Publications

• Identification of myotubularin as the lipid phosphatase catalytic subunit associated with the 3-phosphatase adapter protein, 3-PAP.

  Nandurkar H.H., Layton M., Laporte J., Selan C., Corcoran L., Caldwell K.K., Mochizuki Y., Majerus P.W., Mitchell C.A.

  Myotubularin is a dual-specific phosphatase that dephosphorylates phosphatidylinositol 3-phosphate and phosphatidylinositol (3,5)-bisphosphate. Mutations in myotubularin result in the human disease X-linked myotubular myopathy, characterized by persistence of muscle fibers that retain an immature ... >> More

  Myotubularin is a dual-specific phosphatase that dephosphorylates phosphatidylinositol 3-phosphate and phosphatidylinositol (3,5)-bisphosphate. Mutations in myotubularin result in the human disease X-linked myotubular myopathy, characterized by persistence of muscle fibers that retain an immature phenotype. We have previously reported the identification of the 3-phosphatase adapter protein (3-PAP), a catalytically inactive member of the myotubularin gene family, which coprecipitates lipid phosphatidylinositol 3-phosphate-3-phosphatase activity from lysates of human platelets. We have now identified myotubularin as the catalytically active 3-phosphatase subunit interacting with 3-PAP. A 65-kDa polypeptide, coprecipitating with endogenous 3-PAP, was purified from SDS/PAGE, subjected to trypsin digestion, and analyzed by collision-induced dissociation tandem MS. Three peptides derived from human myotubularin were identified. Association between 3-PAP and myotubularin was confirmed by reciprocal coimmunoprecipitation of both endogenous and recombinant proteins expressed in K562 cells. Recombinant myotubularin localized to the plasma membrane, causing extensive filopodia formation. However, coexpression of 3-PAP with myotubularin led to attenuation of the plasma membrane phenotype, associated with myotubularin relocalization to the cytosol. Collectively these studies indicate 3-PAP functions as an "adapter" for myotubularin, regulating myotubularin intracellular location and thereby altering the phenotype resulting from myotubularin overexpression. << Less

  Proc. Natl. Acad. Sci. U.S.A. 100:8660-8665(2003) [PubMed] [EuropePMC]

• Phosphatidylinositol 5-phosphate links dehydration stress to the activity of ARABIDOPSIS TRITHORAX-LIKE factor ATX1.

  Ndamukong I., Jones D.R., Lapko H., Divecha N., Avramova Z.

  <h4>Background</h4>Changes in gene expression enable organisms to respond to environmental stress. Levels of cellular lipid second messengers, such as the phosphoinositide PtdIns5P, change in response to a variety of stresses and can modulate the localization, conformation and activity of a number ... >> More

  <h4>Background</h4>Changes in gene expression enable organisms to respond to environmental stress. Levels of cellular lipid second messengers, such as the phosphoinositide PtdIns5P, change in response to a variety of stresses and can modulate the localization, conformation and activity of a number of intracellular proteins. The plant trithorax factor (ATX1) tri-methylates the lysine 4 residue of histone H3 (H3K4me3) at gene coding sequences, which positively correlates with gene transcription. Microarray analysis has identified a target gene (WRKY70) that is regulated by both ATX1 and by the exogenous addition of PtdIns5P in Arabidopsis. Interestingly, ATX1 contains a PtdIns5P interaction domain (PHD finger) and thus, phosphoinositide signaling, may link environmental stress to changes in gene transcription.<h4>Principal findings</h4>Using the plant Arabidopsis as a model system, we demonstrate a link between PtdIns5P and the activity of the chromatin modifier ATX1 in response to dehydration stress. We show for the first time that dehydration leads to an increase in cellular PtdIns5P in Arabidopsis. The Arabidopsis homolog of myotubularin (AtMTM1) is capable of generating PtdIns5P and here, we show that AtMTM1 is essential for the induced increase in PtdIns5P upon dehydration. Furthermore, we demonstrate that the ATX1-dependent gene, WRKY70, is downregulated during dehydration and that lowered transcript levels are accompanied by a drastic reduction in H3K4me3 of its nucleosomes. We follow changes in WRKY70 nucleosomal K4 methylation as a model to study ATX1 activity at chromatin during dehydration stress. We found that during dehydration stress, the physical presence of ATX1 at the WRKY70 locus was diminished and that ATX1 depletion resulted from it being retained in the cytoplasm when PtdIns5P was elevated. The PHD of ATX1 and catalytically active AtMTM1 are required for the cytoplasmic localization of ATX1.<h4>Conclusions/significance</h4>The novelty of the manuscript is in the discovery of a mechanistic link between a chromatin modifying activity (ATX1) and a lipid (PtdIns5P) synthesis in a signaling pathway that ultimately results in altered expression of ATX1 dependent genes downregulated in response to dehydration stress. << Less

  PLoS ONE 5:E13396-E13396(2010) [PubMed] [EuropePMC]

• Myotubularin, a phosphatase deficient in myotubular myopathy, acts on phosphatidylinositol 3-kinase and phosphatidylinositol 3-phosphate pathway.

  Blondeau F., Laporte J., Bodin S., Superti-Furga G., Payrastre B., Mandel J.L.

  Myotubular myopathy (MTM1) is an X-linked disease, characterized by severe neonatal hypotonia and generalized muscle weakness, with pathological features suggesting an impairment in maturation of muscle fibres. The MTM1 gene encodes a protein (myotubularin) with a phosphotyrosine phosphatase conse ... >> More

  Myotubular myopathy (MTM1) is an X-linked disease, characterized by severe neonatal hypotonia and generalized muscle weakness, with pathological features suggesting an impairment in maturation of muscle fibres. The MTM1 gene encodes a protein (myotubularin) with a phosphotyrosine phosphatase consensus. It defines a family of at least nine genes in man, including the antiphosphatase hMTMR5/Sbf1 and hMTMR2, recently found mutated in a recessive form of Charcot-Marie-Tooth disease. Myotubularin shows a dual specificity protein phosphatase activity in vitro. We have performed an in vivo test of tyrosine phosphatase activity in Schizosaccharomyces pombe, indicating that myotubularin does not have a broad specificity tyrosine phosphatase activity. Expression of active human myotubularin inhibited growth of S.pombe and induced a vacuolar phenotype similar to that of mutants of the vacuolar protein sorting (VPS) pathway and notably of mutants of VPS34, a phosphatidylinositol 3-kinase (PI3K). In S.pombe cells deleted for the endogenous MTM homologous gene, expression of human myotubularin decreased the level of phosphatidylinositol 3-phosphate (PI3P). We have created a substrate trap mutant which shows relocalization to plasma membrane projections (spikes) in HeLa cells and was inactive in the S.pombe assay. This mutant, but not the wild-type or a phosphatase site mutant, was able to immunoprecipitate a VPS34 kinase activity. Wild-type myotubularin was also able to directly dephosphorylate PI3P and PI4P in vitro. Myotubularin may thus decrease PI3P levels by down-regulating PI3K activity and by directly degrading PI3P. << Less

  Hum. Mol. Genet. 9:2223-2229(2000) [PubMed] [EuropePMC]

• Divergent functions of the myotubularin (MTM) homologs AtMTM1 and AtMTM2 in Arabidopsis thaliana: evolution of the plant MTM family.

  Ding Y., Ndamukong I., Zhao Y., Xia Y., Riethoven J.-J., Jones D.R., Divecha N., Avramova Z.

  Myotubularin and myotubularin-related proteins are evolutionarily conserved in eukaryotes. Defects in their function result in muscular dystrophy, neuronal diseases and leukemia in humans. In contrast to the animal lineage, where genes encoding both active and inactive myotubularins (phosphoinosit ... >> More

  Myotubularin and myotubularin-related proteins are evolutionarily conserved in eukaryotes. Defects in their function result in muscular dystrophy, neuronal diseases and leukemia in humans. In contrast to the animal lineage, where genes encoding both active and inactive myotubularins (phosphoinositide 3-phosphatases) have appeared and proliferated in the basal metazoan group, myotubularin genes are not found in the unicellular relatives of green plants. However, they are present in land plants encoding proteins highly similar to the active metazoan enzymes. Despite their remarkable structural conservation, plant and animal myotubularins have significantly diverged in their functions. While loss of myotubularin function causes severe disease phenotypes in humans it is not essential for the cellular homeostasis under normal conditions in Arabidopsis thaliana. Instead, myotubularin deficiency is associated with altered tolerance to dehydration stress. The two Arabidopsis genes AtMTM1 and AtMTM2 have originated from a segmental chromosomal duplication and encode catalytically active enzymes. However, only AtMTM1 is involved in elevating the cellular level of phosphatidylinositol 5-phosphate in response to dehydration stress, and the two myotubularins differentially affect the Arabidopsis dehydration stress-responding transcriptome. AtMTM1 and AtMTM2 display different localization patterns in the cell, consistent with the idea that they associate with different membranes to perform specific functions. A single amino acid mutation in AtMTM2 (L250W) results in a dramatic loss of subcellular localization. Mutations in this region are linked to disease conditions in humans. << Less

  Plant J. 70:866-878(2012) [PubMed] [EuropePMC]

• Characterization and functional studies of a FYVE domain-containing phosphatase in C. elegans.

  Ma J., Zeng F., Ho W.T., Teng L., Li Q., Fu X., Zhao Z.J.

  The myotubularin (MTM) enzymes are phosphatidylinositol 3-phosphate (PI3P) and phosphatidylinositol 3,5-bisphosphate phosphatases. Mutation of MTM1, the founder member of this family, is responsible for X-linked myotubular myopathy in humans. Here, we have isolated and characterized a Caenorhabdit ... >> More

  The myotubularin (MTM) enzymes are phosphatidylinositol 3-phosphate (PI3P) and phosphatidylinositol 3,5-bisphosphate phosphatases. Mutation of MTM1, the founder member of this family, is responsible for X-linked myotubular myopathy in humans. Here, we have isolated and characterized a Caenorhabditis elegans homology of the enzymes designated ceMTM3. ceMTM3 preferably dephosphorylates PI3P and contains a FYVE lipid-binding domain at its C-terminus which binds PI3P. Immunoblotting analyses revealed that the enzyme is expressed during the early development and adulthood of the animal. Immunofluorescent staining revealed predominant expression of the enzyme in eggs and muscles. Knockdown of the enzyme by using feeding-based RNA interference resulted in an increased level of PI3P and caused severe impairment of body movement of the worms at their post-reproductive ages and significantly shortened their lifespan. This study thus reveals an important role of the MTM phosphatases in maintaining muscle function, which may have clinical implications in prevention and treatment of sarcopenia. << Less

  J. Cell. Biochem. 104:1843-1852(2008) [PubMed] [EuropePMC]

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

  Kim S.-A., Vacratsis P.O., Firestein R., Cleary M.L., Dixon J.E.

  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-Ma ... >> More

  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. << Less

  Proc. Natl. Acad. Sci. U.S.A. 100:4492-4497(2003) [PubMed] [EuropePMC]

• Muscle-specific alternative splicing of myotubularin-related 1 gene is impaired in DM1 muscle cells.

  Buj-Bello A., Furling D., Tronchere H., Laporte J., Lerouge T., Butler-Browne G.S., Mandel J.L.

  The myotubularin-related 1 (MTMR1) gene belongs to a highly conserved family of eucaryotic phosphatases, with at least 11 members in humans. The founder member of this gene family, MTM1, is mutated in X-linked myotubular myopathy, a severe congenital disorder that affects skeletal muscle, and code ... >> More

  The myotubularin-related 1 (MTMR1) gene belongs to a highly conserved family of eucaryotic phosphatases, with at least 11 members in humans. The founder member of this gene family, MTM1, is mutated in X-linked myotubular myopathy, a severe congenital disorder that affects skeletal muscle, and codes for myotubularin, a specific phosphatidylinositol 3-phosphate [PI(3)P] phosphatase. MTM1 and MTMR1 are adjacent on the X chromosome, and the corresponding proteins share 59% sequence identity. In the present study, we investigated the putative role of MTMR1 in myogenesis by analysing its expression pattern in muscle cells during differentiation and in skeletal muscle throughout development. We have identified three novel coding exons in the MTMR1 intron 2 that are conserved between mouse and human, are alternatively spliced, and give rise to six mRNA isoforms. One of the transcripts is muscle-specific and is induced during myogenesis both in vitro and in vivo, and represents the major isoform in adult skeletal muscle. We show that the two main MTMR1 protein muscular isoforms, like myotubularin, efficiently dephosphorylate PI(3)P in vitro. We have also analysed whether MTMR1 alternative splicing is affected in skeletal muscle cells derived from patients with congenital myotonic dystrophy (cDM1), in which mRNA splicing disturbances of specific genes are thought to constitute an important pathogenic mechanism. We found a striking reduction in the level of the muscle-specific isoform and the appearance of an abnormal MTMR1 transcript in differentiated cDM1 muscle cells in culture and in skeletal muscle from cDM1 patients. Our results suggest that MTMR1 plays a role in muscle formation and represents a novel target for abnormal mRNA splicing in myotonic dystrophy. << Less

  Hum. Mol. Genet. 11:2297-2307(2002) [PubMed] [EuropePMC]

• Loss of phosphatase activity in myotubularin-related protein 2 is associated with Charcot-Marie-Tooth disease type 4B1.

  Berger P., Bonneick S., Willi S., Wymann M., Suter U.

  Mutations in the gene encoding myotubularin-related protein 2 (MTMR2) are responsible for autosomal recessive Charcot-Marie-Tooth disease type 4B1 (CMT4B1), a severe hereditary motor and sensory neuropathy characterized by focally folded myelin sheaths and demyelination. MTMR2 belongs to the myotu ... >> More

  Mutations in the gene encoding myotubularin-related protein 2 (MTMR2) are responsible for autosomal recessive Charcot-Marie-Tooth disease type 4B1 (CMT4B1), a severe hereditary motor and sensory neuropathy characterized by focally folded myelin sheaths and demyelination. MTMR2 belongs to the myotubularin family, which is characterized by the presence of a phosphatase domain. Myotubularin (MTM), the archetype member of this family, is mutated in X-linked myotubular myopathy. Although MTMR2 and MTM are closely related, they are likely to have different functions. Recent studies revealed that MTM dephosphorylates specifically phosphatidylinositol 3-phosphate. Here we analyze the biochemical properties of the mouse Mtmr2 protein, which shares 97% amino acid identity with human MTMR2. We show that phosphatidylinositol-3-phosphate is also a substrate for Mtmr2, but, unlike myotubularin, Mtmr2 dephosphorylates phosphatidylinositol 3,5-bisphosphate with high efficiency and peak activity at neutral pH. We demonstrate that the known disease-associated MTMR2 mutations lead to dramatically reduced phosphatase activity, suggesting that the MTMR2 phosphatase activity is crucial for the proper function of peripheral nerves in CMT4B1. Expression analysis of Mtmr2 suggests particularly high levels in neurons. Thus, the demyelinating neuropathy CMT4B1 might be triggered by the malfunction of neural membrane recycling, membrane trafficking, and/or endocytic or exocytotic processes, combined with altered axon-Schwann cell interactions. Furthermore, the different biochemical properties of MTM and MTMR2 offer a potential explanation for the different human diseases caused by mutations in their respective genes. << Less

  Hum. Mol. Genet. 11:1569-1579(2002) [PubMed] [EuropePMC]

  This publication is cited by 1 other entry.

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

  Taylor G.S., Maehama T., Dixon J.E.

  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 ... >> More

  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. << Less

  Proc. Natl. Acad. Sci. U.S.A. 97:8910-8915(2000) [PubMed] [EuropePMC]

  This publication is cited by 1 other entry.

• Characterization of MTMR3. an inositol lipid 3-phosphatase with novel substrate specificity.

  Walker D.M., Urbe S., Dove S.K., Tenza D., Raposo G., Clague M.J.

  Inositol lipids play key roles in many fundamental cellular processes that include growth, cell survival, motility, and membrane trafficking. Recent studies on the PTEN and Myotubularin proteins have underscored the importance of inositol lipid 3-phosphatases in cell function. Inactivating mutatio ... >> More

  Inositol lipids play key roles in many fundamental cellular processes that include growth, cell survival, motility, and membrane trafficking. Recent studies on the PTEN and Myotubularin proteins have underscored the importance of inositol lipid 3-phosphatases in cell function. Inactivating mutations in the genes encoding PTEN and Myotubularin are key steps in the progression of some cancers and in the onset of X-linked myotubular myopathy, respectively. Myotubularin-related protein 3 (MTMR3) shows extensive homology to Myotubularin, including the catalytic domain, but additionally possesses a C-terminal extension that includes a FYVE domain. We show that MTMR3 is an inositol lipid 3-phosphatase, with a so-far-unique substrate specificity. It is able to hydrolyze PtdIns3P and PtdIns3,5P2, both in vitro and when heterologously expressed in S. cerevisiae, and to thereby provide the first clearly defined route for the cellular production of PtdIns5P. Overexpression of a catalytically dead MTMR3 (C413S) in mammalian cells induces a striking formation of vacuolar compartments that enclose membranous structures that are highly concentrated in mutant proteins. << Less

  Curr. Biol. 11:1600-1605(2001) [PubMed] [EuropePMC]

  This publication is cited by 3 other entries.

• Characterization of myotubularin-related protein 7 and its binding partner, myotubularin-related protein 9.

  Mochizuki Y., Majerus P.W.

  Myotubularin-related protein 7 (MTMR7) is a member of the myotubularin (MTM) family. The cDNA encoding the mouse MTMR7 contains 1,983 bp, and the predicted protein has a deduced molecular mass of 75.6 kDa. Northern and Western blot analyses showed that MTMR7 is expressed mainly in brain and mouse ... >> More

  Myotubularin-related protein 7 (MTMR7) is a member of the myotubularin (MTM) family. The cDNA encoding the mouse MTMR7 contains 1,983 bp, and the predicted protein has a deduced molecular mass of 75.6 kDa. Northern and Western blot analyses showed that MTMR7 is expressed mainly in brain and mouse neuroblastoma N1E-115 cells. Recombinant MTMR7 dephosphorylated the D-3 position of phosphatidylinositol 3-phosphate and inositol 1,3-bisphosphate [Ins(1,3)P2]. The substrate specificity of MTMR7 is different than other MTM proteins in that this enzyme prefers the water-soluble substrate. Immunofluorescence showed that MTMR7 is localized in Golgi-like granules and cytosol, and subcellular fractionation showed both cytoplasmic and membrane localization of MTMR7 in N1E-115 cells. An MTMR7-binding protein was found in an anti-MTMR7 immunoprecipitate from N1E-115 cells and identified as MTM-related protein 9 (MTMR9) by tandem mass spectrometry. The coiled-coil domain of MTMR9 was sufficient for binding to MTMR7. The binding of MTMR9 increased the Ins(1,3)P2 phosphatase activity of MTMR7. Our results show that MTMR7 forms a complex with MTMR9 and dephosphorylates phosphatidylinositol 3-phosphate and Ins(1,3)P2 in neuronal cells. << Less

  Proc. Natl. Acad. Sci. U.S.A. 100:9768-9773(2003) [PubMed] [EuropePMC]

  This publication is cited by 1 other entry.

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

  Kim S.A., Taylor G.S., Torgersen K.M., Dixon J.E.

  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, phospha ... >> More

  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. << Less

  J. Biol. Chem. 277:4526-4531(2002) [PubMed] [EuropePMC]

  This publication is cited by 1 other entry.

• SAC1 encodes a regulated lipid phosphoinositide phosphatase, defects in which can be suppressed by the homologous Inp52p and Inp53p phosphatases.

  Hughes W.E., Woscholski R., Cooke F.T., Patrick R.S., Dove S.K., McDonald N.Q., Parker P.J.

  The yeast protein Sac1p is involved in a range of cellular functions, including inositol metabolism, actin cytoskeletal organization, endoplasmic reticulum ATP transport, phosphatidylinositol-phosphatidylcholine transfer protein function, and multiple-drug sensitivity. The activity of Sac1p and it ... >> More

  The yeast protein Sac1p is involved in a range of cellular functions, including inositol metabolism, actin cytoskeletal organization, endoplasmic reticulum ATP transport, phosphatidylinositol-phosphatidylcholine transfer protein function, and multiple-drug sensitivity. The activity of Sac1p and its relationship to these phenotypes are unresolved. We show here that the regulation of lipid phosphoinositides in sac1 mutants is defective, resulting in altered levels of all lipid phos-phoinositides, particularly phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate. We have identified two proteins with homology to Sac1p that can suppress drug sensitivity and also restore the levels of the phosphoinositides in sac1 mutants. Overexpression of truncated forms of these suppressor genes confirmed that suppression was due to phosphoinositide phosphatase activity within these proteins. We have now demonstrated this activity for Sac1p and have characterized its specificity. The in vitro phosphatase activity and specificity of Sac1p were not altered by some mutations. Indeed, in vivo mutant Sac1p phosphatase activity also appeared unchanged under conditions in which cells were drug-resistant. However, under different growth conditions, both drug sensitivity and the phosphatase defect were manifest. It is concluded that SAC1 encodes a novel lipid phosphoinositide phosphatase in which specific mutations can cause the sac1 phenotypes by altering the in vivo regulation of the protein rather than by destroying phosphatase activity. << Less

  J. Biol. Chem. 275:801-808(2000) [PubMed] [EuropePMC]

  This publication is cited by 1 other entry.

• The Arabidopsis chromatin modifier ATX1, the myotubularin-like AtMTM and the response to drought.

  Ding Y., Lapko H., Ndamukong I., Xia Y., Al-Abdallat A., Lalithambika S., Sadder M., Saleh A., Fromm M., Riethoven J.-J., Lu G., Avramova Z.

  Plants respond to environmental stresses by altering transcription of genes involved in the response. The chromatin modifier ATX1 regulates expression of a large number of genes; consequently, factors that affect ATX1 activity would also influence expression from ATX1-regulated genes. Here, we dem ... >> More

  Plants respond to environmental stresses by altering transcription of genes involved in the response. The chromatin modifier ATX1 regulates expression of a large number of genes; consequently, factors that affect ATX1 activity would also influence expression from ATX1-regulated genes. Here, we demonstrate that dehydration is such a factor implicating ATX1 in the plant's response to drought. In addition, we report that a hitherto unknown Arabidopsis gene, At3g10550, encodes a phosphoinositide 3'-phosphatase related to the animal myotubularins (AtMTM1). Myotubularin activities in plants have not been described and herein, we identify an overlapping set of genes co-regulated by ATX1 and AtMTM under drought conditions. We propose that these shared genes represent the ultimate targets of partially overlapping branches of the pathways of the nuclear ATX1 and the cytoplasmic AtMTM1. Our analyses offer first genome-wide insights into the relationship of an epigenetic factor and a lipid phosphatase from the other end of a shared drought responding pathway in Arabidopsis. << Less

  Plant Signal. Behav. 4:1049-1058(2009) [PubMed] [EuropePMC]

  This publication is cited by 1 other entry.