Arginine 343 and 350 are two active residues involved in substrate binding by human Type I D-myo-inositol 1,4,5,-trisphosphate 5-phosphatase

Biological importance of arginine: A comprehensive review of the roles in structure, disorder, and functionality of peptides and proteins

Arginine shows Jekyll and Hyde behavior in several respects. It participates in protein folding via ionic and H-bonds and cation-pi interactions; the charge and hydrophobicity of its side chain make it a disorder-promoting amino acid. Its methylation in histones; RNA binding proteins; chaperones regulates several cellular processes. The arginine-centric modifications are important in oncogenesis and as biomarkers in several cardiovascular diseases. The cross-links involving arginine in collagen and cornea are involved in pathogenesis of tissues but have also been useful in tissue engineering and wound-dressing materials. Arginine is a part of active site of several enzymes such as GTPases, peroxidases, and sulfotransferases. Its metabolic importance is obvious as it is involved in production of urea, NO, ornithine and citrulline. It can form unusual functional structures such as molecular tweezers in vitro and sprockets which engage DNA chains as part of histones in vivo. It has been used in design of cell-penetrating peptides as drugs. Arginine has been used as an excipient in both solid and injectable drug formulations; its role in suppressing opalescence due to liquid-liquid phase separation is particularly very promising. It has been known as a suppressor of protein aggregation during protein refolding. It has proved its usefulness in protein bioseparation processes like ion-exchange, hydrophobic and affinity chromatographies. Arginine is an amino acid, whose importance in biological sciences and biotechnology continues to grow in diverse ways.

Deficiency of TRIM32 Impairs Motor Function and Purkinje Cells in Mid-Aged Mice

Proper functioning of the cerebellum is crucial to motor balance and coordination in adult mammals. Purkinje cells (PCs), the sole output neurons of the cerebellar cortex, play essential roles in cerebellar motor function. Tripartite motif-containing protein 32 (TRIM32) is an E3 ubiquitin ligase that is involved in balance activities of neurogenesis in the subventricular zone of the mammalian brain and in the development of many nervous system diseases, such as Alzheimer's disease, autism spectrum disorder, attention deficit hyperactivity disorder. However, the role of TRIM32 in cerebellar motor function has never been examined. In this study we found that motor balance and coordination of mid-aged TRIM32 deficient mice were poorer than those of wild-type littermates. Immunohistochemical staining was performed to assess cerebella morphology and TRIM32 expression in PCs. Golgi staining showed that the extent of dendritic arborization and dendritic spine density of PCs were decreased in the absence of TRIM32. The loss of TRIM32 was also associated with a decrease in the number of synapses between parallel fibers and PCs, and in synapses between climbing fibers and PCs. In addition, deficiency of TRIM32 decreased Type I inositol 1,4,5-trisphosphate 5-phosphatase (INPP5A) levels in cerebellum. Overall, this study is the first to elucidate a role of TRIM32 in cerebellar motor function and a possible mechanism, thereby highlighting the importance of TRIM32 in the cerebellum.

Inositol triphosphate-triggered calcium release blocks lipid exchange at endoplasmic reticulum-Golgi contact sites

Vesicular traffic and membrane contact sites between organelles enable the exchange of proteins, lipids, and metabolites. Recruitment of tethers to contact sites between the endoplasmic reticulum (ER) and the plasma membrane is often triggered by calcium. Here we reveal a function for calcium in the repression of cholesterol export at membrane contact sites between the ER and the Golgi complex. We show that calcium efflux from ER stores induced by inositol-triphosphate [IP₃] accumulation upon loss of the inositol 5-phosphatase INPP5A or receptor signaling triggers depletion of cholesterol and associated Gb3 from the cell surface, resulting in a blockade of clathrin-independent endocytosis (CIE) of Shiga toxin. This phenotype is caused by the calcium-induced dissociation of oxysterol binding protein (OSBP) from the Golgi complex and from VAP-containing membrane contact sites. Our findings reveal a crucial function for INPP5A-mediated IP₃ hydrolysis in the control of lipid exchange at membrane contact sites.

Cerebellum-enriched protein INPP5A contributes to selective neuropathology in mouse model of spinocerebellar ataxias type 17

Spinocerebellar ataxias 17 (SCA17) is caused by polyglutamine (polyQ) expansion in the TATA box-binding protein (TBP). The selective neurodegeneration in the cerebellum in SCA17 raises the question of why ubiquitously expressed polyQ proteins can cause neurodegeneration in distinct brain regions in different polyQ diseases. By expressing mutant TBP in different brain regions in adult wild-type mice via stereotaxic injection of adeno-associated virus, we found that adult cerebellar neurons are particularly vulnerable to mutant TBP. In SCA17 knock-in mice, mutant TBP inhibits SP1-mediated gene transcription to down-regulate INPP5A, a protein that is highly abundant in the cerebellum. CRISPR/Cas9-mediated deletion of Inpp5a in the cerebellum of wild-type mice leads to Purkinje cell degeneration, and Inpp5a overexpression decreases inositol 1,4,5-trisphosphate (IP₃) levels and ameliorates Purkinje cell degeneration in SCA17 knock-in mice. Our findings demonstrate the important contribution of a tissue-specific protein to the polyQ protein-mediated selective neuropathology.

Structural basis for phosphoinositide substrate recognition, catalysis, and membrane interactions in human inositol polyphosphate 5-phosphatases

SHIP2, OCRL, and INPP5B belong to inositol polyphosphate 5-phophatase subfamilies involved in insulin regulation and Lowes syndrome. The structural basis for membrane recognition, substrate specificity, and regulation of inositol polyphosphate 5-phophatases is still poorly understood. We determined the crystal structures of human SHIP2, OCRL, and INPP5B, the latter in complex with phosphoinositide substrate analogs, which revealed a membrane interaction patch likely to assist in sequestering substrates from the lipid bilayer. Residues recognizing the 1-phosphate of the substrates are highly conserved among human family members, suggesting similar substrate binding modes. However, 3- and 4-phosphate recognition varies and determines individual substrate specificity profiles. The high conservation of the environment of the scissile 5-phosphate suggests a common reaction geometry for all members of the human 5-phosphatase family.

Regulation of neurite growth by spontaneous Ca2+ oscillations in astrocytes

Astrocytes play a pivotal role in the regulation of neurite growth, but the intracellular signaling mechanism in astrocytes that mediates this regulation remains unclarified. We studied the relationship between spontaneous Ca(2+) oscillations in astrocytes and the astrocyte-mediated neurite growth. We generated Ca(2+) signal-deficient astrocytes in which spontaneous Ca(2+) oscillations were abolished by a chronic inhibition of IP(3) signaling. When hippocampal neurons were cultured on a monolayer of Ca(2+) signal-deficient astrocytes, the growth of dendrites and axons was inhibited. Time-lapse imaging of the advancement of axonal growth cones indicated the involvement of membrane-bound molecules for this inhibition. Among six candidate membrane-bound molecules that may modulate neuronal growth, N-cadherin was downregulated in Ca(2+) signal-deficient astrocytes. Although a blocking antibody to N-cadherin suppressed the axonal growth on control astrocytes, extrinsic N-cadherin expression rescued the suppressed axonal growth on Ca(2+) signal-deficient astrocytes. These findings suggest that spontaneous Ca(2+) oscillations regulate the astrocytic function to promote neurite growth by maintaining the expression of specific growth-enhancing proteins on their surface, and that N-cadherin is one of such molecules.

Postsynaptic inositol 1,4,5-trisphosphate signaling maintains presynaptic function of parallel fiber-Purkinje cell synapses via BDNF

The maintenance of synaptic functions is essential for neuronal information processing, but cellular mechanisms that maintain synapses in the adult brain are not well understood. Here, we report an activity-dependent maintenance mechanism of parallel fiber (PF)-Purkinje cell (PC) synapses in the cerebellum. When postsynaptic metabotropic glutamate receptor (mGluR) or inositol 1,4,5-trisphosphate (IP(3)) signaling was chronically inhibited in vivo, PF-PC synaptic strength decreased because of a decreased transmitter release probability. The same effects were observed when PF activity was inhibited in vivo by the suppression of NMDA receptor-mediated inputs to granule cells. PF-PC synaptic strength similarly decreased after the in vivo application of an antibody against brain-derived neurotrophic factor (BDNF). Furthermore, the weakening of synaptic connection caused by the blockade of mGluR-IP(3) signaling was reversed by the in vivo application of BDNF. These results indicate that a signaling cascade comprising PF activity, postsynaptic mGluR-IP(3) signaling and subsequent BDNF signaling maintains presynaptic functions in the mature cerebellum.

Cross talk between metabotropic and ionotropic glutamate receptor-mediated signaling in parallel fiber-induced inositol 1,4,5-trisphosphate production in cerebellar Purkinje cells

In many excitatory glutamatergic synapses, both ionotropic glutamate receptors (iGluRs) and metabotropic glutamate receptors (mGluRs) are closely distributed on the postsynaptic membrane. However, the functional significance of the close distribution of the two types of glutamate receptors has not been fully clarified. In this study, we examined the functional interaction between iGluR and mGluR at parallel fiber (PF)--> Purkinje cell synapses in the generation of inositol 1,4,5-trisphosphate (IP3), a key second messenger that regulates many important cellular functions. We visualized local IP3 dynamics in Purkinje cells using the green fluorescent protein-tagged pleckstrin homology domain (GFP-PHD) as a fluorescent IP3 probe. Purkinje cells were transduced with Sindbis virus encoding GFP-PHD and imaged with a two-photon laser scanning microscope. Translocation of GFP-PHD from the plasma membrane to the cytoplasm attributable to an increase in IP3 concentration was observed on PF stimulation in fine dendrites of Purkinje cells. Surprisingly, this PF-induced IP3 production was blocked not only by the group I mGluR antagonist but also by the AMPA receptor (AMPAR) antagonist. The PF-induced IP3 production was blocked by either the inhibition of G-protein activation by GDP-betaS or intracellular Ca2+ buffering by BAPTA. These results show that IP3 production is mediated cooperatively by group I mGluR and AMPAR through G-protein activation and Ca2+ influx at PF--> Purkinje cell synapses, identifying the robust cross talk between iGluR and mGluR for the generation of IP3 signals.

Role of PRIP-1, a novel Ins(1,4,5)P3 binding protein, in Ins(1,4,5)P3-mediated Ca2+ signaling

PRIP-1 was isolated as a novel inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] binding protein with a domain organization similar to phospholipase C-delta1 (PLC-delta1) but lacking the enzymatic activity. Further studies revealed that the pleckstrin homology (PH) domain of PRIP-1 is the region responsible for binding Ins(1,4,5)P3. In this study we aimed to clarify the role of PRIP-1 at the physiological concentration in Ins(1,4,5)P3-mediated Ca2+ signaling, as we had previously used COS-1 cells overexpressing PRIP-1 (Takeuchi et al., 2000, Biochem J 349:357-368). For this purpose we employed PRIP-1 knock out (PRIP-1-/-) mice generated previously (Kanematsu et al., 2002, EMBO J 21:1004-1011). The increase in free Ca2+ concentration in response to purinergic receptor stimulation was lower in primary cultured cortical neurons prepared from PRIP-1-/- mice than in those from wild type mice. The relative amounts of [3H]Ins(1,4,5)P3 measured in neurons labeled with [3H]inositol was also lower in cells from PRIP-1-/- mice. In contrast, PLC activities in brain cortex samples from PRIP-1-/- mice were not different from those in the wild type mice, indicating that the hydrolysis of Ins(1,4,5)P3 is enhanced in cells from PRIP-1-/- mice. In vitro analyses revealed that type1 inositol polyphosphate 5-phosphatase physically interacted with a PH domain of PRIP-1 (PRIP-1PH) and its enzyme activity was inhibited by PRIP-1PH. However, physical interaction with these two proteins did not appear to be the reason for the inhibition of enzyme activity, indicating that binding of Ins(1,4,5)P3 to the PH domain prevented its hydrolyzation. Together, these results indicate that PRIP-1 plays an important role in regulating the Ins(1,4,5)P3-mediated Ca2+ signaling by modulating type1 inositol polyphosphate 5-phosphatase activity through binding to Ins(1,4,5)P3.

A diverse family of inositol 5-phosphatases playing a role in growth and development in Dictyostelium discoideum

Inositol phosphate-containing molecules play an important role in a broad range of cellular processes. Inositol 5-phosphatases participate in the regulation of these signaling molecules. We have identified four inositol 5-phosphatases in Dictyostelium discoideum, Dd5P1-4, showing a high diversity in domain composition. Dd5P1 possesses only a inositol 5-phosphatase catalytic domain. An unique domain composition is present in Dd5P2 containing a RCC1-like domain. RCC1 has a seven-bladed propeller structure and interacts with G-proteins. Dd5P3 and Dd5P4 have a domain composition similar to human Synaptojanin with a SacI domain and OCRL with a RhoGAP domain, respectively. We have expressed the catalytic domains and show that these inositol 5-phosphatases have different substrate preferences. Single and double gene inactivation suggest a functional redundancy for Dd5P1, Dd5P2, and Dd5P3. Inactivation of the gene coding for Dd5P4 leads to defects in growth and development. These defects are restored by the expression of the complete protein but not by the 5-phosphatase catalytic domain.

Caenorhabditis elegans inositol 5-phosphatase homolog negatively regulates inositol 1,4,5-triphosphate signaling in ovulation

Ovulation in Caenorhabditis elegans requires inositol 1,4,5-triphosphate (IP(3)) signaling activated by the epidermal growth factor (EGF)-receptor homolog LET-23. We generated a deletion mutant of a type I 5-phosphatase, ipp-5, and found a novel ovulation phenotype whereby the spermatheca hyperextends to engulf two oocytes per ovulation cycle. The temporal and spatial expression of IPP-5 is consistent with its proposed inhibition of IP(3) signaling in the adult spermatheca. ipp-5 acts downstream of let-23, and interacts with let-23-mediated IP(3) signaling pathway genes. We infer that IPP-5 negatively regulates IP(3) signaling to ensure proper spermathecal contraction.

A novel receptor-mediated regulation mechanism of type I inositol polyphosphate 5-phosphatase by calcium/calmodulin-dependent protein kinase II phosphorylation

D-myo-inositol 1,4,5-trisphosphate (Ins(1,4,5)P(3)) and D-myo-inositol 1,3,4,5-tetrakisphosphate (Ins(1,3,4,5)P(4)) are both substrates of the 43-kDa type I inositol polyphosphate 5-phosphatase. Transient and okadaic acid-sensitive inhibition by 70-85% of Ins(1,4,5)P(3) and Ins(1,3,4,5)P(4) 5-phosphatase activities was observed in homogenates from rat cortical astrocytes, human astrocytoma 1321N1 cells, and rat basophilic leukemia RBL-2H3 cells after incubation with carbachol. The effect was reproduced in response to UTP in rat astrocytic cells and Chinese hamster ovary cells overexpressing human type I 5-phosphatase. Immunodetection as well as mass spectrometric peptide mass fingerprinting and post-source decay (PSD) sequence data analysis after immunoprecipitation permitted unambiguous identification of the major native 5-phosphatase isoform hydrolyzing Ins(1,4,5)P(3) and Ins(1,3,4,5)P(4) as type I inositol polyphosphate 5-phosphatase. In ortho-(32)P-preincubated cells, the phosphorylated 43 kDa-enzyme could be identified after receptor activation by immunoprecipitation followed by electrophoretic separation. Phosphorylation of type I 5-phosphatase was blocked after cell preincubation in the presence of Ca(2+)/calmodulin kinase II inhibitors (i.e. KN-93 and KN-62). In vitro phosphorylation of recombinant type I enzyme by Ca(2+)/calmodulin kinase II resulted in an inhibition (i.e. 60-80%) of 5-phosphatase activity. In this study, we demonstrated for the first time a novel regulation mechanism of type I 5-phosphatase by phosphorylation in intact cells.

Visualization of IP(3) dynamics reveals a novel AMPA receptor-triggered IP(3) production pathway mediated by voltage-dependent Ca(2+) influx in Purkinje cells

IP(3) signaling in Purkinje cells is involved in the regulation of cell functions including LTD. We have used a GFP-tagged pleckstrin homology domain to visualize IP(3) dynamics in Purkinje cells. Surprisingly, IP(3) production was observed in response not only to mGluR activation, but also to AMPA receptor activation in Purkinje cells in culture. AMPA-induced IP(3) production was mediated by depolarization-induced Ca(2+) influx because it was mimicked by depolarization and was blocked by inhibition of the P-type Ca(2+) channel. Furthermore, trains of complex spikes, elicited by climbing fiber stimulation (1 Hz), induced IP(3) production in Purkinje cells in cerebellar slices. These results revealed a novel IP(3) signaling pathway in Purkinje cells that can be elicited by synaptic inputs from climbing fibers.

Molecular characterization of At5PTase1, an inositol phosphatase capable of terminating inositol trisphosphate signaling

The inositol triphosphate (IP(3))-signaling pathway has been associated with several developmental and physiological processes in plants, but we currently know little about the regulation of this pathway. Inositol 5' phosphatases (5PTases) are enzymes that remove a 5' phosphate from several potential second messengers, including IP(3). In catalyzing the removal of a 5' phosphate from second messenger substrates, 5PTases can act to terminate signal transduction events. We describe the molecular analysis of At5PTase1, a 5PTase gene from Arabidopsis. When expressed transiently in Arabidopsis leaf tissue or ectopically in transgenic plants, At5PTase1 allowed for the increased hydrolysis of I(1,4,5)P(3) and I(1,3,4,5)P(4) substrates. At5PTase1 did not hydrolyze I(1)P, I(1,4)P(2), or PI(4,5)P(2) substrates. This substrate specificity was similar to that of the human Type I 5PTase. We identified 14 other potential At5PTase genes and constructed an unrooted phylogenetic tree containing putative Arabidopsis, human, and yeast 5PTase proteins. This analysis indicated that the Arabidopsis 5PTases were grouped in two separate branches of the tree. The multiplicity of At5PTases indicates that these enzymes may have different substrate specificities and play different roles in signal termination in Arabidopsis.

Investigation of the roles of Ca2+ and InsP3 diffusion in the coordination of Ca2+ signals between connected hepatocytes

Glycogenolytic agonists induce coordinated Ca2+ oscillations in multicellular rat hepatocyte systems as well as in the intact liver. The coordination of intercellular Ca2+ signals requires functional gap-junction coupling. The mechanisms ensuring this coordination are not precisely known. We investigated possible roles of Ca2+ or inositol 1,4,5-trisphosphate (InsP3) as a coordinating messengers for Ca2+ spiking among connected hepatocytes. Application of ionomycin or of supra-maximal concentrations of agonists show that Ca2+ does not significantly diffuse between connected hepatocytes, although gap junctions ensure the passage of small signaling molecules, as demonstrated by FRAP experiments. By contrast, coordination of Ca2+ spiking among connected hepatocytes can be favored by a rise in the level of InsP3, via the increase of agonist concentrations, or by a shift in the affinity of InsP3 receptor for InsP3. In the same line, coordination cannot be achieved if the InsP3 is rapidly metabolized by InsP3-phosphatase in one cell of the multiplet. These results demonstrate that even if small amounts of Ca2+ diffuse across gap junctions, they most probably do not play a significant role in inducing a coordinated Ca2+ signal among connected hepatocytes. By contrast, coordination of Ca2+ oscillations is fully dependent on the diffusion of InsP3 between neighboring cells.

The inositol polyphosphate 5-phosphatases and the apurinic/apyrimidinic base excision repair endonucleases share a common mechanism for catalysis

Inositol polyphosphate 5-phosphatases (5-phosphatase) hydrolyze the 5-position phosphate from the inositol ring of phosphatidylinositol-derived signaling molecules; however, the mechanism of catalysis is only partially characterized. These enzymes play critical roles in regulating cell growth, apoptosis, intracellular calcium oscillations, and post-synaptic vesicular trafficking. The UCLA fold recognition server (threader) predicted that the conserved 300-amino acid catalytic domain, common to all 5-phosphatases, adopts the fold of the apurinic/apyrimidinic (AP) base excision repair endonucleases. PSI-BLAST searches of GENPEPT, using the amino acid sequence of AP endonuclease exonuclease III, identified all members of the 5-phosphatase family with highly significant scores. A sequence alignment between exonuclease III and all known 5-phosphatases revealed six highly conserved motifs containing residues that corresponded to the catalytic residues in the AP endonucleases. Mutation of each of these residues to alanine in the mammalian 43-kDa, or yeast Inp52p 5-phosphatase, resulted in complete loss of enzyme activity. We predict the 5-phosphatase enzymes share a similar mechanism of catalysis to the AP endonucleases, consistent with other common functional similarities such as an absolute requirement for magnesium for activity. Based on this analysis, functional roles have been assigned to conserved residues in all 5-phosphatase enzymes.

Mutations in synaptojanin disrupt synaptic vesicle recycling

Synaptojanin is a polyphosphoinositide phosphatase that is found at synapses and binds to proteins implicated in endocytosis. For these reasons, it has been proposed that synaptojanin is involved in the recycling of synaptic vesicles. Here, we demonstrate that the unc-26 gene encodes the Caenorhabditis elegans ortholog of synaptojanin. unc-26 mutants exhibit defects in vesicle trafficking in several tissues, but most defects are found at synaptic termini. Specifically, we observed defects in the budding of synaptic vesicles from the plasma membrane, in the uncoating of vesicles after fission, in the recovery of vesicles from endosomes, and in the tethering of vesicles to the cytoskeleton. Thus, these results confirm studies of the mouse synaptojanin 1 mutants, which exhibit defects in the uncoating of synaptic vesicles (Cremona, O., G. Di Paolo, M.R. Wenk, A. Luthi, W.T. Kim, K. Takei, L. Daniell, Y. Nemoto, S.B. Shears, R.A. Flavell, D.A. McCormick, and P. De Camilli. 1999. Cell. 99:179-188), and further demonstrate that synaptojanin facilitates multiple steps of synaptic vesicle recycling.

Novel inositol polyphosphate 5-phosphatase localizes at membrane ruffles

We have cloned a novel inositol polyphosphate 5-phosphatase from the rat brain cDNA library. It contains two highly conserved 5-phosphatase motifs, both of which are essential for its enzymatic activity. Interestingly, the proline content of this protein is high and concentrated in its N- and C-terminal regions. One putative SH3-binding motif and six 14-3-3 zeta-binding motifs were found in the amino acid sequence. This enzyme hydrolyzed phosphate at the D-5 position of inositol 1,4,5-trisphosphate, inositol 1,3,4, 5-tetrakisphosphate, and phosphatidylinositol 4,5-bisphosphate, consistent with the substrate specificity of type II 5-phosphatase, OCRL, synaptojanin and synaptojanin 2, already characterized 5-phosphatases. When the Myc-epitope-tagged enzyme was expressed in COS-7 cells and stained with anti-Myc polyclonal antibody, a signal was observed at ruffling membranes and in the cytoplasm. We prepared several deletion mutants and demonstrated that the 123 N-terminal amino acids (311-433) and a C-terminal proline-rich region containing 277 amino acids (725-1001) were essential for its localization to ruffling membranes. This enzyme might regulate the level of inositol and phosphatidylinositol polyphosphates at membrane ruffles.

Underexpression of the 43 kDa inositol polyphosphate 5-phosphatase is associated with spontaneous calcium oscillations and enhanced calcium responses following endothelin-1 stimulation

The 43 kDa inositol polyphosphate 5-phosphatase (5-phosphatase) hydrolyses the signalling molecules inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) and inositol 1,3,4,5-tetrakisphosphate (Ins(1,3,4, 5)P4) and thereby regulates cellular transformation. To investigate the role Ins(1,4,5)P3-mediated Ca2+ oscillations play in cellular transformation, we studied Ins(1,4,5)P3-mediated Ca2+ responses in cells underexpressing the 43 kDa 5-phosphatase. Chronic reduction in 43 kDa 5-phosphatase enzyme activity resulted in a 2.6-fold increase in the resting Ins(1,4,5)P3 concentration and a 4.1-fold increase in basal intracellular Ca2+. The increased Ins(1,4,5)P3 levels resulted in partial emptying (40%) of the Ins(1,4,5)P3-sensitive Ca2+ store, however, store-operated Ca2+ influx remained unchanged. In addition, Ins(1,4,5)P3 receptors were chronically down-regulated in unstimulated cells, as shown by a 53% reduction in [3H]Ins(1,4,5)P3 binding to microsomal receptor sites. Agonist stimulation with endothelin-1 resulted in the rapid rise and fall of Ins(1,4,5)P3 and Ins(1,3,4,5)P4 levels, with no significant differences in the rates of hydrolysis of these second messengers in antisense- or vector-transfected cells. These studies indicate, in contrast to its predicted action, the 43 kDa 5-phosphatase does not metabolise Ins(1, 4,5)P3 and Ins(1,3,4,5)P4 post agonist stimulation. Cells with decreased 43 kDa 5-phosphatase activity exhibited spontaneous Ca2+ oscillations in the absence of any agonist stimulation, and increased sensitivity and amplitude of intracellular Ca2+ responses to both high and low dose endothelin-1 stimulation. We conclude the 43 kDa 5-phosphatase exerts a profound influence on Ins(1,4, 5)P3-induced Ca2+ spiking, both in the unstimulated cell and following agonist stimulation. We propose the enhanced Ca2+ oscillations may mediate cellular transformation in cells underexpressing the 43 kDa 5-phosphatase.

Use of phosphorofluoridate analogues of D-myo-inositol 1,4,5-trisphosphate to assess the involvement of ionic interactions in its recognition by the receptor and metabolising enzymes

D-myo-inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] analogues fluoridated at 4- or 5-phosphate or both were analysed to assess the involvement of ionic interactions between the phosphates of Ins(1,4,5)P3 and the proteins that recognize it, such as metabolic enzymes and the InsP3 receptor. These analogues were effective in inhibiting type I Ins(1,4,5)P3 5-phosphatase activity with much the same potency as Ins(1,4,5)P3, although the enzyme showed a lower Km value as pH values increased. In contrast, the analogues were less potent ligands than Ins(1,4,5)P3 in both the assay of [3H]Ins(1,4,5)P3 binding to the receptors and the phosphorylation of [3H]Ins(1,4,5)P3 catalysed by Ins(1,4,5)P3 3-kinase. These results suggest that ionic interactions with the dianionic 4- and 5-phosphates of Ins(1,4,5)P3 are involved in recognition by the receptor and the kinase, but not by the phosphatase.

The diversity and possible functions of the inositol polyphosphate 5-phosphatases

Distinct forms of inositol and phosphatidylinositol polyphosphate 5-phosphatases selectively remove the phosphate from the 5-position of the inositol ring from both soluble and lipid substrates, i.e., inositol 1,4,5-trisphosphate (Ins(1,4,5)P3), inositol 1,3,4, 5-tetrakisphosphate (Ins(1,3,4,5)P4), phosphatidylinositol 4, 5-bisphosphate (PtdIns(4,5)P2) or phosphatidylinositol 3,4, 5-trisphosphate (PtdIns(3,4,5)P3). In mammalian cells, this family contains a series of distinct genes and splice variants. All inositol polyphosphate 5-phosphatases share a 5-phosphatase domain and various protein modules probably responsible for specific cell localisation or recruitment (SH2 domain, proline-rich sequences, prenylation sites, etc.). Type I Ins(1,4,5)P3 5-phosphatase also uses Ins(1,3,4,5)P4 but not the phosphoinositides as substrates. This enzyme is targeted to specific membranes by means of a prenylation site. Type II 5-phosphatases can use both PtdIns(4,5)P2 and PtdIns(3,4,5)P3 as substrates. Five mammalian enzymes and multiple splice variants are known: INPP5P or inositol polyphosphate 5-phosphatase II, OCRL (a Golgi protein implicated in the Lowe oculocerebrorenal syndrome), synaptojanin (a protein involved in the recycling of synaptic vesicles), SHIP 1 and SHIP 2 (or SH2-containing inositol 5-phosphatases). As discussed in this review, the substrate specificity, regulatory mechanisms, subcellular localisation and tissue specificity indicate that the different 5-phosphatase isoforms may play specific roles. As known in the dephosphorylation of tyrosine containing substrates by the tyrosine protein phosphatases or in the metabolism of cyclic nucleotides by the cyclic nucleotide phosphodiesterases, inositol polyphosphate 5-phosphatases directly participate in the control of second messengers in response to both activation or inhibitory cell signalling.

INP51, a yeast inositol polyphosphate 5-phosphatase required for phosphatidylinositol 4,5-bisphosphate homeostasis and whose absence confers a cold-resistant phenotype

Sequence analysis of Saccharomyces cerevisiae chromosome IX identified a 946 amino acid open reading frame (YIL002C), designated here as INP51, that has carboxyl- and amino-terminal regions similar to mammalian inositol polyphosphate 5-phosphatases and to yeast SAC1. This two-domain primary structure resembles the mammalian 5-phosphatase, synaptojanin. We report that Inp51p is associated with a particulate fraction and that recombinant Inp51p exhibits intrinsic phosphatidylinositol 4,5-bisphosphate 5-phosphatase activity. Deletion of INP51 (inp51) results in a "cold-tolerant" phenotype, enabling significantly faster growth at temperatures below 15 degreesC as compared with a parental strain. Complementation analysis of an inp51 mutant strain demonstrates that the cold tolerance is strictly due to loss of 5-phosphatase catalytic activity. Furthermore, deletion of PLC1 in an inp51 mutant does not abrogate cold tolerance, indicating that Plc1p-mediated production of soluble inositol phosphates is not required. Cells lacking INP51 have a 2-4-fold increase in levels of phosphatidylinositol 4,5-bisphosphate and inositol 1,4, 5-trisphosphate, whereas cells overexpressing Inp51p exhibit a 35% decrease in levels of phosphatidylinositol 4,5-bisphosphate. We conclude that INP51 function is critical for proper phosphatidylinositol 4,5-bisphosphate homeostasis. In addition, we define a novel role for a 5-phosphatase loss of function mutant that improves the growth of cells at colder temperatures without alteration of growth at normal temperatures, which may have useful commercial applications.

Arginyl residues are involved in acyl-CoA binding to the elongase from etiolated leek seedlings

The C18:0-CoA elongase from etiolated leek seedling microsomes was inactivated by treatment with phenylglyoxal, a reagent which specifically modifies arginyl residues. In the presence of 20 mM phenylglyoxal, 95% of the C18:0-CoA elongation was inhibited. The condensation and dehydration reactions of the overall elongation were totally inhibited, whereas enoyl-CoA reductase activity was diminished by 75%, but the nature of the final elongation product was unchanged. Phenylglyoxal did not modify the C18:0-CoA partition between membrane and aqueous compartments; moreover, [1-14C]phenylglyoxal labeling experiments showed a covalent binding of the inhibitor to membrane proteins. The ability of several substrates to prevent the inactivation by phenylglyoxal was investigated. NADH and NADPH had no effect. CoA led to a 75% protection, and the incorporation of [14C]phenylglyoxal was strongly affected by 10 mM CoA. The acyl chain length of the acyl-CoAs played also a crucial role in preventing the binding of phenylglyoxal. The maximal prevention of phenylglyoxal inhibition was obtained with C18:0-CoA. This suggests that arginyl residues could be present in the vicinity of the acyl-CoA binding site of the subunits of C18:0-CoA elongase.

Distinct membrane and cytosolic forms of inositol polyphosphate 5-phosphatase II. Efficient membrane localization requires two discrete domains

The 75-kDa inositol polyphosphate 5-phosphatase (5-phosphatase II) hydrolyzes various signaling molecules including the following: inositol 1,4,5-trisphosphate, inositol 1,3,4,5-tetrakisphosphate, phosphatidylinositol 4,5-bisphosphate, and phosphatidylinositol 3,4, 5-trisphosphate. Although studied extensively, a demonstrably full-length cDNA encoding 5-phosphatase II has yet to be isolated. In this study we used a human partial 2.3-kilobase pair (kb) cDNA to screen mouse brain and kidney cDNA libraries, resulting in the isolation of a 3.7-kb cDNA (M5), which by multiple criteria represents a full-length cDNA encoding a 115-kDa 5-phosphatase II. We also isolated a smaller cDNA (M22) with a unique N terminus that encodes a 104-kDa polypeptide. Analysis of these cDNAs suggests a further 87-kDa isoform may arise from differential splicing resulting in translation at methionine 234 in M5. RNA analysis of tissues demonstrates expression of two mRNA species of approximately 4.0 or 3.0 kb, respectively. Probes unique to the 5' end of M5 or M22 hybridized to the 4.0- or 3.0-kb transcripts, respectively. RNA analysis using probes derived from sequence 3' to the potential splice site in M5 and M22 hybridized to both transcripts. Expression of the recombinant 115-kDa protein, or a smaller recombinant protein lacking the N terminus transiently in COS-7 cells, showed localization of enzyme activity to the membrane. Removal of the C-terminal CAAX motif resulted in a significant translocation of the protein lacking the N terminus but not the 115-kDa 5-phosphatase to the cytosol. Western blot analysis of membrane and cytosolic fractions of multiple mouse tissues confirmed the 115-kDa 5-phosphatase II was located in the membrane, whereas the 104- and 87-kDa isoforms were prominent in the cytosol. Collectively these studies demonstrate the widespread expression of at least three isoforms of 5-phosphatase II derived from RNA splicing events. This allows differential distribution of the 5-phosphatase II activity between the membrane and cytosol of the cell and thereby may regulate enzyme access to phosphoinositide-derived signaling molecules.

Identification and characterization of an essential family of inositol polyphosphate 5-phosphatases (INP51, INP52 and INP53 gene products) in the yeast Saccharomyces cerevisiae

We recently demonstrated that the S. cerevisiae INP51 locus (YIL002c) encodes an inositol polyphosphate 5-phosphatase. Here we describe two related yeast loci, INP52 (YNL106c) and INP53 (YOR109w). Like Inp51p, the primary structures of Inp52p and Inp53p resemble the mammalian synaptic vesicle-associated protein, synaptojanin, and contain a carboxy-terminal catalytic domain and an amino-terminal SAC1-like segment. Inp51p (108 kD), Inp52p (136 kD) and Inp53p (124 kD) are membrane-associated. Single null mutants (inp51, inp52, or inp53) are viable. Both inp51 inp52 and inp52 inp53 double mutants display compromised cell growth, whereas an inp51 inp53 double mutant does not. An inp51 inp52 inp53 triple mutant is inviable on standard medium, but can grow weakly on media supplemented with an osmotic stabilizer (1 M sorbitol). An inp51 mutation, and to a lesser degree an inp52 mutation, confers cold-resistant growth in a strain background that cannot grow at temperatures below 15 degrees. Analysis of inositol metabolites in vivo showed measurable accumulation of phosphatidylinositol 4,5-bisphosphate in the inp51 mutant. Electron microscopy revealed plasma membrane invaginations and cell wall thickening in double mutants and the triple mutant grown in sorbitol-containing medium. A fluorescent dye that detects endocytic and vacuolar membranes suggests that the vacuole is highly fragmented in inp51 inp52 double mutants. Our observations indicate that Inp51p, Inp52p, and Inp53p have distinct functions and that substrates and/or products of inositol polyphosphate 5-phosphatases may have roles in vesicle trafficking, membrane structure, and/or cell wall formation.

Activation of the 43 kDa inositol polyphosphate 5-phosphatase by 14-3-3zeta

The 43 kDa inositol polyphosphate 5-phosphatase (5-phosphatase) hydrolyzes and thereby inactivates the second messenger molecules inositol 1,4,5-trisphosphate -Ins(1,4,5)P3- and inositol 1,3,4,5-tetrakisphosphate in a signal terminating reaction. Recent studies have shown that the platelet protein pleckstrin forms a complex with the 43 kDa 5-phosphatase and activates Ins(1,4,5)P3 hydrolysis 2-fold [Auethavekiat, V., Abrams, C. S., & Majerus, P. W. (1997) J. Biol. Chem. 272, 1786-1790]. We now show that another platelet protein, 14-3-3zeta, forms a complex with the 43 kDa 5-phosphatase and thereby activates the hydrolysis of Ins(1,4,5)P3. Both pleckstrin and 14-3-3zeta contain one or more pleckstrin-homology domains, both are present in platelet cytosol, and both dimerize and form complexes with other signalling proteins. Purified platelet pleckstrin and 14-3-3zeta enhanced the rate of the hydrolysis of Ins(1,4,5)P3 by the 43 kDa 5-phosphatase 1.9- and 3.8-fold, respectively, but did not activate the 75 kDa 5-phosphatase. We have demonstrated that the mechanism of 5-phosphatase activation by 14-3-3zeta results from specific complex formation between the 43 kDa 5-phosphatase and 14-3-3zeta. Recombinant 43 kDa 5-phosphatase bound to recombinant glutathione S-transferase (GST)/14-3-3zeta fusion protein, but not GST alone, immobilized on glutathione-Sepharose. A potential 14-3-3 binding motif was located in the 43 kDa, but not the 75 kDa, 5-phosphatase. The motif "363RSESEE" is present in close proximity to the proposed catalytic domain of the 43 kDa 5-phosphatase. A synthetic peptide corresponding to the putative 14-3-3 binding motif demonstrated specific, saturable binding to purified 125I-14-3-3, with a Kd of 92 nM. In addition, platelet cytosolic 5-phosphatase bound to recombinant 14-3-3zeta immobilized on glutathione-Sepharose. Thus, 14-3-3zeta serves in human platelets to activate the 43 kDa 5-phosphatase and may thereby function to prevent generation of Ins(1,4,5)P3 -mediated calcium release in unstimulated platelets.

Identification of a second SH2-domain-containing protein closely related to the phosphatidylinositol polyphosphate 5-phosphatase SHIP

Distinct inositol and phosphatidylinositol polyphosphates 5-phosphatases have recently been cloned. Primers have been designed coding for highly conserved amino acid regions that are shared between sequences of 5-phosphatases. One of the PCR fragment referred to as 51 C, shows 99% identity to a previously reported sequence (INPPL-1) present in the database. We report here the identification of cDNAs for a new SH2-domain-containing protein showing homology to the inositol 5-phosphatase SHIP and therefore referred to as SHIP2. SHIP2 differs at both N- and C-terminal ends with the sequence of INPPL-1. The translated sequence of SHIP2 encodes a 1258 amino acid protein with a predicted molecular mass of 142 kDa. Particularly high levels of SHIP2 were found in human heart, skeletal muscle and placenta as shown by Northern blot analysis. SHIP2 was also expressed in dog thyroid cells in primary culture where the expression was enhanced in TSH and EGF-stimulated cells.

D-myo-inositol 1,4,5-trisphosphate 3-kinase A is activated by receptor activation through a calcium:calmodulin-dependent protein kinase II phosphorylation mechanism

D-myo-inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] 3-kinase, the enzyme responsible for production of D-myo-inositol 1,3,4,5-tetrakisphosphate, was activated 3- to 5-fold in homogenates of rat brain cortical slices after incubation with carbachol. The effect was reproduced in response to UTP in Chinese hamster ovary (CHO) cells overexpressing Ins(1,4,5)P3 3-kinase A, the major isoform present in rat and human neuronal cells. In ortho-32P-labelled cells, the phosphorylated 53 kDa enzyme could be identified after receptor activation by immunoprecipitation. The time course of phosphorylation was very similar to that observed for carbachol (or UTP)-induced enzyme activation. Enzyme phosphorylation was prevented in the presence of okadaic acid. Calmodulin (CaM) kinase II inhibitors (i.e. KN-93 and KN-62) prevented phosphorylation of Ins(1,4,5)P3 3-kinase. Identification of the phosphorylation site in transfected CHO cells indicated that the phosphorylated residue was Thr311. This residue of the human brain sequence lies in an active site peptide segment corresponding to a CaM kinase II-mediated phosphorylation consensus site, i.e. Arg-Ala-Val-Thr. The same residue in Ins(1,4,5)P3 3-kinase A was also phosphorylated in vitro by CaM kinase II. Phosphorylation resulted in 8- to 10-fold enzyme activation and a 25-fold increase in sensitivity to the Ca2+:CaM complex. In this study, direct evidence is provided for a novel regulation mechanism for Ins(1,4,5)P3 3-kinase (isoform A) in vitro and in intact cells.

Identification of an active site cysteine residue in human type I Ins(1,4,5)P3 5-phosphatase by chemical modification and site-directed mutagenesis

Chemical modification using thiol-directed agents and site-directed mutagenesis have been used to investigate the crucial role of an active site cysteine residue within the substrate-binding domain of human type I Ins(1,4,5)P3 5-phosphatase. Irreversible inhibition of enzymic activity is provoked by chemical modification of the enzyme by N-ethylmaleimide (NEM), 5,5'-dithio-2-nitrobenzoic acid, iodoacetate and to a much smaller extent by iodoacetämide. The alkylation reaction by NEM is prevented in the presence of Ins(1,4,5)P3. The results indicate that NEM binds at the active site of the enzyme with a stoichiometry of 0.9 mol of NEM per mol of enzyme. A single [14C]NEM-modified peptide was isolated after alpha-chymotrypsin proteolysis of the radiolabelled enzyme and reverse-phase HPLC. Sequence analysis of the active site-labelled peptide (i.e. MNTRCPAWCD) demonstrated that Cys348 contained the radiolabel. Furthermore two mutant enzymes were obtained by site-directed mutagenesis of the cysteine residue to serine and alanine respectively. Both mutant enzymes had identical UV CD spectra. The two mutants (i.e. Cys348-->Ser and Cys348-->Ala) show a marked loss of enzymic activity (more than 98% compared with the wild-type enzyme). Thus we have directly identified a reactive cysteine residue as part of the active site, i.e. the substrate-binding domain, of Ins(1,4,5)P3 5-phosphatase. This cysteine residue is part of a sequence 10 amino acids long that is well conserved among the primary structures of inositol and phosphatidylinositol polyphosphate 5-phosphatases.