Localization of a high-affinity inositol 1,4,5-trisphosphate/inositol 1,4,5,6-tetrakisphosphate binding domain to the pleckstrin homology module of a new 130 kDa protein: characterization of the determinants of structural specificity

PTB domain of insulin receptor substrate-1 binds inositol compounds

We examined whether a phosphotyrosine binding (PTB) domain from the human insulin receptor substrate-1 (hIRS-1) is capable of binding inositol phosphates/phosphoinositides. The binding specificity was compared with that of the pleckstrin homology (PH) domain derived from the same protein because the three dimensional structure was found to be very similar to that of the PH domain, despite the lack of sequence similarity. We also attempted to locate the site of binding of the inositol compounds. The PTB domain bound [3H]Ins(1,4, 5)P3, which was displaced most strongly by Ins(1,3,4,5,6)P5 and InsP6, indicating that these inositol polyphosphates show the highest affinity. The PTB domain bound to liposomes containing PtdIns(4,5)P2, PtdIns(3,4,5)P3 and PtdIns(3,4)P2, but not phosphatidylinositol. In contrast, the PH domain showed a preference for Ins(1,4,5)P3, the polar head of PtdIns(4,5)P2. Site-directed mutagenesis studies were performed to map the binding site for inositol phosphates in the PTB domain. Mutation of K169Q, K171Q or K177Q, located in the loop connecting the beta1 and beta2 strands, which is partially responsible for binding inositol phosphates/phosphoinositides in the PH domains of several other proteins, reduced binding activity, probably because of a reduction in affinity. Mutation of R212Q or R227Q, shown to be involved in the binding of a phosphotyrosine, had little effect on the binding capacity. These results indicate that the PTB domain of hIRS-1 can bind inositol phosphates/phosphoinositides. Therefore signalling through the PTB domain could be regulated by the binding not only of proteins with phosphotyrosine but also of inositol phosphates/phosphoinositides, implying that PTB domains could be involved in a myriad of interconnections between intracellular signalling pathways.

Pleckstrin homology domain as an inositol compound binding module

Many of the proteins that participate in cell signalling contain structural modules involved in regulatory interactions between components of signal transduction cascades. One of such modules is the pleckstrin homology (PH) domain, a region of approximately 120 amino acids that can form an electrostatically polarized tertiary structure. Several molecules such as inositol 1,4,5-trisphosphate/phosphatidylinositol 4,5-bisphosphate, the betagamma-subunits of heterotrimeric G proteins and protein kinase C have been proposed as common ligands for the PH domain. Through these potential interactions, the PH domain has been proposed to play a role in membrane recruitment of proteins containing the PH domain, thus targeting them to appropriate cellular compartment or enabling them to interact with other components of the signal transduction pathway. In this review, we mainly focus on membrane targeting through the binding to inositol phosphates/phosphoinositides.

Smooth muscle aldolase C-bound inositol 1,4,5-trisphosphate studied in vitro under physiological conditions

Our goal was to quantitate inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) binding to aldolase C tetramer (aldolase4) and its displacement by inositol 1,3,4-trisphosphate (Ins(1,3,4)P3) under conditions which approximated the in vivo state. Anions were found to have major effects. Decreasing [KCl] from 100 to 10mM, at 0 degrees C and pH 7.0, increased maximal Ins(1,4,5)P3 binding to 1.0 to 2.4mol per mol aldolase4. At 10 and 30mEq/l [Cl-], an additional high affinity site was detected (Kds = 0.43 and 0.86 microM, respectively). Increasing concentrations of other anions (SO42-, propanoate-, HCO3-, acetate-) also inhibited binding, but effects would be minimal at concentrations of these anions present in the cytoplasm of living cells. Ins(1,3,4)P3 displacement of aldolase C-bound Ins(1,4,5)P3 was sensitive to [Cl-]; at 30mEq/l [Cl-] and 37 degrees C, Ins(1,3,4)P3 released 20% of bound Ins(1,4,5)P3 at concentrations of 100nM. Changing temperature from 0 to 37 degrees C increased Kds for Ins(1,4,5)P3 binding. Changes in free [Ca2+], [Mg2+], [Na+] and [K+] and changes in osmolality had no effect on Ins(1,4,5)P3 binding to aldolase C. In vivo Ins(1,4,5)P3-aldolase4 binding at 30mEq/l [Cl-] and 37 degrees C were calculated for different [Ins(1,4,5)P3]free over the range 0.2 to 1.0 microM. For different cytoplasmic [Ins(1,4,5)P3]free. Ins(1,4,5)P3 binding to aldolase4 was sufficient, if acutely released, to nearly double cytoplasmic [Ins(1,4,5)P3]free. We proposed a schema whereby release of aldolase C-bound Ins(1,4,5)P3 evoked by Ins(1,3,4)P3 amplifies effects of phospholipase C-formed Ins(1,4,5)P3.

Replacements of single basic amino acids in the pleckstrin homology domain of phospholipase C-delta1 alter the ligand binding, phospholipase activity, and interaction with the plasma membrane

The pleckstrin homology (PH) domain of phosphatidylinositol-specific phospholipase C-delta1 (PLC-delta1) binds to both D-myo-inositol 1,4, 5-trisphosphate (Ins(1,4,5)P3) and phosphatidylinositol 4, 5-bisphosphate (PtdIns(4,5)P2) with high affinities. We have previously identified a region rich in basic amino acids within the PH domain critical for ligand binding (Yagisawa, H., Hirata, M., Kanematsu, T., Watanabe, Y., Ozaki, S., Sakuma, K., Tanaka, H., Yabuta, N., Kamata, H., Hirata, H., and Nojima, H. (1994) J. Biol. Chem. 269, 20179-20188; Hirata, M., Kanematsu, T., Sakuma, K., Koga, T., Watanabe, Y., Ozaki, S., and Yagisawa, H. (1994) Biochem. Biophys. Res. Commun. 205, 1563-1571). To investigate the role of these basic residues, we have performed site-directed mutagenesis replacing each of the basic amino acid in the N-terminal 60 residues of PLC-delta1 (Lys24, Lys30, Lys32, Arg37, Arg38, Arg40, Lys43, Lys49, Arg56, Lys57, and Arg60) with a neutral or an acidic amino acid. The effects of these mutations on the PH domain ligand binding properties and their consequence for substrate hydrolysis and membrane interactions of PLC-delta1 were analyzed using several assay systems. Analysis of [3H]-Ins(1,4,5)P3 binding, measurement of the binding affinities, and measurements of phospholipase activity using PtdIns(4,5)P2-containing phospholipid vesicles, demonstrated that residues Lys30, Lys32, Arg37, Arg38, Arg40, and Lys57 were required for these PLC-delta1 functions; in comparison, other mutations resulted in a moderate reduction. A subset of selected mutations was further analyzed for the enzyme activity toward substrate present in cellular membranes of permeabilized cells and for interaction with the plasma membrane after microinjection. These experiments demonstrated that mutations affecting ligand binding and PtdIns(4,5)P2 hydrolysis in phospholipid vesicles also resulted in reduction in the hydrolysis of cellular polyphosphoinositides and loss of membrane attachment. All residues (with the exception of the K43E substitution) found to be critical for the analyzed PLC-delta1 functions are present at the surface of the PH domain shown to contain the Ins(1,4,5)P3 binding pocket.

D-myo-Inositol 1,4,5,6-tetrakisphosphate produced in human intestinal epithelial cells in response to Salmonella invasion inhibits phosphoinositide 3-kinase signaling pathways

Several inositol-containing compounds play key roles in receptor-mediated cell signaling events. Here, we describe a function for a specific inositol polyphosphate, D-myo-inositol 1,4,5,6-tetrakisphosphate [Ins(1,4,5,6)P4], that is produced acutely in response to a receptor-independent process. Thus, infection of intestinal epithelial cells with the enteric pathogen Salmonella, but not with other invasive bacteria, induced a multifold increase in Ins(1,4,5,6)P4 levels. To define a specific function of Ins(1,4,5,6)P4, a membrane-permeant, hydrolyzable ester was used to deliver it to the intracellular compartment, where it antagonized epidermal growth factor (EGF)-induced inhibition of calcium-mediated chloride (Cl-) secretion (CaMCS) in intestinal epithelia. This EGF function is likely mediated through a phosphoinositide 3-kinase (PtdIns3K)-dependent mechanism because the EGF effects are abolished by wortmannin, and three different membrane-permeant esters of the PtdIns3K product phosphatidylinositol 3,4,5-trisphosphate mimicked the EGF effect on CaMCS. We further demonstrate that Ins(1,4,5,6)P4 antagonized EGF signaling downstream of PtdIns3K because Ins(1,4,5, 6)P4 interfered with the PtdInsP3 effect on CaMCS without affecting PtdIns3K activity. Thus, elevation of Ins(1,4,5,6)P4 in Salmonella-infected epithelia may promote Cl- flux by antagonizing EGF inhibition mediated through PtdIns3K and PtdInsP3.

Distinct specificity in the binding of inositol phosphates by pleckstrin homology domains of pleckstrin, RAC-protein kinase, diacylglycerol kinase and a new 130 kDa protein

The pleckstrin homology domains (PH domains) derived from four different proteins, the N-terminal part of pleckstrin, RAC-protein kinase, diacylglycerol kinase and the 130 kDa protein originally cloned as an inositol 1,4,5-trisphosphate binding protein, were analysed for binding of inositol phosphates and derivatives of inositol lipids. The PH domain from pleckstrin bound inositol phosphates according to a number of phosphates on the inositol ring, i.e. more phosphate groups, stronger the binding, but a very limited specificity due to the 2-phosphate was also observed. On the other hand, the PH domains from RAC-protein kinase and diacylglycerol kinase specifically bound inositol 1,3,4,5,6-pentakisphosphate and inositol 1,4,5,6-tetrakisphosphate most strongly. The PH domain from the 130 kDa protein, however, had a preference for inositol 1,4,5-trisphosphate and 1,4,5,6-tetrakisphosphate. Comparison was also made between binding of inositol 1,4,5-trisphosphate, inositol 1,3,4,5-tetrakisphosphate and soluble derivatives of their corresponding phospholipids. The PH domains examined, except that from pleckstrin, showed a 8- to 42-times higher affinity for inositol 1,4,5-trisphosphate than that for corresponding phosphoinositide derivative. However, all PH domains had similar affinity for inositol 1,3,4,5-tetrakisphosphate compared to the corresponding lipid derivative. The present study supports our previous proposal that inositol phosphates and/or inositol lipids could be important ligands for the PH domain, and therefore inositol phosphates/inositol lipids may have the considerable versatility in the control of diverse cellular function. Which of these potential ligands are physiologically relevant would depend on the binding affinities and their cellular abundance.

Molecular cloning and expression of a rat hepatic multiple inositol polyphosphate phosphatase

The characterization of the multiple inositol polyphosphate phosphatase (MIPP) is fundamental to our understanding of how cells control the signalling activities of 'higher' inositol polyphosphates. We now describe our isolation of a 2.3 kb cDNA clone of a rat hepatic form of MIPP. The predicted amino acid sequence of MIPP includes an 18 amino acid region that aligned with approximately 60% identity with the catalytic domain of a fungal inositol hexakisphosphate phosphatase (phytase A); the similarity encompassed conservation of the RHGXRXP signature of the histidine acid phosphatase family. A histidine-tagged, truncated form of MIPP was expressed in Escherichia coli and the enzymic specificity of the recombinant protein was characterized: Ins(1,3,4,5,6)P5 was hydrolysed, first to Ins(1,4,5,6)P4 and then to Ins(1,4,5)P3, by consecutive 3- and 6-phosphatase activities. Inositol hexakisphosphate was catabolized without specificity towards a particular phosphate group, but in contrast, MIPP only removed the beta-phosphate from the 5-diphosphate group of diphosphoinositol pentakisphosphate. These data, which are consistent with the substrate specificities of native (but not homogeneous) MIPP isolated from rat liver, provide the first demonstration that a single enzyme is responsible for this diverse range of specific catalytic activities. A 2.5 kb transcript of MIPP mRNA was present in all rat tissues that were examined, but was most highly expressed in kidney and liver. The predicted C-terminus of MIPP is comprised of the tetrapeptide SDEL, which is considered a signal for retaining soluble proteins in the lumen of the endoplasmic reticulum; the presence of this sequence provides a molecular explanation for our earlier biochemical demonstration that the endoplasmic reticulum contains substantial MIPP activity [Ali, Craxton and Shears (1993) J. Biol. Chem. 268, 6161-6167].