Inositol hexakisphosphate receptor identified as the clathrin assembly protein AP-2

Inositol polyphosphate-mediated iron transport in Pseudomonas aeruginosa

It has previously been shown that myo-inositol hexakisphosphate (myo-InsP6) mediates iron transport into Pseudomonas aeruginosa and overcomes iron-dependent growth inhibition. In this study, the iron transport properties of myo-inositol trisphosphate and tetrakisphosphate regio-isomers were studied. Pseudomonas aeruginosa accumulated iron (III) at similar rates whether complexed with myo-Ins(1,2,3)P3 or myo-InsP6. Iron accumulation from other compounds, notably D/L myo-Ins(1,2,4,5)P4 and another inositol trisphosphate regio-isomer, D-myo-Ins(1,4,5)P3, was dramatically increased. Iron transport profiles from myo-InsP6 into mutants lacking the outer membrane porins oprF, oprD and oprP were similar to the wild-type, indicating that these porins are not involved in the transport process. The rates of reduction of iron (III) to iron (II) complexed to any of the compounds by a Ps. aeruginosa cell lysate were similar, suggesting that a reductive mechanism is not the rate-determining step.

The versatility of inositol phosphates as cellular signals

Cells from across the phylogenetic spectrum contain a variety of inositol phosphates. Many different functions have been ascribed to this group of compounds. However, it is remarkable how frequently several of these different inositol phosphates have been linked to various aspects of signal transduction. Therefore, this review assesses the evidence that inositol phosphates have evolved into a versatile family of second messengers.

Inositol polyphosphates regulate the membrane interactions of the endosomal p100, G-protein-related protein

The protein, p100, was previously identified as a G-protein related protein that cycles on and off the cytoplasmic face of the endosome membrane (Traub et al., Biochem. J. 280 (1991) 171-178). Here we present evidence that the inositol polyphosphates, inositol 1,4, 5-trisphosphate (IP3) and inositol hexakisphosphate (IP6), release p100 from light-density microsomal membranes and inhibit rebinding of p100 through receptors, which are specific for IP3 or for IP6. These receptors can be co-extracted with p100 from the microsomes by 0.5 M Tris-HCl and, in the soluble state, they exhibit similar binding activity towards the inositol polyphosphates as do untreated microsomes. Soluble p100 self-aggregates and this aggregation is blocked by both IP3 and IP6. Stimulation of permeabilized rat basophilic leukemia (RBL-2H3) cells with carbachol, via transfected muscarinic m1 receptors, results in increased levels of inositol polyphosphates and the quantitative release of p100 into the cytosol. This effect is reversible and cytosolic p100 rebinds to the membrane as the levels of inositol polyphosphates decline. These findings suggest that p100 may belong to a family of IP-binding proteins whose intracellular localization is determined by extracellular signals.

Probing the phosphoinositide binding site of the clathrin assembly protein AP-2 with photoaffinity labels

The relative binding specificities of the subunitsof bovine assembly protein AP-2 for the phosphatidylinositol polyphosphates (PtdInsPn) and inositol polyphosphates (InsPn) were determined by photoaffinitylabeling. Three types of benzophenone-containing photoprobes were employed: (i) the water-solubleP-1- or P-2-tethered p-benzoyldihydrocinnamoyl-InsPn (BZDC-InsPn) analogs, (ii) P-1-linked phosphotriester PtdInsPn analogs that sampled the interface between the water and lipid phases, and (iii) sn-1-O-acyl-linked PtdInsPn analogs that interacted with proteins penetrating the bilayer. The InsPn and PtdInsPn probes bind with highest selectivity and affinity to the two alpha subunit isoforms, with certain probes and conditions resulting in strong labeling of the 50-kDa mu subunit. Three main conclusions were reached: (i) head group recognition predominated over acyl chain recognition, (ii) the PtdInsPn binding site of alpha-AP-2 prefers more highly phosphorylated species, and (iii) the protein-acyl chain interactions showed high capacity but low selectivity.

ATM binds to beta-adaptin in cytoplasmic vesicles

Inherited mutations in the ATM gene lead to a complex clinical phenotype characterized by neuronal degeneration, oculocutaneous telangiectasias, immune dysfunction, and cancer predisposition. Using the yeast two-hybrid system, we demonstrate that ataxia telangiectasia mutated (ATM) binds to beta-adaptin, one of the components of the AP-2 adaptor complex, which is involved in clathrin-mediated endocytosis of receptors. The interaction between ATM and beta-adaptin was confirmed in vitro, and coimmunoprecipitation and colocalization studies show that the proteins also associate in vivo. ATM also interacts in vitro with beta-NAP, a neuronal-specific beta-adaptin homolog that was identified as an autoantigen in a patient with cerebellar degeneration. Our data describing the association of ATM with beta-adaptin in vesicles indicate that ATM may play a role in intracellular vesicle and/or protein transport mechanisms.

Overproduction and immuno-affinity purification of myelin proteolipid protein (PLP), an inositol hexakisphosphate-binding protein, in a baculovirus expression system

Myelin proteolipid protein (PLP) is a major integral membrane protein of central nervous system myelin and is considered to play a significant role in myelination. PLP has a four-transmembrane structure, judging from the hydropathy profile. In addition, it has InsP6 binding activity. Here, we have succeeded in producing PLP in large quantities of 3.9 pg/cell (6 mg/L) by using a baculovirus expression system and developing an efficient purification method, maintaining InsP6 binding activity. The recombinant PLP (rPLP) was purified by ion-exchange and immunoaffinity chromatography in a nonorganic solvent. The final yield of purified rPLP was 36%. The Kd and Bmax values for the InsP6-PLP binding were 55 nM and 33 pmol/microgram protein, respectively. The Kd value of purified rPLP is equal to that of mouse brain PLP. These results indicate that purified rPLP keeps its native conformation and binds InsP6 in an almost one-to-one ratio.

Regulation of AP-2-synaptotagmin interaction by inositol high polyphosphates

The inositol high-polyphosphate series (IHPS) inhibits neurotransmission through binding to the second C2 domain of synaptotagmins I and II(Syt), synaptic vesicle membrane proteins. We have revealed that several proteins, including alpha adaptins which are specific subunits of clathrin assembly protein, AP2, were eluted from mouse brain by affinity elution chromatography from the C2 domains of Syt II-immobilized Sepharose using 50 microM of InsP6. The interaction between Syt II and AP2 was more markedly inhibited by IHPS than by the same concentration of InsP3. Limited digestion of mouse crude synaptosomal fractions with trypsin revealed different cleavage patterns in the presence and absence of 50 microM InsP6. These results suggest that IHPS-binding to the C2B domain of synaptotagmin alters the state of protein-protein interaction including the synaptotagmin-AP2 interaction, possibly resulting in the inhibition of events involved in the synaptic vesicle trafficking.

Expression and subcellular localization of p42IP4/centaurin-alpha, a brain-specific, high-affinity receptor for inositol 1,3,4,5-tetrakisphosphate and phosphatidylinositol 3,4,5-trisphosphate in rat brain

Recently emerging evidence suggests important roles for inositol polyphosphates and inositol phospholipids in neuronal Ca2+ signalling, membrane vesicle trafficking and cytoskeletal rearrangement. A prerequisite for a detailed physiological characterization of the signalling of both potential second messengers inositol-(1,3,4,5)-tetrakisphosphate (InsP4) and phosphatidylinositol-3,4,5-trisphosphate (PtdInsP3) in the nervous system is the precise cellular localization of their receptors. Based on the cDNA sequence of a recently cloned brain-specific receptor with high affinity for both InsP4 and PtdInsP3 (InsP4-PtdInsP3R), p42IP4/centaurin-alpha, we localized the mRNA and the protein in rat brain. In situ hybridization revealed a widespread expression of the InsP4-PtdInsP3R with prominent labelling in cerebellum, hippocampus, cortex and thalamus, which moreover is developmentally regulated. Using peptide-specific antibodies, the immunoreactivity was localized in the adult brain in the vast majority of neuronal cell types and probably also in some glial cells. Prominent immunoreactivity was found in axonal processes and in cell types characterized by extensive neurites. In the hypothalamus a subpopulation of parvocellular neurons in the peri- and paraventricular nuclei was most heavily labelled. This was confined by strong immunoreactivity in the lamina externa of the median eminence in close proximity to portal plexus blood vessels. Electron microscopy revealed that the InsP4-PtdInsP3R was frequently associated with presynaptic vesicular structures. Further studies should identify the role of the InsP4-PtdInsP3R in cellular neural processes.

Structural and functional requirements for agonist-induced internalization of the human platelet-activating factor receptor

The receptor for platelet-activating factor (PAF) is a member of the G-protein-coupled receptor family. To study the structural elements and mechanisms involved in the internalization of human PAF receptor (hPAFR), we used the following mutants: a truncated mutant in the C-terminal tail of the receptor (Cys317 --> Stop) and mutations in the (D/N)P(X)2,3Y motif (Asp289 --> Asn,Ala and Tyr293 --> Phe,Ala). Chinese hamster ovary cells expressing the Cys317 --> Stop mutant exhibited a marked reduction in their capacity to internalize PAF, suggesting the existence of determinants important for endocytosis in the last 26 amino acids of the cytoplasmic tail. Substitution of Asp289 to alanine abolished both internalization and G-protein coupling, whereas substitution of Tyr293 to alanine abolished coupling but not internalization. Inhibition or activation of protein kinase C did not significantly affect the internalization process. Receptor sequestration and ligand uptake was, at least in part, blocked by concanavalin A and blockers of endocytosis mediated by clathrin-coated pits. Our data suggest that the internalization of a G-protein-coupled receptor and coupling to a G-protein can be two independent events. Moreover, the C terminus tail of hPAFR, but not the putative internalization motifs, may be involved in the internalization of hPAFR.

The function of inositol high polyphosphate binding proteins

The inositol phosphate metabolism network has been found to be much more complex than previously thought, as more and more inositol phosphates and their metabolizing enzymes have been discovered. Some of the inositol phosphates have been shown to have biological activities, but little is known about their signal transduction mechanisms except for that of inositol 1,4,5-trisphosphate. The recent discovery, however, of a number of binding proteins for inositol high polyphosphate [inositol 1,3,4,5-tetrakisphosphate (IP4), inositol 1,3,4,5,6-pentakisphosphate, or inositol hexakisphosphate] enables us to speculate on the physiological function of these compounds. In this article we focus on two major issues: (1) the roles of inositol high polyphosphates in vesicular trafficking, especially exocytosis, and (2) pleckstrin homology domain-containing IP4 binding proteins involved in the Ras signaling pathway.

Inhibition of phospholipase D by clathrin assembly protein 3 (AP3)

In the accompanying paper (Chung, J.-K., Sekiya, F., Kang, H.-S., Lee, C., Han, J.-S., Kim, S. R., Bae, Y. S., Morris, A. J., and Rhee, S. G. (1997) J. Biol. Chem. 272, 15980-15985), synaptojanin is identified as a protein that inhibits phospholipase D (PLD) activity stimulated by ADP-ribosylation factor and phosphatidylinositol 4, 5-bisphosphate (PI(4,5)P2). Here, the purification from rat brain cytosol of another PLD-inhibitory protein that is immunologically distinct from synaptojanin is described, and this protein is identified as clathrin assembly protein 3 (AP3) by peptide sequencing and immunoblot analysis. AP3 binds both inositol hexakisphosphate and preassembled clathrin cages with high affinity. However, neither inositol hexakisphosphate binding nor clathrin cage binding affected the ability of AP3 to inhibit PLD. AP3 also binds to PI(4,5)P2 with low affinity. But the PI(4,5)P2 binding was not responsible for PLD inhibition, because the potency and efficacy of AP3 as an inhibitor of PLD were similar in the absence and presence of PI(4,5)P2. A bacterially expressed fusion protein, glutathione S-transferase-AP3 (GST-AP3), also inhibited PLD with a potency equal to that of brain AP3. The inhibitory effect of AP3 appeared to be the result of direct interaction between AP3 and PLD because PLD bound GST-AP3 in an in vitro binding assay. Using GST fusion proteins containing various AP3 sequences, we found that the sequence extending from residues Pro-290 to Lys-320 of AP3 is critical for both inhibition of and binding to PLD. The fact that AP3 is a synapse-specific protein indicates that the AP3-dependent inhibition of PLD might play a regulatory role that is restricted to the rapid cycling of synaptic vesicles.

Regulatory interactions in the recognition of endocytic sorting signals by AP-2 complexes

Many plasma membrane proteins destined for endocytosis are concentrated into clathrin-coated pits through the recognition of a tyrosine-based motif in their cytosolic domains by an adaptor (AP-2) complex. The mu2 subunit of isolated AP-2 complexes binds specifically, but rather weakly, to proteins bearing the tyrosine-based signal. We now demonstrate, using peptides with a photoreactive probe, that this binding is strengthened significantly when the AP-2 complex is present in clathrin coats, indicating that there is cooperativity between receptor-AP-2 interactions and coat formation. Phosphoinositides with a phosphate at the D-3 position of the inositol ring, but not other isomers, also increase the affinity of the AP-2 complex for the tyrosine-based motif. AP-2 is the first protein known (in any context) to interact with phosphatidylinositol 3-phosphate. Our findings indicate that receptor recruitment can be coupled to clathrin coat assembly and suggest a mechanism for regulation of membrane traffic by lipid products of phosphoinositide 3-kinases.

Regulation of AP-3 function by inositides. Identification of phosphatidylinositol 3,4,5-trisphosphate as a potent ligand

As part of the growing effort to understand the role inositol phosphates and inositol lipids play in the regulation of vesicle traffic within nerve terminals, we determined whether or not the synapse-specific clathrin assembly protein AP-3 can interact with inositol lipids. We found that soluble dioctanoyl-phosphatidylinositol 3,4,5-trisphosphate (DiC8PtdIns(3,4, 5)P3) was only 7.5-fold weaker a ligand than D-myo-inositol hexakisphosphate in assays that measured the displacement of D-myo-[3H]inositol hexakisphosphate. In functional assays we found that both of these ligands inhibited clathrin assembly, but DiC8-PtdIns(3,4,5)P3 was more potent and exhibited a larger maximal effect. We also examined the structural features of DiC8-PtdIns(3,4, 5)P3 that establish specificity. Dioctanoyl-phosphatidylinositol 3, 4-bisphosphate, which does not have a 5-phosphate, and 4, 5-O-bisphosphoryl-D-myo-inosityl 1-O-(1, 2-O-diundecyl)-sn-3-glycerylphosphate, which does not have a 3-phosphate, were, respectively, 2-fold and 4-fold less potent than DiC8-PtdIns(3,4,5)P3 as inhibitors of clathrin assembly. Deacylation of DiC8-PtdIns(3,4,5)P3 reduced its affinity for AP-3 almost 20-fold, and also dramatically lowered its ability to inhibit clathrin assembly. The deacylated products of the soluble derivatives of phosphatidylinositol 3,4-bisphosphate and phosphatidylinositol 4, 5-bisphosphate were both not significant inhibitors of clathrin assembly. It therefore appears that the interactions of inositides with AP-3 should not be considered simply in terms of electrostatic effects of the highly charged phosphate groups. Ligand specificity appears also to be mediated by hydrophobic interactions with the fatty-acyl chains of the inositol lipids.

Selective photoaffinity labeling of the inositol polyphosphate binding C2B domains of synaptotagmins

Synaptotagmin (Syt) II, a synaptic vesicle protein containing two copies of highly conserved protein kinase C homology regions known as the C2A and C2B domains, acts as a Ca2+ sensor and provides both phospholipid and inositol polyphosphate (IPn) recognition domains important in endo- and exocytosis. Four photoaffinity analogues of IP3, IP4, and IP6 containing a P-1- or P-2-linked 4-benzoyldihydrocinnamidyl (BZDC) photophore were used to label glutathione S-transferase (GST) fusion constructs of the Syt II-C2A and C2B domains. The P-2-linked [3H]BZDC-IP6 showed efficient, IP6-displaceable labeling of the GST-Syt II-C2B. The rank order of photocovalent modification paralleled the order of competitive displacement: IP6 (P-2-linked) > IP4 > IP3. The P-1-linked [3H]BZDC-IP6 failed to label the C2B domains. The GST-Syt III-C2B domain, which lacks IP6 binding affinity, also failed to undergo labeling by P-2-linked [3H]BZDC-IP6. When mixtures of the 32-amino acid basic peptide corresponding to the essential IPn binding region of the Syt II-C2B domain and GST-Syt II-C2B were labeled by a stoichiometric amount of P-2-linked [3H]BZDC-IP6, the two polypeptides showed equivalent affinity for the photolabel. Although the CD spectrum of this 32-mer at two pH values showed a random coil, the photoaffinity analogue of IP6 appeared to induce a binding-compatible structure in the short peptide.

Benzophenone photoprobes for phosphoinositides, peptides and drugs

Benzophenones (BP) and related aryl ketone photophores have become established as the photoactivatable group of choice for high-efficiency covalent modification of hydrophobic regions of binding proteins, including enzymes and receptors that recognize peptide hormones, (oligo) nucleotides and nucleosides, phosphoinositides, inositol polyphosphates and a wide variety of therapeutic molecules. This review presents the advantages of BP as photoaffinity labels and provides specific examples from the last 3 years of applications of BP-containing ligands used in biochemistry.

Neuronal cytotoxicity of inositol hexakisphosphate (phytate) in the rat hippocampus

D-myo-Inositol hexakisphosphate (InsP6, phytate), a normal cellular constituent, was found to be toxic to neuronal perikarya when injected into the rat hippocampus. However, the extrinsic cholinergic innervation of the hippocampus (as estimated by staining for acetylcholinesterase) was unaffected. Its potency as a toxin was approximately equal to that of the excitotoxin quinolinate. Other highly charged derivatives of inositol (inositol hexakissulphate, inositol monophosphate) were not toxic. The cytotoxicity of InsP6 was not due to a high osmolality, or to seizure-induced lesions, but was reduced by calcium. Nevertheless, the toxicity was not due to chelation of brain calcium by InsP6, as another calcium chelator with a higher affinity for calcium, 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA), produced only a very mild lesion. Thus, abnormal metabolism of InsP6 might possibly contribute to neuronal death in neurodegenerative diseases.

Eps15 is a component of clathrin-coated pits and vesicles and is located at the rim of coated pits

Eps15, a phosphorylation substrate of the epidermal growth factor (EGF) receptor kinase, has been shown to bind to the alpha-subunit of the clathrin-associated protein complex AP-2. Here we report that in cells, virtually all Eps15 interacts with the cytosol and membrane-bound forms of AP-2. This association is not affected by the treatment of cells with EGF. Immunofluorescence microscopy reveals nearly absolute co-localization of Eps15 with AP-2 and clathrin, and analysis by immunoelectron microscopy shows that the localization of membrane-associated Eps15 is restricted to the profiles corresponding to endocytic coated pits and vesicles. Unexpectedly, Eps15 was found at the edge of forming coated pits and at the rim of budding coated vesicles. This asymmetric distribution is in sharp contrast to the localization of AP-2 that shows an even distribution along the same types of clathrin-coated structures. These findings suggest several possible regulatory roles of Eps15 during the formation of coated pits.

Myelin proteolipid protein (PLP), but not DM-20, is an inositol hexakisphosphate-binding protein

Myelin proteolipid protein (PLP) and its alternatively spliced isoform, DM-20, are the major integral membrane proteins of central nervous system myelin. It is known that PLP and DM-20 are delivered to myelin by a finely regulated vesicular transport system in oligodendrocytes. Evolutionarily, it is believed that ancestral DM-20 acquired a PLP-specific exon to create PLP, after which PLP/DM-20 became a major component of central nervous system myelin. We purified PLP as an inositol 1,3,4,5-tetrakisphosphate-binding protein after solubilization in a non-organic solvent. However, under the isotonic condition, PLP binds inositol hexakisphosphate (InsP6) significantly, not inositol 1,3,4,5-tetrakisphosphate. Most of the InsP6-binding proteins are involved in vesicular transport, suggesting the involvement of PLP in vesicular transport. We separated DM-20 from PLP by CM-52 chromatography and showed that DM-20 has no InsP6 binding activity. These findings indicate that the PLP-specific domain confers the InsP6 binding activity and this interaction may be important for directing PLP transport to central nervous system myelin.

Characterization of inositolpolyphosphate binding to myocardial membranes

Although it is well-accepted that the phosphatidylinositol signalling transduction pathway, producing inositol-1,4,5-P3 (InsP3) and inositol-1,3,4,5-P4 (InsP4) as second messengers, functions in heart muscle, virtually nothing is known about the roles of the higher inositol polyphosphates such as inositolhexakisphosphate (InsP6). This study demonstrates that InsP6 has the ability to bind intracellularly, with different binding characteristics, to different myocardial membranes. Binding to purified sarcoplasmic reticulum (SR) membranes, purified sarcolemmal (SL) membranes as well as to viable mitochondria were characterized. Binding to all these membranes display high as well as low affinity binding sites, with differing affinities. Kd values of binding to SR were 32 and 383 nM, to SL 61 and 1312 nM, while those of mitochondrial binding were 230 and 2200 nM respectively. InsP4 binding was also investigated and displayed the following characteristics: to SR, one low affinity binding site (Kd = 203 nM) and to SL, a high as well as a low affinity binding site with Kd values of 41 and 2075 nM respectively. Presence of InsP3, the second messenger for SR calcium release, at concentrations of 1 nM, elevated the binding of InsP4 to SR and SL by a mean of 30% and 20% respectively. Fractionation of SR and SL membranes on sucrose density gradients, after solubilization with CHAPS, indicated that InsP6 bound to two separate protein peaks in both these membranes, while InsP4 bound to only one. In SR membranes, InsP4 bound preferentially to a protein separating at high sucrose density while it bound to a protein separating at low sucrose density in SL membranes.

A functional phosphatidylinositol 3,4,5-trisphosphate/phosphoinositide binding domain in the clathrin adaptor AP-2 alpha subunit. Implications for the endocytic pathway

Clathrin-coated pits are sites of concentration of ligand-bound signaling receptors. Several such receptors are known to recruit, bind, and activate the heterodimeric phosphatidylinositol-3-kinase, resulting in the generation of phosphatidylinositol 3,4, 5-trisphosphate. We report here that dioctanoyl-phosphatidylinositol-3,4,5-P3 binds specifically and saturably to soluble AP-2 and with greater affinity to AP-2 within assembled coat structures. Soluble -myo-inositol hexakisphosphate shows converse behavior. Binding to bovine brain clathrin-coated vesicles is evident only after detergent extraction. These observations and evidence for recognition of the diacylglyceryl backbone as well as the inositol phosphate headgroup are consistent with AP-2 interaction with membrane phosphoinositides in coated vesicles and with soluble inositol phosphates in cytoplasm. A discrete binding domain is identified near the N terminus of the AP-2 alpha subunit, and an expressed fusion protein containing this sequence exhibits specific, high affinity binding that is virtually identical to the parent protein. This region of the AP-2 alpha sequence also shows the greatest conservation between a Caenorhabditis elegans homolog and mammalian alpha, consistent with a function in recognition of an evolutionarily unchanging low molecular weight ligand. Binding of phosphatidylinositol 3,4, 5-trisphosphate to AP-2 inhibits the protein's clathrin binding and assembly activities. These findings are discussed in the context of the potential roles of phosphoinositides and AP-2 in the internalization and trafficking of cell surface receptors.

Identification and cloning of centaurin-alpha. A novel phosphatidylinositol 3,4,5-trisphosphate-binding protein from rat brain

Using an affinity resin and photoaffinity label based on phospholipid analogs of inositol 1,3,4,5-tetrakisphosphate (InsP4), we have isolated, characterized, and cloned a 46-kDa protein from rat brain, which we have named centaurin-alpha. Binding specificity was determined using displacement of 1-O-[3H](3-[4-benzoyldihydrocinnamidyl]propyl)-InsP4 photoaffinity labeling. Centaurin-alpha displayed highest affinity for phosphatidylinositol 3,4,5-trisphosphate (PtdInsP3) (IC50 = 120 nM), whereas InsP4, PtdInsP2, and InsP3 bound with 5-, 12-, and >50-fold lower affinity, respectively. Screening a rat brain cDNA library with a polymerase chain reaction product, generated using partial amino acid sequence from tryptic peptides, yielded a full-length clone. The 2,450-base pair cDNA contained an open reading frame (ORF) encoding a novel protein of 419 amino acids. Northern analysis revealed a 2.5-kilobase transcript that is highly expressed in brain. The deduced sequence contains a novel putative zinc finger motif, 10 ankyrin-like repeats, and shows homology to recently identified yeast and mammalian Arf GTPase-activating proteins. Given the specificity of binding and enrichment in brain, centaurin-alpha is a candidate PtdInsP3 receptor that may link the activation of phosphoinositide 3-kinase to downstream responses in the brain.

Isolation of InsP4 and InsP6 binding proteins from human platelets: InsP4 promotes Ca2+ efflux from inside-out plasma membrane vesicles containing 104 kDa GAP1IP4BP protein

A low-density membrane fraction from human platelets contained the plasma membrane marker glycoprotein Ib (GpIb) and selective binding sites for InsP4 and InsP6. It was separated from the bulk of InsP3-receptor-containing membranes, but was heterogeneous, probably also containing surface-connected canalicular system and some lighter elements of the internal dense tubule system. After loading with calcium oxalate and re-centrifugation on Percoll gradients, this mixed fraction was subfractionated into light membranes containing all of the GpIb, high-affinity InsP4 binding sites (KD = 18 nM) and phosphate-stimulated Ca2+ transport activity. InsP4 (EC50 0.6 microM), but not InsP3 or InsP6, released up to 35% of the accumulated Ca2+ from these vesicles, which were shown to be inside-out plasma membrane vesicles by a biotinylation labelling technique and selective removal of right-side-out plasma membrane vesicles with streptavidin-agarose. Most of the InsP4, and all of the InsP6, binding was present in the much denser calcium oxalate-loaded subfractions, which were free of GpIb. InsP6 binding activity was chromatographically purified as a 116 kDa protein (KD for InsP6 = 5.9 nM), with an amino acid content and two internal peptide sequences identical to those of 116 kDa vinculin. A 104 kDa InsP4 binding protein (KD for InsP4 = 12 nM), probably identical to GAP1IP4BP described by Cullen, Hsuan, Truong, Letcher, Jackson, Dawson and Irvine [(1995) Nature (London) 376, 527-530], was also isolated. This InsP4 receptor may mediate Ca2+ influx in platelets that occurs subsequent to receptor-stimulated production of InsP3 and unloading of internal Ca2+ stores.

Purified inositol hexakisphosphate kinase is an ATP synthase: diphosphoinositol pentakisphosphate as a high-energy phosphate donor

Diphosphoinositol pentakisphosphate (PP-IP5) and bis(diphospho)inositol tetrakisphosphate (bis-PP-IP4) are recently identified inositol phosphates that possess pyrophosphate bonds. We have purified an inositol hexakisphosphate (IP6) kinase from rat brain supernatants. The pure protein, a monomer of 54 kDa, displays high affinity (Km = 0.7 microM) and selectivity for inositol hexakisphosphate as substrate. It can be dissociated from bis(diphospho)inositol tetrakisphosphate synthetic activity. The purified enzyme transfers a phosphate from PP-IP5 to ADP to form ATP. This ATP synthase activity indicates the high phosphate group transfer potential of PP-IP5 and may represent a physiological role for PP-IP5.

Phosphoinositides as regulators in membrane traffic

Phosphorylated products of phosphatidylinositol play critical roles in the regulation of membrane traffic, in addition to their classical roles as second messengers in signal transduction at the cell surface. Growing evidence suggests that phosphorylation-dephosphorylation of the polar heads of phosphoinositides (polyphosphorylated inositol lipids) in specific intracellular locations signals either the recruitment or the activation of proteins essential for vesicular transport. Cross talk between phosphatidylinositol metabolites and guanosine triphosphatases is an important feature of these regulatory mechanisms.

Characterization of inositol hexakisphosphate (InsP6)-mediated priming in human neutrophils: lack of extracellular [3H]-InsP6 receptors

1. Inositol hexakisphosphate (InsP6) is a ubiquitous and abundant cytosolic inositol phosphate that has been reported to prime human neutrophils for enhanced agonist-stimulated superoxide anion generation. This led to the proposal that the release of InsP6 from necrotic cells may augment the functional responsiveness of neutrophils at an inflammatory focus. The aim of this study was to examine whether the functional effects of InsP6 in neutrophils are receptor-mediated and establish the magnitude of this priming effect relative to other better characterized priming agents. 2. Analysis of [3H]-InsP6 binding to human neutrophil membranes in 20 mM Tris, 20 mM NaCl, 100 mM KCl, 5 mM EDTA (pH 7.7) buffer using 0.1 mg ml-1 membrane protein and 2.5 nM [3H]-InsP6 (90 min, 4 degrees C), demonstrated specific low affinity [3H]-InsP6 binding that was non-saturable up to a radioligand concentration of 10 nM. 3. [3H]-InsP6 displacement by InsP6 gave a Hill coefficient of 0.55 and best fitted a two-site logistic model (53% KD 150 nM, 47% KD 5 microM). [3H]-InsP6 binding also displayed low (3 fold) selectivity for InsP6 over Ins(1,3,4,5,6)P5. 4. The specific [3H]-InsP6 binding displayed a pH optimum of 8, was abolished by pre-boiling the membranes, and was enhanced by Ca2+, Mg2+ and Na+. 5. In incubations with intact neutrophils, where high levels of specific [3H]-LTB4 binding was observed, no [3H]-InsP6 binding could be identified. 6. Preincubation of neutrophils with 100 microM InsP6 had no effect on resting cell morphology, but caused a minor and transient (maximal at 30 s) enhancement of (0.1 nM) fMLP-induced shape change (% cells shape changed: fMLP 53 +/- 3%, fMLP+InsP6 66 +/- 4%). Similarly, InsP6 (100 microM, 30 s) had no effect on basal superoxide anion generation and, compared to lipopolysaccharide (LPS, 100 ng ml-1, 60 min), tumour necrosis factor-alpha (TNF alpha, 200 u ml-1, 30 min) or platelet-activating factor (PAF, 100 nM, 5 min) caused only a small enhancement of 100 nM fMLP-stimulated superoxide anion generation (fold-increase in superoxide anion generation over fMLP alone: InsP6 1.8 +/- 0.3, LPS 6.8 +/- 0.6, TNF alpha 5.2 +/- 0.7, PAF 5.8 +/- 0.6). 7. While these data support the presence of a specific, albeit low affinity, [3H]-InsP6 binding site in human neutrophil membrane preparations, the lack of binding to intact cells implies that the functional effects of InsP6 (ie. enhanced fMLP-stimulated superoxide anion generation and shape change) are not receptor-mediated.

Membrane-associated inositol hexakisphosphate binding in bovine retina

We investigated the InsP6 binding proteins in bovine retinal membranes and rod outer segments (ROS) by radioligand binding assay and western blotting. The relative affinity of InsP6 for the binding protein was determined by competitive binding of [3H]-InsP6 with increasing concentrations of the unlabeled InsP6 or other isomers. InsP6 specifically binds to both bovine retinal membranes and ROS; maximum binding was achieved after one-hour incubation at 4 degrees C and was unchanged up to 2 h. Tris-HCl or acetate buffer was equally suitable for the binding assay over a broad range of pH, although specific binding was slightly increased at acidic pH. The order of potencies of displacement was InsP6 > Ins(1,3,4,5,6)P5 > Ins(1,3,4,6)P4 = Ins(1,3,4,5)P4, whereas Ins(1,4,5)P3, Ins(1,4)P2, Ins(4,5)P2, and Ins(1)P were not effective displacers. Scatchard analyses of the binding data were consistent with an equilibrium dissociation constant (Kd) of 2.5 +/- 0.2 microM and maximal binding capacity (Bmax) of 123.7 +/- 25.0 pmol/mg at pH 7.4. Western blotting was used to detect whether AP-2 (an InsP6 binding protein) is present in the retina. Immunoreactivity to AP-2 alpha and beta subunits was found in retinal membranes and ROS. Thus, bovine retinal membranes and ROS contain membrane-associated InsP6 binding protein(s) which is distinct from proteins that bind InsP5, InsP4, or InsP3.

Inositol hexakisphosphate binds to clathrin assembly protein 3 (AP-3/AP180) and inhibits clathrin cage assembly in vitro

We have isolated an inositol hexakisphosphate binding protein from rat brain by affinity elution chromatography from Mono S cation exchange resin using 0.1 mM inositol hexakisphosphate (InsP6). The amino acid sequences of six tryptic peptides from the protein were identical to the sequences predicted from the cDNA encoding a previously isolated protein designated as AP-3 or AP180. This protein is localized in nerve endings and promotes assembly of clathrin into coated vesicles. The isolated protein-bound InsP6 with a dissociation constant of 1.2 microM and a stoichiometry of 0.9 mol of InsP6 bound/mol of AP-3. Recombinant AP-3 expressed in Escherichia coli also bound InsP6 with a similar affinity. InsP6 inhibited clathrin cage assembly mediated by AP-3, in an in vitro assay, but had little effect AP-3 binding to preformed cages. We speculate that InsP6 and perhaps highly phosphorylated inositol lipids may play a role in coated vesicle formation.

Analysis of successive endocytic compartments isolated from Dictyostelium discoideum by magnetic fractionation

A colloidal iron probe was fed to the amoeba Dictyostelium discoideum and chased for different intervals. Successive segments of the endocytic pathway were then isolated magnetically at high yield and purity. There were approx. 500 endocytic vacuoles per cell; their diameters increased from approx. 0.1-0.2 microns after 3 min of feeding to approx. 2 microns after 15 min of feeding and 60 min of chase. The wave-like progression of ingested probes along the endocytic pathway suggested that the transfer of cargo involved a maturation mechanism rather than the shuttling of cargo between stable compartments. The lifetime of primary pinosomes was calculated to be approx. 1 s. Multivesicular bodies were common in the 3 min fraction and abundant in 15 min lysosomes. alpha- and beta-adaptins of molecular masses of approx. 89 and 83 kDa were richer in the 3 min vesicles than in plasma membranes and later endocytic vacuoles. Acid phosphatase, intrinsic vacuole acidity, the vacuolar proton pump protein and pump activity were present at all endocytic stages but rose between the 3 min and 15 min vacuoles and declined thereafter. Bis(monoacyglycero)phosphate or BMP, a lipid characteristic of lysosomes, followed a similar time course; it contributed up to half of the total lipid in lysosomal vacuoles. We conclude that there is both continuity and differentiation along this endocytic pathway.

The intracellular distribution of inositol polyphosphates in HL60 promyeloid cells

1. HL60 promyeloid cells contain high intracellular concentrations of inositol polyphosphates, notably inositol 1,3,4,5,6-pentakisphosphate (InsP5) and inositol hexakisphosphate (InsP6). To determine their intracellular location(s), we studied the release of inositol (poly)phosphates, of ATP, and of cytosolic and granule-enclosed enzymes from cells permeabilized by four different methods. 2. When cells were treated with digitonin, all of the inositol phosphates were released in parallel with the cytosolic constituents. Most of the InsP5 and InsP6 was released before significant permeabilization of azurophil granules. 3. Similar results were obtained from cells preloaded with ethylene glycol and permeabilized by osmotic lysis. 4. Electroporation at approximately 500 V/cm caused rapid release of free inositol. Higher field strengths provoked release of most of the ATP, InsP5 and InsP6, but only slight release of the intracellular enzymes. Multiple discharges released approximately 80-90% of total InsP5 and InsP6. In the absence of bivalent-cation chelators, InsP5 and InsP6 were released less readily than ATP. 5. Treatment of cells with Staphylococcus aureus alpha-toxin caused quantitative release of inositol and ATP, without release of intracellular enzymes. However, inositol phosphates were released much less readily than inositol or ATP. Even after prolonged incubation with a high concentration of alpha-toxin, only approximately 50-70% of InsP2, InsP3 and InsP4 and < or = 20% of InsP5 and InsP6 were released, indicating that the high charge or large hydrated radius of InsP5 and InsP6 might limit their release through small toxin-induced pores. 6. These results indicate that most intracellular inositol metabolites are either in, or in rapid exchange with, the cytosolic compartment of HL60 cells. However, they leave open the possibility that a small proportion of cellular InsP5 and InsP6 (< or = 10-20%) might be in some intracellular bound form.

Phosphoinositides and calcium signaling New aspects and diverse functions in cell regulation

Numerous circulating and locally produced hormones bind to specific cell-surface receptors and activate a variety of second-messenger pathways that evoke characteristic phenotypic responses in their target cells. One of the most ubiquitous signal transduction mechanisms is the phosphoinositide-calcium messenger system, which is activated by hormones, neurotransmitters, and growth factors. Stimulation of these receptors by their ligands causes a characteristic change in the metabolism of membrane phospholipids with production of diacylglycerol and a rapid increase in cytoplasmic Ca(2+) concentration, due to the release of stored intracellular Ca(2+) and stimulated Ca(2+) entry from the extracellular space. These intracettular signals act in concert to activate protein kinases that phosphorylate a variety of regulatory proteins. The link between phosphoinositide turnover and Ca(2+) mobilization is inositol 1,4,5-trisphosphate, the major Ca(2+)-mobilizing second messenger, which is produced from membrane phosphoinositides by activated phospholipase C enzymes. The mechanisms of ligand-regulated Ca(2+) influx and the additional regulatory role(s) of phosphoinositides and inositol phosphates are still being unfolded. This review and the following article summarize some recent developments and unsolved issues about this major signal transduction cascade that links calcium-mobilizing hormone receptors to the regulation of endocrine cell function.

Regulation of 1,4,5-IP3, 1,3,4,5-IP4 and IP6 binding sites following entorhinal cortex lesions in rat brain

A lesion of the entorhinal cortex produces a loss of more than 80% of the synapses in the outer molecular layer of the hippocampus in the rat. However, this synaptic loss is transient. Beginning a few days after denervation, new synapses are formed, virtually replacing the lost inputs within two months. Synaptic remodelling induced by entorhinal cortex lesion is associated with specific modifications of various neurotransmitters, hormones and growth factors. Many of these substances act at membrane bound-receptors to induce the hydrolysis of phosphatidylinositols generating various inositol phosphates. Some of the key members of this family include inositol 1,4,5-trisphosphate, inositol 1,3,4,5-tetrakisphosphate and inositol hexakisphosphate which are all associated with the maintenance Ca2+ homeostasis. To investigate the potential roles and/or alterations of inositol phosphates in entorhinal cortex lesions-induced neuronal plasticity, we quantified specific receptor sites for inositol 1,4,5-trisphosphate, inositol 1,3,4,5-tetrakisphosphate and inositol hexakisphosphate using their respective tritiated ligands, at different periods post-lesion corresponding to the degenerative and subsequent reinnervation phases. [3H]inositol 1,4,5-trisphosphate binding sites are maximally increased (30%) between two and eight days post-lesion in the hippocampal formation on both sides of the lesion. In the cortex, [3H]inositol 1,4,5-trisphosphate binding increased also bilaterally following the lesion. Changes in [3H]inositol 1,3,4,5-tetrakisphosphate binding are delayed and reduced (20% increase) in magnitude compared to these seen for [3H]inositol 1,4,5-trisphosphate binding. The maximal peak in [3H]inositol 1,3,4,5-tetrakisphosphate binding is observed between eight and 14 days after the lesion in the hippocampal formation and the cortex.(ABSTRACT TRUNCATED AT 250 WORDS)

Siderophore activity of myo-inositol hexakisphosphate in Pseudomonas aeruginosa

myo-Inositol hexakisphosphate (InsP6), which is found in soil and most, if not all, plant and animal cells, has been estimated to have an affinity for Fe3+ in the range of 10(25) to 10(30) M-1. In this report, we demonstrate that the Fe-InsP6 complex has siderophore activity and is able to reverse the iron-restricted growth inhibition of Pseudomonas aeruginosa by ethylene diamine di(o-hydroxyphenyl)acetic acid. With 55Fe-InsP6 in transport studies, iron uptake is strongly iron regulated, being repressed after growth in iron-replete conditions and inhibited by treatment with potassium cyanide and carbonyl cyanide m-chlorophenylhydrazone. The kinetics of iron transport revealed a Km of 100 nM. Self-displacement of binding of [3H]InsP6 to isolated membranes by InsP6 revealed a single class of binding sites (Kd = 143 +/- 6 nM; Hill coefficient, 1.1 +/- 0.1). The binding of [3H]InsP6 to membranes was not dependent on whether cells had been grown under conditions of high or low iron concentrations. We believe that this is the first report of inositol polyphosphate activity in prokaryotic cells.

Differential localization and pH dependency of phosphoinositide 1,4,5-IP3, 1,3,4,5-IP4 and IP6 receptors in rat and human brains

It is well established that the inositol lipids mediate signal transduction in several cellular populations. Many neurotransmitters, hormones and growth factors act at plasma membrane receptors to induce the hydrolysis of phosphatidylinositols and hence the generation of various inositol phosphates (IP). The best known member of this family is 1,4,5-IP3, which is associated with the release of Ca2+ from intracellular pools. It has also been proposed that two others inositides, 1,3,4,5-IP4 and IP6, may be involved in Ca2+ homeostasis. In order to study the possible relevance of these various inositides in neuronal tissues, we have localized the respective receptors in rat and human brain under both acidic and basic pH conditions. In the hippocampal formation, [3H]1,3,4,5-IP4 binding sites are concentrated in the hilus and the molecular layer while a clearly different pattern of distribution is seen for [3H]1,4,5-IP3, its highest concentration of labelling being concentrated in the oriens and radiatum laminae. This contrasting profile of distribution is also observed in other brain areas such as the caudate-putamen, the septo-hippocampal area, and the molecular and granular layers of the cerebellum. Moreover, while highest amounts of specific [3H]1,4,5-IP3 binding are obtained at pH 8.5, the opposite is found for [3H]1,3,4,5-IP4, with high binding levels seen under acidic conditions. [3H]IP6 binding sites are broadly distributed with specific labelling concentrated in areas enriched with neuronal perikarya such as the granular cell layer of the dentate gyrus, the pyramidal cell layers of the hippocampus and the granular cell layer of the cerebellum.(ABSTRACT TRUNCATED AT 250 WORDS)

The inositol trisphosphate receptor gene family: implications for normal and abnormal brain function

1. The phosphatidyl inositol (PI) second messenger system has been extensively investigated in the past decade. This complex pathway results in the production of two second messengers, one of which, inositol 1,4,5-trisphosphate, will be the focus of this review. 2. The intracellular receptor for this second messenger (IP3R) has been purified, reconstituted and extensively characterized in both brain and peripheral tissues. 3. Localization and functional studies show that IP3 binding causes the receptor to release portions of the intracellular calcium stores. 4. Multiple modulators of the receptor have been identified, including pH, calcium concentration, adenine nucleotide concentration and phosphorylation. 5. The cDNA for this molecule has been cloned from a number of sources. Studies of the molecular structure of the receptor have revealed additional levels of complexity including multiple alternative splicing events in the initially cloned cerebellar (Type I) receptor, as well as the existence of highly related but distinct cDNAs which likely reflect a gene family. 6. There is suggestive evidence linking the PI system, and thus the IP3R, to bipolar disorder and the actions of lithium.

Novel inositol containing phospholipids and phosphates: their synthesis and possible new roles in cellular signalling

Details of the widely employed PtdIns(4,5)P2 hydrolysis receptor-stimulated signalling pathway continue to be elucidated rapidly. However, it has recently become apparent that numerous other inositol lipids and phosphates are widespread and are likely to have important cellular functions. In this review, we focus particularly on three rapidly progressing areas: the synthesis and possible functions of 3-phosphorylated inositol lipids, particularly phosphatidylinositol 3,4,5-trisphosphate; the roles of inositol 1,4,5-trisphosphate and inositol 1,3,4,5-tetrakisphosphate in coordinating intracellular Ca2+ mobilization and Ca2+ influx in stimulated cells; and the metabolism and possible functions of other inositol polyphosphates and of inositol polyphosphate pyrophosphates.