A presynaptic inositol-5-phosphatase

Clathrin-coated vesicle formation and protein sorting: an integrated process

Clathrin-coated vesicles were the first discovered and remain the most extensively characterized transport vesicles. They mediate endocytosis of transmembrane receptors and transport of newly synthesized lysosomal hydrolases from the trans-Golgi network to the lysosome. Cell-free assays for coat assembly, membrane binding, and coated vesicle budding have provided detailed functional and structural information about how the major coat constituents, clathrin and the adaptor protein complexes, interact with each other, with membranes, and with the sorting signals found on cargo molecules. Coat constituents not only serve to shape the budding vesicle, but also play a direct role in the packaging of cargo, suggesting that protein sorting and vesicle budding are functionally integrated. The functional interplay between the coated vesicle machinery and its cargo could ensure sorting fidelity and packaging efficiency and might enable modulation of vesicular trafficking in response to demand.

The inositol 5'-phosphatase SHIP binds to immunoreceptor signaling motifs and responds to high affinity IgE receptor aggregation

Immunoreceptors such as the high affinity IgE receptor, FcepsilonRI, and T-cell receptor-associated proteins share a common motif, the immunoreceptor tyrosine-based activation motif (ITAM). We used the yeast tribrid system to identify downstream effectors of the phosphorylated FcepsilonRI ITAM-containing subunits beta and gamma. One novel cDNA was isolated that encodes a protein that is phosphorylated on tyrosine, contains a Src-homology 2 (SH2) domain, inositolpolyphosphate 5-phosphatase activity, three NXXY motifs, several proline-rich regions, and is called SHIP. Mutation of the conserved tyrosine or leucine residues within the FcepsilonRI beta or gamma ITAMs eliminates SHIP binding and indicates that the SHIP-ITAM interaction is specific. SHIP also binds to ITAMs from the CD3 complex and T cell receptor zeta chain in vitro. SHIP protein possesses both phosphatidylinositol-3,4,5-trisphosphate 5'-phosphatase and inositol-1,3,4,5-tetrakisphosphate 5'-phosphatase activity. Phosphorylation of SHIP by a protein-tyrosine kinase, Lck, results in a reduction in enzyme activity. FcepsilonRI activation induces the association of several tyrosine phosphoproteins with SHIP. SHIP is constitutively tyrosine-phosphorylated and associated with Shc and Grb2. These data suggest that SHIP may serve as a multifunctional linker protein in receptor activation.

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.

Endocytosis and molecular sorting

Endocytosis in eukaryotic cells is characterized by the continuous and regulated formation of prolific numbers of membrane vesicles at the plasma membrane. These vesicles come in several different varieties, ranging from the actin-dependent formation of phagosomes involved in particle uptake, to smaller clathrin-coated vesicles responsible for the internalization of extracellular fluid and receptor-bound ligands. In general, each of these vesicle types results in the delivery of their contents to lysosomes for degradation. The membrane components of endocytic vesicles, on the other hand, are subject to a series of highly complex and iterative molecular sorting events resulting in their targeting to specific destinations. In recent years, much has been learned about the function of the endocytic pathway and the mechanisms responsible for the molecular sorting of proteins and lipids. This review attempts to integrate these new concepts with long-established views of endocytosis to present a more coherent picture of how the endocytic pathway is organized and how the intracellular transport of internalized membrane components is controlled. Of particular importance are emerging concepts concerning the protein-based signals responsible for molecular sorting and the cytosolic complexes responsible for the decoding of these signals.

Tissue-specific alternative splicing generates two synaptojanin isoforms with differential membrane binding properties

Synaptojanin is an Src homology 3 domain-binding inositol 5-phosphatase that is thought to function in synaptic vesicle endocytosis. It is encoded by a cDNA with two open reading frames separated by an in-frame stop codon. The first open reading frame encodes a 145-kDa form of the protein, whereas a 170-kDa isoform appears to be composed of both open reading frames and contains additional Src homology 3 domain-binding consensus sequences. Here, we demonstrate that the two synaptojanin isoforms are generated by the alternative use of an exon containing the stop codon. Whereas the 145-kDa isoform is highly enriched in adult brain, the 170-kDa isoform is excluded from this tissue and has a widespread distribution in non-neuronal cells. Unlike the 145-kDa isoform, which can be removed from membranes by a low salt wash, the 170-kDa isoform remains membrane-associated, even in the presence of 1 salt. Further, the 170-kDa form, but not the 145-kDa form, can be isolated from membranes as part of a large molecular weight complex. These properties may allow the 170-kDa isoform of synaptojanin to play a unique and perhaps more general role in endocytosis as compared with the 145-kDa isoform.

Dynamin, endocytosis and intracellular signalling (review)

Dynamin is a neuronal phosphoprotein and a GTPase enzyme which mediates late stages of endocytosis in both neural and non-neural cells. Current knowledge about dynamin is reviewed with particular emphasis on its structure and regulation with respect to phosphorylation, protein-protein interactions and phospholipid binding. The major themes are the biochemical regulation of dynamin, its effects on dynamin's GTPase activity and how this might relate to assembling the 'fission ring' that brings about vesicle retrieval. Dynamin I is an isoform of the enzyme primarily located in the central and peripheral nervous systems, where it is enriched in areas of abundant synaptic contacts. Dynamin I undergoes protein-protein interactions via its proline-rich domain at the C-terminus and these can elevate its N-terminal GTPase activity. Dynamin I interacts with multiple proteins in the nerve terminal, including SH3 domain-containing proteins such as amphiphysin and potentially with other proteins such as betagamma subunits. These regulate its role in endocytosis by targeting dynamin I to specific subcellular locations of retrieval. Dynamin I is phosphorylated in vivo by PKC and dephosphorylated on depolarization and calcium influx into nerve terminals in parallel with the coupled events of exocytosis and endocytosis. In late stages of synaptic vesicle retrieval dynamin I undergoes stimulated assembly into a collar, or fission ring, that surrounds the neck of recycling synaptic vesicles. Activation of GTP hydrolysis probably then generates the free synaptic vesicle, which can be refilled with neurotransmitters. This targeting and assembly may involve sequential steps including recruitment of AP-2 to synaptotagmin on the synaptic vesicle, and recruitment of amphiphysin, dynamin I, and synaptojanin. In addition to synaptic vesicle retrieval, dynamin has been associated with intracellular events mediated by growth factor receptors, insulin receptors and the beta-adrenergic receptor. This is likely to reflect targeting of these receptors for endocytosis soon after their activation. However, does it also suggest a broader role for dynamin in other aspects of intracellular signalling pathways?

Tyrosine phosphorylation modulates the activity of clostridial neurotoxins

Clostridial neurotoxins' metalloprotease domain selectively cleaves proteins implicated in the process of synaptic vesicle fusion with the plasma membrane and, accordingly, blocks neurotransmitter release into the synaptic cleft. Here we investigate the potential modulation of these neurotoxins by intracellular cascades triggered by environmental signals, which in turn may alter its activity on target substrates. We report that the nonreceptor tyrosine kinase Src phosphorylates botulinum neurotoxins A, B, and E and tetanus neurotoxin. Protein tyrosine phosphorylation of serotypes A and E dramatically increases both their catalytic activity and thermal stability, while dephosphorylation reverses the effect. This suggests that the biologically significant form of the neurotoxins inside neurons is phosphorylated. Indeed, in PC12 cells in which tyrosine kinases such as Src and PYK2 are highly abundant, stimulation by membrane depolarization in presence of extracellular calcium induces rapid and selective tyrosine phosphorylation of internalized light chain, the metalloprotease domain, of botulinum toxin A. These findings provide a conceptual framework to connect intracellular signaling pathways involving tyrosine kinases, G-proteins, phosphoinositides, and calcium with the action of botulinum neurotoxins in abrogating vesicle fusion and neurosecretion.

ARF and PITP restore GTP gamma S-stimulated protein secretion from cytosol-depleted HL60 cells by promoting PIP2 synthesis

BACKGROUND

In many cell types, including neutrophils and HL60 cells, there is an absolute requirement for a GTP-dependent step to elicit Ca(2+)-regulated secretion. Neutrophils and HL60 cells secrete lysosomal enzymes from azurophilic granules; this secretion is inhibited by 1% ethanol, indicating that phosphatidate (PA) produced by phospholipase D (PLD) activity may be involved. PLD can use primary alcohols in preference to water during the hydrolytic step, generating the corresponding phosphatidylalcohol instead of PA, its normal product. As ARF (ADP-ribosylation factor) proteins regulate PLD activity and are implicated in constitutive vesicular traffic, we have investigated whether ARF is also required for GTP-dependent secretion in HL60 cells.

RESULTS

We have used a cell-permeabilization protocol that allows HL60 cells to become refractory to stimulation with GTP gamma S plus 10 microM Ca2+ with regard to secretion and PLD activity. Permeabilization with streptolysin O for 10 minutes permitted the loss of freely diffusable cytosolic proteins, including ARF proteins. Fractions derived from brain cytosol, enriched in ARF proteins, restored secretory function and PLD activity. The major contaminating protein present in these ARF-enriched fractions was identified as phosphatidylinositol transfer protein (PITP). Unexpectedly, PITP was also found to restore GTP gamma S-dependent secretion. Restoration of secretory function was characterized using recombinant proteins, rARF1 and rPITP alpha and rPITP beta. The rARF1 protein restored both secretory function and PLD activity, whereas PITP only restored secretory function. However, both ARF and PITP were capable of stimulating phosphatidylinositol bis phosphate (PIP2) synthesis.

CONCLUSIONS

ARF and PITP restore secretory function in cytosol-depleted cells when stimulated with GTP gamma S plus Ca2+. We have previously shown that PITP participates in the synthesis of PIP2. In comparison, ARF1 activates PLD, producing PA, which is a known activator of phosphatidylinositol-4-phosphate 5 kinase, the enzyme responsible for PIP2 synthesis. We propose that ARF and PITP both restore exocytosis by a common mechanism-promoting PIP2 synthesis.

Inositols do it all

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A role for ADP-ribosylation factor 1, but not COP I, in secretory vesicle biogenesis from the trans-Golgi network

A synthetic N-myristoylated peptide corresponding to the amino-terminal domain of ADP-ribosylation factor 1 (ARF1) markedly increases, in a cell-free system using post-nuclear supernatant from PC12 cells, the biogenesis of constitutive secretory vesicles and immature secretory granules from the trans-Golgi network (TGN). The related N-myristoylated ARF4 peptide only weakly stimulates, and the non-myristoylated ARF1 and ARF4 peptides inhibit, the biogenesis of these secretory vesicles. In a modified cell-free system using TGN membranes, coatomer-depleted cytosol supports the biogenesis of TGN-derived secretory vesicles to the same extent as control cytosol. These results suggest a role for ARF1, but not the COP I coat, in secretory vesicle biogenesis from the TGN, possibly via the activation of phospholipase D.

Multiple forms of an inositol polyphosphate 5-phosphatase form signaling complexes with Shc and Grb2

BACKGROUND

Shc and Grb2 form a complex in cells in response to growth factor stimulation and link tyrosine kinases to Ras during the resulting signaling process. Shc and Grb2 each contain domains that mediate interactions with other unidentified intracellular proteins. For example, the Shc PTB domain binds to 130 kDa and 145 kDa tyrosine-phosphorylated proteins in response to stimulation of cells by growth factors, cytokines and crosslinking of antigen receptors. The Grb2 SH3 domains bind to an unidentified 116 kDa protein in T cells. We have identified three proteins, of 110 kDa, 130 kDa and 145 kDa, as a new family of molecules encoded by the same gene. In vivo studies show that these proteins form signal transduction complexes with Shc and with Grb2.

RESULTS

The 130 kDa and 145 kDa tyrosine-phosphorylated proteins that associate with the Shc PTB domain were purified by conventional chromatographic methods. Partial peptide and cDNA sequences corresponding to these proteins, termed SIP-145 and SIP-130 (SIP for signaling inositol polyphosphate 5-phosphatase), identified them as SH2 domain-containing products of a single gene and as members of the inositol polyphosphate 5-phosphatase family. The SIP-130 and SIP-145 proteins and inositol polyphosphate 5-phosphatase activity associated with Shc in vivo in response to B-cell activation. By using an independent approach, expression cloning, we found that the Grb2 SH3 domains bind specifically to SIP-110, a 110 kDa splice variant of SIP-145 and SIP-130, which lacks the SH2 domain. The SIP proteins hydrolyzed phosphatidylinositol (3,4,5)-trisphosphate (PtdIns (3,4,5)-P3) and Ins (1,3,4,5)-P4, but not PtdIns (4,5)-P2 or Ins (1,4,5)-P3.

CONCLUSIONS

These findings strongly implicate the inositol polyphosphate 5-phosphatases in Shc- and Grb2-mediated signal transduction. Furthermore, SIP-110, SIP-130 and SIP-145 prefer 3-phosphorylated substrates, suggesting a link to the phosphatidylinositol 3-kinase signaling pathway.

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.