-binding protein implicated in translational repression of histone mRNAs, provides a strategy for regulating posttranscriptional gene expression.

PTEN protects p53 from Mdm2 and sensitizes cancer cells to chemotherapy

The PTEN tumor suppressor protein inhibits phosphatidylinositol 3-kinase (PI3K)/Akt signaling that promotes translocation of Mdm2 into the nucleus. When restricted to the cytoplasm, Mdm2 is degraded. The ability of PTEN to inhibit the nuclear entry of Mdm2 increases the cellular content and transactivation of the p53 tumor suppressor protein. Retroviral transduction of PTEN into U87MG (PTEN null) glioblastoma cells increases p53 activity and expression of p53 target genes and induces cell cycle arrest. U87MG/PTEN glioblastoma cells are more sensitive than U87MG/PTEN null cells to death induced by etoposide, a chemotherapeutic agent that induces DNA damage. Previously, tumor suppressor proteins have been supposed to act individually to suppress cancers. Our results establish a direct connection between the activities of two major tumor suppressors and show that they act together to respond to stresses and malignancies. PTEN protects p53 from survival signals, permitting p53 to function as a guardian of the genome. By virtue of its capacity to protect p53, PTEN can sensitize tumor cells to chemotherapy that relies on p53 activity. p53 induces PTEN gene expression, and here it is shown that PTEN protects p53, indicating that a positive feedback loop may amplify the cellular response to stress, damage, and cancer.

Myotubularin and MTMR2, phosphatidylinositol 3-phosphatases mutated in myotubular myopathy and type 4B Charcot-Marie-Tooth disease

Myotubularin is the archetype of a family of highly conserved protein-tyrosine phosphatase-like enzymes. The myotubularin gene, MTM1, is mutated in the genetic disorder, X-linked myotubular myopathy. We and others have previously shown that myotubularin utilizes the lipid second messenger, phosphatidylinositol 3-phosphate (PI(3)P), as a physiologic substrate. We demonstrate here that the myotubularin-related protein MTMR2, which is mutated in the neurodegenerative disorder, type 4B Charcot-Marie-Tooth disease, is also highly specific for PI(3)P as a substrate. Furthermore, the MTM-related phosphatases MTMR1, MTMR3, and MTMR6 also dephosphorylate PI(3)P, suggesting that activity toward this substrate is common to all myotubularin family enzymes. A direct comparison of the lipid phosphatase activities of recombinant myotubularin and MTMR2 demonstrates that their enzymatic properties are indistinguishable, indicating that the lack of functional redundancy between these proteins is likely to be due to factors other than the utilization of different physiologic substrates. To this end, we have analyzed myotubularin and MTMR2 transcripts during induced differentiation of cultured murine C2C12 myoblasts and find that their expression is divergently regulated. In addition, myotubularin and MTMR2 enhanced green fluorescent protein fusion proteins exhibit overlapping but distinct patterns of subcellular localization. Finally, we provide evidence that myotubularin, but not MTMR2, can modulate the levels of endosomal PI(3)P. From these data, we conclude that the developmental expression and subcellular localization of myotubularin and MTMR2 are differentially regulated, resulting in their utilization of specific cellular pools of PI(3)P.

A unique carbohydrate binding domain targets the lafora disease phosphatase to glycogen

Lafora disease (progressive myoclonus epilepsy of Lafora type) is an autosomal recessive neurodegenerative disorder resulting from defects in the EPM2A gene. EPM2A encodes a 331-amino acid protein containing a carboxyl-terminal phosphatase catalytic domain. We demonstrate that the EPM2A gene product also contains an amino-terminal carbohydrate binding domain (CBD) and that the CBD is critical for association with glycogen both in vitro and in vivo. The CBD domain localizes the phosphatase to specific subcellular compartments that correspond to the expression pattern of glycogen processing enzyme, glycogen synthase. Mutations in the CBD result in mis-localization of the phosphatase and thereby suggest that the CBD targets laforin to intracellular glycogen particles where it is likely to function. Thus naturally occurring mutations within the CBD of laforin likely result in progressive myoclonus epilepsy due to mis-localization of phosphatase expression.

The sorting nexin, DSH3PX1, connects the axonal guidance receptor, Dscam, to the actin cytoskeleton

Dock, an adaptor protein that functions in Drosophila axonal guidance, consists of three tandem Src homology 3 (SH3) domains preceding an SH2 domain. To develop a better understanding of axonal guidance at the molecular level, we used the SH2 domain of Dock to purify a protein complex from fly S2 cells. Five proteins were obtained in pure form from this protein complex. The largest protein in the complex was identified as Dscam (Down syndrome cell adhesion molecule), which was subsequently shown to play a key role in directing neurons of the fly embryo to correct positions within the nervous system (Schmucker, D., Clemens, J. C., Shu, H., Worby, C. A., Xiao, J., Muda, M., Dixon, J. E., and Zipursky, S. L. (2000) Cell 101, 671-684). The smallest protein in this complex (p63) has now been identified. We have named p63 DSH3PX1 because it appears to be the Drosophila orthologue of the human protein known as SH3PX1. DSH3PX1 is comprised of an NH(2)-terminal SH3 domain, an internal PHOX homology (PX) domain, and a carboxyl-terminal coiled-coil region. Because of its PX domain, DSH3PX1 is considered to be a member of a growing family of proteins known collectively as sorting nexins, some of which have been shown to be involved in vesicular trafficking. We demonstrate that DSH3PX1 immunoprecipitates with Dock and Dscam from S2 cell extracts. The domains responsible for the in vitro interaction between DSH3PX1 and Dock were also identified. We further show that DSH3PX1 interacts with the Drosophila orthologue of Wasp, a protein component of actin polymerization machinery, and that DSH3PX1 co-immunoprecipitates with AP-50, the clathrin-coat adapter protein. This evidence places DSH3PX1 in a complex linking cell surface receptors like Dscam to proteins involved in cytoskeletal rearrangements and/or receptor trafficking.

RNA interference of gene expression (RNAi) in cultured Drosophila cells

RNA interference (RNAi) can be used to silence genes in a number of taxa, including plants, nematodes, protozoans, flies, and mammals represented by mouse embryos and cultured mammalian cells. To investigate signal transduction pathways, we used RNAi on Drosophila-cultured cells, which affords the opportunity to study protein function in a simple, well-defined cell culture system. Furthermore, the results obtained from experiments performed on cultured cells can be confirmed and extended in the whole organism, which, in the case of Drosophila, is also RNAi responsive. RNAi takes advantage of the unique ability of double-stranded RNA (dsRNA) molecules to induce posttranscriptional gene silencing in a highly specific manner. This silencing is efficacious and long-lived, as it is passed to subsequent generations in insect cell culture. To date, all Drosophila cell lines tested (S2, KC, BG2-C6, and Shi) respond to dsRNAs by ablating expression of the target protein. Furthermore, all dsRNAs tested (more than 15) have been effective at silencing the target gene. Drosophila cell cultures are simple, easily manipulated model systems that will facilitate loss-of-function studies applicable to a wide variety of questions.

Regulation of KChIP2 potassium channel beta subunit gene expression underlies the gradient of transient outward current in canine and human ventricle

Expression of four members of the KChIP family of potassium channel beta subunits was examined in canine heart. Only one member of the gene family, KChIP2, was expressed in heart. There was a steep gradient of KChIP2 mRNA expression across the canine ventricular free wall. KChIP2 mRNA was 25-fold more abundant in the epicardium than in the endocardium, and this gradient paralleled the gradient in transient outward current (Ito) expression. In contrast, Kv4.3 potassium channel alpha subunit mRNA was expressed at equal levels across the ventricular wall. There was no difference in the pharmacological sensitivity of epicardial and endocardial Ito channels to flecainide, suggesting that the current is produced by the same channel in the two tissues. A similar gradient of KChIP2 expression was found across the ventricular wall of human heart, but not rat heart. It is concluded that transcriptional regulation of the KChIP2 beta subunit gene, rather than the Kv4.3 [alpha] subunit gene, is the primary determinant regulating the transmural gradient of Ito expression in the ventricular free wall of canine and human heart.

ERK7 is an autoactivated member of the MAPK family

Extracellular signal-regulated kinase 7 (ERK7) shares significant sequence homology with other members of the ERK family of signal transduction proteins, including the signature TEY activation motif. However, ERK7 has several distinguishing characteristics. Unlike other ERKs, ERK7 has been shown to have significant constitutive activity in serum-starved cells, which is not increased further by extracellular stimuli that typically activate other members of the mitogen-activated protein kinase (MAPK) family. On the other hand, ERK7's activation state and kinase activity appear to be regulated by its ability to utilize ATP and the presence of its extended C-terminal region. In this study, we investigated the mechanism of ERK7 activation. The results suggest that 1) MAPK kinase (MEK) inhibitors do not suppress ERK7 kinase activity; 2) intramolecular autophosphorylation is sufficient for activation of ERK7 in the absence of an upstream MEK; and 3) multiple regions of the C-terminal domain of ERK7 regulate its kinase activity. Taken together, these results indicate that autophosphorylation is sufficient for ERK7 activation and that the C-terminal domain regulates its kinase activity through multiple interactions.

The PTEN tumor suppressor protein inhibits tumor necrosis factor-induced nuclear factor kappa B activity

Nuclear factor kappaB (NF-kappaB) transcriptionally activates genes that promote immunity and cell survival. Activation of NF-kappaB is induced by an IkappaB kinase (IKK) complex that phosphorylates and promotes dissociation of IkappaB from NF-kappaB, which then translocates into the nucleus. Activation of phosphatidylinositol (PI) 3-kinase/Akt signaling by tumor necrosis factor (TNF) activates IKK and NF-kappaB. The present study shows that PTEN, a tumor suppressor that inhibits PI 3-kinase function, impairs TNF activation of Akt and the IKK complex in 293 cells. Transient expression of PTEN suppressed IKK activation and TNF-induced NF-kappaB DNA binding and transactivation. Studies were conducted with PC-3 prostate cancer cells that do not express PTEN and DU145 prostate cancer cells that express PTEN. TNF activated Akt in PC-3 cells, but not in DU145 cells, and the ability of TNF to activate NF-kappaB was blocked by pharmacological inhibition of PI 3-kinase activity in PC-3 cells, but not in DU145 cells. Expression of PTEN in PC-3 cells to a level comparable with that endogenously present in DU145 cells inhibited TNF activation of NF-kappaB. The cell type-specific ability of PTEN to negatively regulate the PI 3-kinase/AKT/NF-kappaB pathway may be important to its tumor suppressor activity.

A phosphatidylinositol 3-kinase/Akt/mTOR pathway mediates and PTEN antagonizes tumor necrosis factor inhibition of insulin signaling through insulin receptor substrate-1

Tyrosine phosphorylation of insulin receptor substrate-1 (IRS-1) by the insulin receptor permits this docking protein to interact with signaling proteins that promote insulin action. Serine phosphorylation uncouples IRS-1 from the insulin receptor, thereby inhibiting its tyrosine phosphorylation and insulin signaling. For this reason, there is great interest in identifying serine/threonine kinases for which IRS-1 is a substrate. Tumor necrosis factor (TNF) inhibited insulin-promoted tyrosine phosphorylation of IRS-1 and activated the Akt/protein kinase B serine-threonine kinase, a downstream target for phosphatidylinositol 3-kinase (PI 3-kinase). The effect of TNF on insulin-promoted tyrosine phosphorylation of IRS-1 was blocked by inhibition of PI 3-kinase and the PTEN tumor suppressor, which dephosphorylates the lipids that mediate PI 3-kinase functions, whereas constitutively active Akt impaired insulin-promoted IRS-1 tyrosine phosphorylation. Conversely, TNF inhibition of IRS-1 tyrosine phosphorylation was blocked by kinase dead Akt. Inhibition of IRS-1 tyrosine phosphorylation by TNF was blocked by rapamycin, an inhibitor of the mammalian target of rapamycin (mTOR), a downstream target of Akt. mTOR induced the serine phosphorylation of IRS-1 (Ser-636/639), and such phosphorylation was inhibited by rapamycin. These results suggest that TNF impairs insulin signaling through IRS-1 by activation of a PI 3-kinase/Akt/mTOR pathway, which is antagonized by PTEN.

PTEN and myotubularin phosphoinositide phosphatases: bringing bioinformatics to the lab bench

Phosphoinositides play an integral role in a diverse array of cellular signaling processes. Although considerable effort has been directed toward characterizing the kinases that produce inositol lipid second messengers, the study of phosphatases that oppose these kinases remains limited. Current research is focused on the identification of novel lipid phosphatases such as PTEN and myotubularin, their physiologic substrates, signaling pathways and links to human diseases. The use of bioinformatics in conjunction with genetic analyses in model organisms will be essential in elucidating the roles of these enzymes in regulating phosphoinositide-mediated cellular signaling.

Alternative splicing of KCNQ2 potassium channel transcripts contributes to the functional diversity of M-currents

The region of alternative splicing in the KCNQ2 potassium channel gene was determined by RNase protection analysis of KCNQ2 mRNA transcripts. Systematic analysis of KCNQ2 alternative splice variant expression in rat superior cervical ganglia revealed multiple variant isoforms. One class of KCNQ2 splice variants, those that contained exon 15a, was found to have significantly different kinetics to those of the other isoforms. These transcripts encoded channel subunits that, when co-expressed with the KCNQ3 subunit, activated and deactivated approximately 2.5 times more slowly than other isoforms. Deletion of exon 15a in these isoforms produced a reversion to the faster kinetics. Comparison of the kinetic properties of the cloned channel splice variants with those of the native M-current suggests that alternative splicing of the KCNQ2 gene may contribute to the variation in M-current kinetics seen in vivo.

Disruption of signaling by Yersinia effector YopJ, a ubiquitin-like protein protease

Homologs of the Yersinia virulence effector YopJ are found in both plant and animal bacterial pathogens, as well as plant symbionts. These YopJ family members were shown to act as cysteine proteases. The catalytic triad of the protease was required for inhibition of the mitogen-activated protein kinase (MAPK) and nuclear factor kappaB (NF-kappaB) signaling in animal cells and for induction of localized cell death in plants. The substrates for YopJ were shown to be highly conserved ubiquitin-like molecules, which are covalently added to numerous regulatory proteins. YopJ family members exert their pathogenic effect on cells by disrupting this posttranslational modification.

A distinctive role for the Yersinia protein kinase: actin binding, kinase activation, and cytoskeleton disruption

The bacterial pathogens of the genus Yersinia deliver several virulence factors into target cells using a type III secretion system. We demonstrate that Yersinia protein kinase A (YpkA), an essential bacterial virulence factor, is produced as an inactive serine/threonine kinase. The inactive kinase is activated within the host cell by a cytosolic eukaryotic activator. Using biochemical purification techniques, we demonstrate that actin is a cellular activator of YpkA. This stimulation of YpkA kinase activity by actin depends on the presence of the C-terminal twenty amino acids of YpkA, because deletion of these 20 aa not only obliterates YpkA activity, but it also destroys the interaction between YpkA and actin. Activated YpkA functions within cultured epithelial cells to disrupt the actin cytoskeleton. The disruption of the actin cytoskeleton by YpkA would be expected to inhibit macrophage function and phagocytosis of Yersinia.

The crystal structure of adenylosuccinate lyase from Pyrobaculum aerophilum reveals an intracellular protein with three disulfide bonds

Adenylosuccinate lyase catalyzes two separate reactions in the de novo purine biosynthetic pathway. Through its dual action in this pathway, adenylosuccinate lyase plays an integral part in cellular replication and metabolism. Mutations in the human enzyme can result in severe neurological disorders, including mental retardation with autistic features. The crystal structure of adenylosuccinate lyase from the hyperthermophilic archaebacterium Pyrobaculum aerophilum has been determined to 2.1 A resolution. Although both the fold of the monomer and the architecture of the tetrameric assembly are similar to adenylosuccinate lyase from the thermophilic eubacterium Thermotoga maritima, the archaebacterial lyase contains unique features. Surprisingly, the structure of adenylosuccinate lyase from P. aerophilum reveals that this intracellular protein contains three disulfide bonds that contribute significantly to its stability against thermal and chemical denaturation. The observation of multiple disulfide bonds in the recombinant form of the enzyme suggests the need for further investigations into whether the intracellular environment of P. aerophilum, and possibly other hyperthermophiles, may be compatible with protein disulfide bond formation. In addition, the protein is shorter in P. aerophilum than it is in other organisms. This abbreviation results from an internal excision of a cluster of helices that may be involved in protein-protein interactions in other organisms and may relate to the observed clinical effects of human mutations in that region.

Myotubularin, a protein tyrosine phosphatase mutated in myotubular myopathy, dephosphorylates the lipid second messenger, phosphatidylinositol 3-phosphate

The lipid second messenger phosphatidylinositol 3-phosphate [PI(3)P] plays a crucial role in intracellular membrane trafficking. We report here that myotubularin, a protein tyrosine phosphatase required for muscle cell differentiation, is a potent PI(3)P phosphatase. Recombinant human myotubularin specifically dephosphorylates PI(3)P in vitro. Overexpression of a catalytically inactive substrate-trapping myotubularin mutant (C375S) in human 293 cells increases PI(3)P levels relative to that of cells overexpressing the wild-type enzyme, demonstrating that PI(3)P is a substrate for myotubularin in vivo. In addition, a Saccharomyces cerevisiae strain in which the myotubularin-like gene (YJR110w) is disrupted also exhibits increased PI(3)P levels. Both the recombinant yeast enzyme and a human myotubularin-related protein (KIAA0371) are able to dephosphorylate PI(3)P in vitro, suggesting that this activity is intrinsic to all myotubularin family members. Mutations in the MTM1 gene that cause human myotubular myopathy dramatically reduce the ability of the phosphatase to dephosphorylate PI(3)P. Our findings provide evidence that myotubularin exerts its effects during myogenesis by regulating cellular levels of the inositol lipid PI(3)P.

Drosophila Dscam is an axon guidance receptor exhibiting extraordinary molecular diversity

A Drosophila homolog of human Down syndrome cell adhesion molecule (DSCAM), an immunoglobulin superfamily member, was isolated by its affinity to Dock, an SH3/SH2 adaptor protein required for axon guidance. Dscam binds directly to both Dock's SH2 and SH3 domains. Genetic studies revealed that Dscam, Dock and Pak, a serine/threonine kinase, act together to direct pathfinding of Bolwig's nerve, containing a subclass of sensory axons, to an intermediate target in the embryo. Dscam also is required for the formation of axon pathways in the embryonic central nervous system. cDNA and genomic analyses reveal the existence of multiple forms of Dscam with a conserved architecture containing variable Ig and transmembrane domains. Alternative splicing can potentially generate more than 38,000 Dscam isoforms. This molecular diversity may contribute to the specificity of neuronal connectivity.

Use of double-stranded RNA interference in Drosophila cell lines to dissect signal transduction pathways

We demonstrate the efficacy of double-stranded RNA-mediated interference (RNAi) of gene expression in generating "knock-out" phenotypes for specific proteins in several Drosophila cell lines. We prove the applicability of this technique for studying signaling cascades by dissecting the well-characterized insulin signal transduction pathway. Specifically, we demonstrate that inhibiting the expression of the DSOR1 (mitogen-activated protein kinase kinase, MAPKK) prevents the activation of the downstream ERK-A (MAPK). In contrast, blocking ERK-A expression results in increased activation of DSOR1. We also show that Drosophila AKT (DAKT) activation depends on the insulin receptor substrate, CHICO (IRS1-4). Finally, we demonstrate that blocking the expression of Drosophila PTEN results in the activation of DAKT. In all cases, the interference of the biochemical cascade by RNAi is consistent with the known steps in the pathway. We extend this powerful technique to study two proteins, DSH3PX1 and Drosophila ACK (DACK). DSH3PX1 is an SH3, phox homology domain-containing protein, and DACK is homologous to the mammalian activated Cdc42 tyrosine kinase, ACK. Using RNAi, we demonstrate that DACK is upstream of DSH3PX1 phosphorylation, making DSH3PX1 an identified downstream target/substrate of ACK-like tyrosine kinases. These experiments highlight the usefulness of RNAi in dissecting complex biochemical signaling cascades and provide a highly effective method for determining the function of the identified genes arising from the Drosophila genome sequencing project.

A sodium channel signaling complex: modulation by associated receptor protein tyrosine phosphatase beta

Voltage-gated sodium channels in brain neurons were found to associate with receptor protein tyrosine phosphatase beta (RPTPbeta) and its catalytically inactive, secreted isoform phosphacan, and this interaction was regulated during development. Both the extracellular domain and the intracellular catalytic domain of RPTPbeta interacted with sodium channels. Sodium channels were tyrosine phosphorylated and were modulated by the associated catalytic domains of RPTPbeta. Dephosphorylation slowed sodium channel inactivation, positively shifted its voltage dependence, and increased whole-cell sodium current. Our results define a sodium channel signaling complex containing RPTPbeta, which acts to regulate sodium channel modulation by tyrosine phosphorylation.

Form, function, and regulation of protein tyrosine phosphatases and their involvement in human diseases

Protein tyrosine phosphatases (PTPs) are a family of enzymes that modulate the cellular level of tyrosine phosphorylation. Based on cellular location, they are classified as receptor like or intracellular PTPs. Structure and function studies have led to the understanding of the enzymatic mechanism of this class of enzymes. Proper targeting of PTPs is essential for many cellular signalling events including antigen induced proliferative responses of B and T cells. The physiological significance of PTPs is further unveiled through mice gene knockout studies and human genome sequencing and mapping projects. Several PTPs are shown to be critical in the pathogenesis of human diseases.

ERBB-2 overexpression confers PI 3' kinase-dependent invasion capacity on human mammary epithelial cells

Amplification and overexpression of ERBB-2 in human breast cancer is thought to play a significant role in the progression of the disease; however, its precise role in the aetiology of altered phenotypes associated with human breast cancer is unknown. We have previously shown that exogenous overexpression of ERBB-2 conferred growth factor independence on human mammary epithelial cells. In this study, we show that ERBB-2 overexpression also causes the cells to acquire other characteristics exhibited by human breast cancer cells, such as anchorage-independent growth and invasion capabilities. ERBB-2-induced invasion is dependent on fibronectin and correlates with the down-regulation of cell surface alpha4 integrin. In addition ERBB-2 co-immunoprecipitates with focal adhesion kinase (FAK) in these cells. We have also shown, by use of exogenously expressed PTEN and by treatment with the PI3'-kinase inhibitor LY294002, that ERBB-2-induced invasion is dependent on the PI3'-kinase pathway; however, PTEN does not dephosphorylate FAK in these cells.

The human Cdc14 phosphatases interact with and dephosphorylate the tumor suppressor protein p53

The yeast Cdc14 phosphatase has been shown to play an important role in cell cycle regulation by dephosphorylating proteins phosphorylated by the cyclin-dependent kinase Cdc28/clb. We recently cloned two human orthologs of the yeast CDC14, termed hCDC14A and -B, the gene products of which share approximately 80% amino acid sequence identity within their N termini and phosphatase domains. Here we report that the hCdc14A and hCdc14B proteins interact with the tumor suppressor protein p53 both in vitro and in vivo. This interaction is dependent on the N termini of the hCdc14 proteins and the C terminus of p53. Furthermore, the hCdc14 phosphatases were found to dephosphorylate p53 specifically at the p34(Cdc2)/clb phosphorylation site (p53-phosphor-Ser(315)). Our findings that hCdc14 is a cyclin-dependent kinase substrate phosphatase suggest that it may play a role in cell cycle control in human cells. Furthermore, the identification of p53 as a substrate for hCdc14 indicates that hCdc14 may regulate the function of p53.

Crystal structure of the PTEN tumor suppressor: implications for its phosphoinositide phosphatase activity and membrane association

The PTEN tumor suppressor is mutated in diverse human cancers and in hereditary cancer predisposition syndromes. PTEN is a phosphatase that can act on both polypeptide and phosphoinositide substrates in vitro. The PTEN structure reveals a phosphatase domain that is similar to protein phosphatases but has an enlarged active site important for the accommodation of the phosphoinositide substrate. The structure also reveals that PTEN has a C2 domain. The PTEN C2 domain binds phospholipid membranes in vitro, and mutation of basic residues that could mediate this reduces PTEN's membrane affinity and its ability to suppress the growth of glioblastoma tumor cells. The phosphatase and C2 domains associate across an extensive interface, suggesting that the C2 domain may serve to productively position the catalytic domain on the membrane.