Cloning, characterization, and expression of a novel Zn2+-binding FYVE finger-containing phosphoinositide kinase in insulin-sensitive cells

Localized PtdIns 3,5-P2synthesis to regulate early endosome dynamics and fusion

Perturbations in the intracellular PtdIns 3,5-P2pool or the downstream transmission of PtdIns 3,5-P2signals often result in a gradual development of gross morphological changes in the pleiomorphic multivesicular endosomes, culminating with the appearance of cytoplasmic vacuoles. To identify the onset of PtdIns 3,5-P2functional requirements along the endocytic system, in this study we characterized the morphological changes associated with early expression of the dominant-negative kinase-deficient form (K1831E) of the PtdIns 3,5-P2-producing kinase PIKfyve, before the formation of cytoplasmic vacuoles in transfected COS cells. Enlarged PIKfyveK1831E-positive vesicles co-localizing with dilated EEA1- and Rab5aWT-positive perinuclear endosomes were observed (WT, wild type). This was dependent on the presence of active forms of Rab5 and the generation of PtdIns 3-P-enriched platforms on early endosomess. Because PIKfyveWTdid not substantially colocalize with EEA1- or Rab5-positive endosomes in COS cells, the dynamic PIKfyve-catalyzed PtdIns 3-to-PtdIns 3,5-P2switch was suggested to drive away PIKfyveWTfrom early endosomes toward later compartments. Late endosomes/lysosomes marked by LAMP1 or Rab7 were dislocated from their typical perinuclear position upon PIKfyveK1831Eearly expression. Cytosols derived from cells stably expressing PIKfyveK1831Estimulated endosome fusion in vitro, whereas PIKfyveWT-enriched cytosols had the opposite effect, consistent with PtdIns 3,5-P2production negatively regulating the endosome fusion. Together, our data indicate that PtdIns 3,5-P2defines specific endosome platforms at the onset of the degradation pathway to regulate the complex process of membrane remodeling and dynamics.

Cloning and subcellular localization of a human phosphatidylinositol 3-phosphate 5-kinase, PIKfyve/Fab1

Yeast Fab1 is a phosphatidylinositol 3-phosphate 5-kinase involved in endocytic membrane traffic and vacuole homeostasis. Here we have cloned and sequenced the cDNA for the human homologue of Fab1, PIKfyve. The cDNA has an open reading frame of 6294 bp and encodes a 2098-amino acid protein with a calculated molecular mass of 237 kDa, containing a phosphatidylinositol 3-phosphate-binding FYVE domain, a DEP domain, a chaperonin-like domain, and a phosphoinositide kinase domain. The human genome contains a single PIKfyve gene, which comprises 38 exons on chromosomal locus 2q34. PIKfyve is expressed as a single molecular species in a number of human cell lines derived from different tissues. The exogenously expressed protein was found to localize mainly to early endosomes containing two other FYVE domain proteins, EEA1 and Hrs. The endosomal membrane localization of PIKfyve was studied in more detail by examining cells transfected with a constitutively active mutant of the small GTPase Rab5, whose expression results in the enlargement of early endosomes. We show that PIKfyve is distributed in microdomains that are distinct from those occupied by EEA1 and Hrs.

Acquisition of Unprecedented Phosphatidylinositol 3,5-Bisphosphate Rise in Hyperosmotically Stressed 3T3-L1 Adipocytes, Mediated by ArPIKfyve-PIKfyve Pathway

Unlike yeast, where hyperosmotic stress induces a dramatic increase in phosphatidylinositol 3,5-bisphosphate (PtdIns 3,5-P(2)) synthesis, in mammalian cells, although activating a complex array of signaling events, hyperosmotic stress fails to up-regulate PtdIns 3,5-P(2), indicating the PtdIns 3,5-P(2) pathway is not involved in mammalian osmo-protective responses. Here we report an unexpected and marked PtdIns 3,5-P(2) increase in response to hyperosmotic stress in differentiated 3T3-L1 adipocytes. Because this effect was not observed in the precursor preadipocytes, a specific role during acquisition of the adipocyte phenotype and transition into insulin-responsive cells could be suggested. However, acute insulin action did not result in a measurable PtdIns 3,5-P(2) rise, indicating the PtdIns 3,5-P(2) pathway is a specific hyperosmotically activated signaling cascade selectively operating in differentiated 3T3-L1 adipocytes. Hyperosmolarity activates different components of several kinase cascades, including p38 mitogen-activated protein and tyrosine kinases, but these appear to be separate from the activated PtdIns 3,5-P(2) pathway. Because PtdIns 3,5-P(2) is primarily produced by PIKfyve-catalyzed synthesis and requires the upstream activator hVac14 (called herein ArPIKfyve) that physically associates with and activates PIKfyve, we examined the contribution of ArPIKfyve-PIKfyve for the hyperosmotic stress-induced rise in PtdIns 3,5-P(2). Small interfering RNA-directed gene silencing to selectively deplete ArPIKfyve or PIKfyve in 3T3-L1 adipocytes determined the ArPIKfyve-PIKfyve axis fully accountable for the hyperosmotically activated PtdIns 3,5-P(2). Together these results reveal a previously uncharacterized PtdIns 3,5-P(2) pathway activated selectively in hyperosmotically stressed 3T3-L1 adipocytes and suggest a plausible role for PtdIns 3,5-P(2) in the osmo-protective response mechanism in this cell type.

A mammalian ortholog of Saccharomyces cerevisiae Vac14 that associates with and up-regulates PIKfyve phosphoinositide 5-kinase activity

Multivesicular body morphology and size are controlled in part by PtdIns(3,5)P(2), produced in mammalian cells by PIKfyve-directed phosphorylation of PtdIns(3)P. Here we identify human Vac14 (hVac14), an evolutionarily conserved protein, present in all eukaryotes but studied principally in yeast thus far, as a novel positive regulator of PIKfyve enzymatic activity. In mammalian cells and tissues, Vac14 is a low-abundance 82-kDa protein, but its endogenous levels could be up-regulated upon ectopic expression of hVac14. PIKfyve and hVac14 largely cofractionated, populated similar intracellular locales, and physically associated. A small-interfering RNA-directed gene-silencing approach to selectively eliminate endogenous hVac14 rendered HEK293 cells susceptible to morphological alterations similar to those observed upon expression of PIKfyve mutants deficient in PtdIns(3,5)P(2) production. Largely decreased in vitro PIKfyve kinase activity and unaltered PIKfyve protein levels were detected under these conditions. Conversely, ectopic expression of hVac14 increased the intrinsic PIKfyve lipid kinase activity. Concordantly, intracellular PtdIns(3)P-to-PtdIns(3,5)P(2) conversion was perturbed by hVac14 depletion and was elevated upon ectopic expression of hVac14. These data demonstrate a major role of the PIKfyve-associated hVac14 protein in activating PIKfyve and thereby regulating PtdIns(3,5)P(2) synthesis and endomembrane homeostasis in mammalian cells.

Phosphoinositides in Constitutive Membrane Traffic

Proteins that make, consume, and bind to phosphoinositides are important for constitutive membrane traffic. Different phosphoinositides are concentrated in different parts of the central vacuolar pathway, with phosphatidylinositol 4-phosphate predominate on Golgi, phosphatidylinositol 4,5-bisphosphate predominate at the plasma membrane, phosphatidylinositol 3-phosphate the major phosphoinositide on early endosomes, and phosphatidylinositol 3,5-bisphosphate found on late endocytic organelles. This spatial segregation may be the mechanism by which the direction of membrane traffic is controlled. Phosphoinositides increase the affinity of membranes for peripheral membrane proteins that function for sorting protein cargo or for the docking and fusion of transport vesicles. This implies that constitutive membrane traffic may be regulated by the mechanisms that control the activity of the enzymes that produce and consume phosphoinositides. Although the lipid kinases and phosphatases that function in constitutive membrane traffic are beginning to be identified, their regulation is poorly understood.

Homotypic secretory vesicle fusion induced by the protein tyrosine phosphatase MEG2 depends on polyphosphoinositides in T cells

Sec14p homology domains are found in a large number of proteins from plants, yeast, invertebrates, and higher eukaryotes. We report that the N-terminal Sec14p homology domain of the human protein tyrosine phosphatase PTP-MEG2 binds phosphatidylinositol-3,4,5-trisphosphate (PtdIns(3,4,5)P(3)) in vitro and colocalizes with this lipid on secretory vesicle membranes in intact cells. Point mutations that prevented PtdIns(3,4,5)P(3) binding abrogated the capacity of PTP-MEG2 to induce homotypic secretory vesicle fusion in cells. Inhibition of cellular PtdIns(3,4,5)P(3) synthesis also rapidly reversed the effect of PTP-MEG2 on secretory vesicles. Finally, we show that several different phosphoinositide kinases colocalize with PTP-MEG2, thus allowing for local synthesis of PtdIns(3,4,5)P(3) in secretory vesicle membranes. We suggest that PTP-MEG2 through its Sec14p homology domain couples inositide phosphorylation to tyrosine dephosphorylation and the regulation of intracellular traffic of the secretory pathway in T cells.

Endofin recruits TOM1 to endosomes

Endofin is an endosomal protein implicated in regulating membrane trafficking. It is characterized by the presence of a phosphatidylinositol 3-phosphate-binding FYVE domain positioned in the middle of the molecule. To determine its potential effectors or binding partners, we used the carboxyl-terminal half of endofin as bait to screen a human brain cDNA library in a yeast two-hybrid system. Three clones that encode TOM1 were recovered. TOM1 is a protein closely related to the VHS (VPS-27, Hrs, and STAM) domain-containing GGA family. Although the function of the GGAs in mediating Golgi to lysosomal trafficking is well established, the subcellular localization and function of TOM1 remain unknown. Glutathione S-transferase pull-down assays as well as co-immunoprecipitation experiments confirmed that the carboxyl-terminal half of endofin binds specifically to the carboxyl-terminal region of TOM1. Neither SARA nor Hrs, two other FYVE domain proteins, interact with this region of TOM1. Moreover, endofin does not interact with the analogous region of two other members of the TOM1 protein family, namely, TOM1-like 1 (TOM1-L1) or TOM1-like 2 (TOM1-L2). The carboxyl-terminal region of TOM1 was used as immunogen to generate TOM1-specific antibody. This antibody can distinguish TOM1 from the other family members as well as coimmunoprecipitate endogenous endofin. It also revealed the primarily cytosolic distribution of TOM1 in a variety of cell types by immunofluorescence analyses. In addition, sucrose density gradient analysis showed that both TOM1 and endofin can be detected in cellular compartments marked by the early endosomal marker EEA1. A marked recruitment of TOM1 to endosomes was observed in cells overexpressing endofin or its carboxyl-terminal fragment, indicating TOM1 to be an effector for endofin and suggesting a possible role for TOM1 in endosomal trafficking.

Active PIKfyve associates with and promotes the membrane attachment of the late endosome-to-trans-Golgi network transport factor Rab9 effector p40

PIKfyve, a kinase that displays specificity for phosphatidylinositol (PtdIns), PtdIns 3-phosphate (3-P), and proteins, is important in multivesicular body/late endocytic function. Enzymatically inactive PIKfyve mutants elicit enormous dilation of late endocytic structures, suggesting a role for PIKfyve in endosome-to-trans-Golgi network (TGN) membrane retrieval. Here we report that p40, a Rab9 effector reported previously to bind Rab9-GTP and stimulate endosome-to-TGN transport, interacts with PIKfyve as determined by yeast two-hybrid assays, glutathione S-transferase (GST) pull-down assays, and co-immunoprecipitation in doubly transfected HEK293 cells. The interaction engages the PIKfyve chaperonin domain and four out of the six C-terminally positioned kelch repeats in p40. Differential centrifugation in a HEK293 cell line, stably expressing PIKfyveWT, showed the membrane-associated immunoreactive p40 co-sedimenting with PIKfyve in the high speed pellet (HSP) fraction. Remarkably, similar analysis in a HEK293 cell line stably expressing dominant-negative kinase-deficient PIKfyveK1831E demonstrated a marked depletion of p40 from the HSP fraction. GST-p40 failed to specifically associate with the PIKfyve lipid products PtdIns 5-P and PtdIns 3,5-P2 in a liposome binding assay but was found to be an in vitro substrate of the PIKfyve serine kinase activity. A band with the p40 electrophoretic mobility was found to react with a phosphoserine-specific antibody mainly in the PIKfyveWT-containing fractions obtained by density gradient sedimentation of total membranes from PIKfyveWT-expressing HEK293 cells. Together these results identify the Rab9 effector p40 as a PIKfyve partner and suggest that p40-PIKfyve interaction and the subsequent PIKfyve-catalyzed p40 phosphorylation anchor p40 to discrete membranes facilitating late endosome-to-TGN transport.

Identification of mammalian Vps24p as an effector of phosphatidylinositol 3,5-bisphosphate-dependent endosome compartmentalization

Phosphatidylinositol 3,5-bisphosphate is a membrane lipid found in all eukaryotes so far studied but downstream effector proteins of this lipid have yet to be identified. Here we report the use of cDNA phage libraries in conjunction with synthetic biotinylated derivatives of phosphatidylinositol 3,5-bisphosphate in the identification of a mammalian phosphatidylinositol 3,5-bisphosphate-binding protein, mVps24p. This protein is orthologous to the Saccharomyces cerevisiae protein, Vps24p, a class-E vacuolar protein-sorting protein. Using in vitro liposome binding and competition assays, we demonstrate that mVps24p selectively binds to phosphatidylinositol 3,5-bisphosphate and phosphatidylinositol 3,4-bisphosphate in preference to other phosphoinositides tested. When expressed in cultured mammalian cells, full-length mVps24p is cytosolic. However, when cells expressing the full-length mVps24p are co-transfected with a mutated form of mVps4p (which is defective in ATP hydrolysis), or when a N-terminal construct of mVps24p is expressed, the class-E cellular phenotype with swollen vacuoles is induced and mVps24p is membrane-associated. Furthermore, the accumulation of the N-terminal mVps24p construct on the swollen endosomal membranes is abrogated when phosphatidylinositol 3,5-bisphosphate synthesis is blocked with wortmannin. These data provide the first direct link between phosphatidylinositol 3,5-bisphosphate and the protein machinery involved in the production of the class-E cellular phenotype. We hypothesize that accumulation of Vps24 on membranes occurs when membrane association (dependent on interaction of phosphatidylinositol 3,5-bisphosphate with the N-terminal domain of the protein) is uncoupled from membrane disassociation (driven by Vps4p).

PIKfyve controls fluid phase endocytosis but not recycling/degradation of endocytosed receptors or sorting of procathepsin D by regulating multivesicular body morphogenesis

The mammalian phosphatidylinositol (PtdIns) 5-P/PtdIns 3,5-P2-producing kinase PIKfyve has been implicated in maintaining endomembrane homeostasis in mammalian cells. To address the role of PIKfyve in trafficking processes, we examined the functioning of the biosynthetic, endocytic, and recycling pathways in stable human embryonic kidney 293 cell lines inducibly expressing the wild-type or kinase-defective dominant-negative form. PIKfyveWT or PIKfyveK1831E expression did not affect the processing and lysosomal targeting of newly synthesized procathepsin D. Likewise the rates of transferrin uptake/recycling or epidermal growth factor receptor degradation were not altered upon expression of either protein. In contrast, PIKfyveK1831E but not PIKfyveWT expression markedly impaired the late uptake of fluid phase marker horseradish peroxidase. Inspection of the organelle morphology by confocal microscopy with specific markers in COS cells transiently expressing PIKfyveK1831E showed the Golgi apparatus, end lysosomes, and the recycling compartment indistinguishable from nontransfected cells, despite the dramatic PIKfyveK1831E-induced endomembrane vacuolation. In contrast, we observed a striking effect on the late endocytic compartment, marked by disruption of the dextran-labeled perinuclear endosomal compartment and formation of dispersed enlarged vesicles. Electron microscopy identified the cytoplasmic vacuoles in the PIKfyveK1831E-expressing human embryonic kidney 293 cells as enlarged multivesicular body-like structures with substantially lower number of internal vesicles and membrane whorls. Together, these data indicate that PIKfyve selectively regulates the sorting and traffic of peripheral endosomes containing lysosomaly directed fluid phase cargo through controlling the morphogenesis and function of multivesicular bodies.

The phox homology (PX) domain-dependent, 3-phosphoinositide-mediated association of sorting nexin-1 with an early sorting endosomal compartment is required for its ability to regulate epidermal growth factor receptor degradation

Recent studies have shown that phox homology (PX) domains act as phosphoinositide-binding motifs. The majority of PX domains studied show binding to phosphatidylinositol 3-monophosphate (PtdIns(3)P), an association that allows the host protein to localize to membranes of the endocytic pathway. One issue, however, is whether PX domains may have alternative phosphoinositide binding specificities that could target their host protein to distinct subcellular compartments or allow their allosteric regulation by phosphoinositides other than PtdIns(3)P. It has been reported that the PX domain of sorting nexin 1 (SNX1) specifically binds phosphatidylinositol 3,4,5-trisphosphate (PtdIns(3,4,5)P(3)) (Zhong, Q., Lazar, C. S., Tronchere, H., Sato, T., Meerloo, T., Yeo, M., Songyang, Z., Emr, S. D., and Gill, G. N. (2002) Proc. Natl. Acad. Sci. U. S. A. 99, 6767-6772). In the present study, we have shown that whereas SNX1 binds PtdIns(3,4,5)P(3) in protein:lipid overlay assays, in liposomes-based assays, binding is observed to PtdIns(3)P and phosphatidylinositol 3,5-bisphosphate (PtdIns(3,5)P(2)) but not to PtdIns(3,4,5)P(3). To address the significance of PtdIns(3,4,5)P(3) binding, we examined the subcellular localization of SNX1 under conditions in which plasma membrane PtdIns(3,4,5)P(3) levels were significantly elevated. Under these conditions, we failed to observe association of SNX1 with this membrane. However, consistent with the binding to PtdIns(3)P and PtdIns(3,5)P(2) being of more physiological significance was the observation that the association of SNX1 with an early endosomal compartment was dependent on a 3-phosphoinositide-binding PX domain and the presence of PtdIns(3)P on this compartment. Finally, we have shown that the PX domain-dependent/early endosomal association of SNX1 is important for its ability to regulate the targeting of internalized epidermal growth factor receptor for lysosomal degradation.

Requirement for PIKfyve enzymatic activity in acute and long-term insulin cellular effects

PIKfyve is a phosphoinositide 5-kinase that can also act as a protein kinase. PIKfyve's role in acute insulin action has been suggested on the basis of its association with the insulin stimulatable phosphatidylinositol-3-kinase and the ability of acute insulin to recruit and phosphorylate PIKfyve on intracellular membranes of 3T3-L1 adipocytes. Here we have examined several classical insulin-regulated long- and short-term responses in insulin-sensitive cells expressing high levels of either active PIKfyve or kinase-dead mutants with a dominant-negative effect. Up-regulation of PIKfyve protein expression was documented in the early stages of differentiation of cultured 3T3-L1 fibroblasts into adipocytes and a kinase-dead mutant, PIKfyveDeltaK, introduced into the preadipocyte stage profoundly delayed the hormone-induced adipogenesis. Next, insulin-induced mitogenesis was markedly inhibited in HEK293 stable cell lines, inducibly expressing the dominant-negative kinase-dead PIKfyve(K1831E) mutant but not in cells expressing PIKfyve(WT). Similarly, expression of the dominant negative mutants PIKfyve(K1831E) or PIKfyveDeltaK strongly inhibited insulin-stimulated translocation of GLUT4 in 3T3-L1 adipocytes, or GLUT1-mediated glucose uptake in Chinese hamster ovary T cells expressing the human insulin receptor. Expression of PIKfyveDeltaK and PIKfyve(WT) in Chinese hamster ovary T cells decreased or increased, respectively, insulin-stimulated Akt phosphorylation at Ser473 but not at Thr308. Furthermore, a powerful inhibition of PIKfyve was documented at a very low concentration (ID(50) = 6 micro M) of the cell-permeable kinase inhibitor curcumin. When introduced into 3T3-L1 adipocytes, curcumin markedly inhibited insulin-induced GLUT4 translocation and glucose transport. Together these data indicate that PIKfyve enzymatic activity functions as a positive regulatory intermediate in insulin acute and long-term biological responses and identify Ser473 in Akt as one potential PIKfyve downstream target.

PIKfyve Kinase and SKD1 AAA ATPase define distinct endocytic compartments. Only PIKfyve expression inhibits the cell-vacoulating activity of Helicobacter pylori VacA toxin

The mammalian phosphatidylinositol (PtdIns)- 5-P/PtdIns-3,5-P(2)-producing kinase PIKfyve and AAA ATPase SKD1, as their yeast counterparts, are implicated in the formation and function of multivesicular bodies/late endosomes. Point mutations inhibiting the enzyme activities convert PIKfyve and SKD1 into dominant-negative mutants (PIKfyve(K1831E) and SKD1(E235Q)), whose expression in cells of kidney origin induces a vacuolation phenotype. This phenotype closely resembles the changes in late endosomal-lysosomal morphology that occur following cell exposure to the vacuolating cytotoxin (VacA) from Helicobacter pylori. Here we have examined the possible functional relationship between PIKfyve and SKD1 as well as the role of these enzymes in the molecular mechanism of VacA-induced intracellular vacuolation. When co-expressed in COS cells, PIKfyve(WT) reduced SKD1(E235Q)dependent vacuole formation, whereas SKD1(WT) did not alter the vacuolation induced by PIKfyve(K1831E). In addition, SKD1(E235Q) disrupted the normal distribution of PIKfyve(WT). Expression of PIKfyve(WT) in COS and HEK293 cells inhibited vacuolation induced by subsequent intoxication with VacA, and microinjection of the PIKfyve lipid product PtdIns-3,5-P(2) produced a similar inhibitory effect. In contrast, in COS cells expressing SKD1(WT), VacA induced the formation of characteristic vacuoles with an efficiency similar to that in the control cells. These observations demonstrate that, although PIKfyve and SKD1 are functionally related, only PIKfyve regulates VacA action, and suggest that the inhibition of PIKfyve PtdIns-3,5-P(2)-producing activity is a key molecular event in VacA-induced cellular vacuolation.

Phosphatidylinositol 3,5-bisphosphate: metabolism and function

Phosphatidylinositol 3,5-bisphosphate (PtdIns(3,5)P(2)) is the most recently discovered PtdInsP(2) isomer. It is likely that PtdIns(3,5)P(2) is ubiquitous to eukaryotes, and that it performs a number of important cellular functions, including vacuolar homeostasis, retrograde trafficking from the vacuole, and protein sorting at the multivesicular body. This review describes the metabolism of PtdIns(3,5)P(2) and discusses the potential functions for this lipid.

Phosphatidylinositol 5-phosphate biosynthesis is linked to PIKfyve and is involved in osmotic response pathway in mammalian cells

The cellular functions, regulation and enzymology of phosphatidylinositol (PtdIns) 5-P, the newest addition to the family of phosphoinositides (PI), are still elusive. Whereas a kinase that uses PtdIns-5-P as an intracellular substrate has been assigned, a kinase that produces it remained to be identified. Here we report that PIKfyve, the enzyme found to synthesize PtdIns-5-P in vitro and PtdIns-3,5-P(2) in vitro and in vivo, is responsible for PtdIns-5-P production in a cellular context. Evidence is based on examination of two groups of cell types by two independent approaches. First, [(32)P]orthophosphate-labeled cells (Sf9, 3T3-L1 fibroblasts, and 3T3-L1 adipocytes) that show a high pressure liquid chromatography (HPLC)-detectable peak of the PtdIns-5-P head group at basal conditions demonstrated a 20-50% increase in radioactive PtdIns-5-P amounts upon expression of PIKfyve(WT). Second, cell types (HEK293), in which the basal levels of radioactive PtdIns-5-P were undetectable by HPLC head group analysis, demonstrated higher in vitro type II PIP kinase-directed conversion of the endogenous PtdIns-5-P pool into PtdIns-4,5-P(2), when induced to express PIKfyve(WT). Conversely, a decrease by 60% in the conversion of PtdIns-5-P to PtdIns-4,5-P(2) was associated with induced expression of the dominant-negative kinase-deficient PIKfyve(K1831E) mutant in HEK293 cells. When 3T3-L1 fibroblasts and 3T3-L1 adipocytes were subjected to osmotic shock, levels of PtdIns-5-P measured by both approaches were found to decrease profoundly upon a hypo-osmotic stimulus. Together, these results identify PIKfyve as an enzyme responsible for PtdIns-5-P biosynthesis and indicate a role for PtdIns-5-P in osmotic response pathways in mammalian cells.

Phagosome maturation: aging gracefully

Foreign particles and apoptotic bodies are eliminated from the body by phagocytic leucocytes. The initial stage of the elimination process is the internalization of the particles into a plasma membrane-derived vacuole known as the phagosome. Such nascent phagosomes, however, lack the ability to kill pathogens or to degrade the ingested targets. These properties are acquired during the course of phagosomal maturation, a complex sequence of reactions that result in drastic remodelling of the phagosomal membrane and contents. The determinants and consequences of the fusion and fission reactions that underlie phagosomal maturation are the topic of this review.

Phosphoinositide-binding domains: Functional units for temporal and spatial regulation of intracellular signalling

Inositol phospholipid (phosphoinositide) is a versatile lipid characterized by its isomer-specific localization, as well as its molecular diversity attributable to phosphorylation events. Phosphoinositides act as signal mediators in a spatially and temporally controlled manner. Information about the timing and location of their production is received by phosphoinositide-binding proteins and transmitted to multiple lines of intracellular events such as signal transduction, cytoskeletal rearrangement, and membrane trafficking. Among those proteins, a significant portion possess globular structural units, called domains, which are specialized for phosphoinositide binding. The pleckstrin homology (PH) domain was the first phosphoinositide-binding domain identified. It contains the largest number of members and is associated with the formation of signalling complexes on the plasma membrane. Recent studies identified other novel phosphoinositide-binding domains (Fab1p, YOTB, Vps27p, EEA1 (FYVE), Phox homology (PX), and epsin N-terminal homology (ENTH)), thus extending our knowledge of how the functional versatility of phosphoinositides is achieved.

Defective Hyphal induction of a Candida albicans phosphatidylinositol 3-phosphate 5-kinase null mutant on solid media does not lead to decreased virulence

A phosphatidylinositol 3-phosphate [PI(3)P] 5-kinase gene (CaFAB1) of the most important human pathogenic yeast, Candida albicans, was cloned and sequenced. An open reading frame was detected which encodes a 2,369-amino-acid protein with a calculated molecular mass of 268 kDa and a relative isoelectric point of 6.76. This protein exhibits 38% overall amino acid sequence identity with Saccharomyces cerevisiae Fab1p. We localized the CaFAB1 gene on chromosome R. To determine the influence of the PI(3)P 5-kinase CaFab1p on processes involved in C. albicans morphogenesis and pathogenicity, we sequentially disrupted both copies of the gene. Homozygous deletion of C. albicans CaFAB1 resulted in a mutant strain which exhibited defects in morphogenesis. A Cafab1 null mutant had enlarged vacuoles, an acidification defect, and increased generation times and was unable to form hyphae on different solid media. The sensitivities to hyperosmotic and high-temperature stresses, adherence, and virulence compared to those of wild-type strain SC5314 were not affected.

Synthesis and function of 3-phosphorylated inositol lipids

The 3-phosphorylated inositol lipids fulfill roles as second messengers by interacting with the lipid binding domains of a variety of cellular proteins. Such interactions can affect the subcellular localization and aggregation of target proteins, and through allosteric effects, their activity. Generation of 3-phosphoinositides has been documented to influence diverse cellular pathways and hence alter a spectrum of fundamental cellular activities. This review is focused on the 3-phosphoinositide lipids, the synthesis of which is acutely triggered by extracellular stimuli, the enzymes responsible for their synthesis and metabolism, and their cell biological roles. Much knowledge has recently been gained through structural insights into the lipid kinases, their interaction with inhibitors, and the way their 3-phosphoinositide products interact with protein targets. This field is now moving toward a genetic dissection of 3-phosphoinositide action in a variety of model organisms. Such approaches will reveal the true role of the 3-phosphoinositides at the organismal level in health and disease.

Functional dissection of lipid and protein kinase signals of PIKfyve reveals the role of PtdIns 3,5-P2 production for endomembrane integrity

PIKfyve enzymatic activity is required in maintaining late endocytic membrane integrity. PIKfyve is a dual specificity enzyme that phosphorylates phosphatidylinositol (PtdIns) and PtdIns 3-P at the 5-hydroxyl and unidentified endogenous protein substrate(s). To determine which of these activities (lipid versus protein kinase activity) is responsible for endomembrane homeostasis we analyzed a double mutant PIKfyve(K1999E/K2000E). These substitutions in the putative lipid-substrate activation loop nearly completely abrogated the lipid kinase activity without any significant effect on the protein kinase activity of PIKfyve(K1999E/K2000E). Expression of PIKfyve(K1999E/K2000E) in COS cells induced a dramatic dominant-negative effect in the form of endomembrane swelling and vacuolation. In addition, the lipid-substrate specificity of PIKfyve was modified by introducing single mutations in Lys-1999 or Lys-2000. This yielded proteins with preferentially abrogated synthesis of PtdIns 5-P (PIKfyve(K2000E)) or PtdIns 3,5-P(2) (PIKfyve(K1999E)), of which only the PIKfyve(K1999E) mutant induced the characteristic endomembrane defects upon cell transfection. Furthermore, phosphoinositide microinjection into cells demonstrated a selective ability of PtdIns 3,5-P(2) to correct the endomembrane defects induced by the dominant-negative PIKfyve lipid kinase-deficient mutants. Thus, PtdIns 3,5-P(2) production by PIKfyve is crucial for endomembrane integrity, and Lys-1999 most likely directs the PIKfyve interactions with the 3-phosphate group in PtdIns 3-P.

Phosphatidylinositol 3-phosphate-interacting domains in PIKfyve. Binding specificity and role in PIKfyve. Endomenbrane localization

PIKfyve is a phosphatidylinositol (PtdIns) 3-phosphate (P)-metabolizing enzyme, which, in addition to a C-terminally positioned catalytic domain, harbors several evolutionarily conserved domains, including a FYVE finger. The FYVE finger domains are thought to direct the protein localization to intracellular membrane PtdIns 3-P. Recent studies with several FYVE domain proteins challenge this general concept. Here we have examined the binding of PIKfyve's FYVE domain to PtdIns 3-P in vitro and in vivo and a plausible contribution of this binding mechanism for the intracellular localization of the full-length protein. We document now a specific and high affinity interaction of a recombinantly produced PIKfyve FYVE domain peptide fragment with PtdIns 3-P-containing liposomes that requires the presence of the conservative core of basic residues within the FYVE domain. PIKfyve localization to membranes of the late endocytic pathway was found to be absolutely dependent on the presence of an intact FYVE finger. Cell treatment with PI 3-kinase inhibitor wortmannin dissociated endosome-bound PIKfyve, indicating that the protein targeted the membrane PtdIns 3-P. An enzymatically inactive peptide fragment of the PIKfyve catalytic domain was found to also specifically bind to PtdIns 3-P-containing liposomes, with residue Lys-1999 being critical in the interaction. This binding, however, was of relatively low affinity and, in the cellular context, was found ineffective in directing the molecule to PtdIns 3-P-enriched endosomes. Collectively, these results demonstrate that interaction of the FYVE domain with PtdIns 3-P is absolutely necessary for PIKfyve targeting to the membranes of the late endocytic pathway and determine PIKfyve as a downstream effector of PtdIns 3-P.

The phosphatidylinositol 3-phosphate-binding FYVE finger

The FYVE zinc finger domain is conserved from yeast (five proteins) to man (27 proteins). It functions in the membrane recruitment of cytosolic proteins by binding to phosphatidylinositol 3-phosphate (PI3P), which is found mainly on endosomes. Here we review recent work that sheds light on the targeting of FYVE finger proteins to PI3P-containing membranes, and how these proteins serve to regulate multiple cellular functions.

Localization of mRNAs for phosphatidylinositol phosphate kinases in the mouse brain during development

The gene expression for seven phosphatidylinositol phosphate kinases (PIPKs)-types Ialpha, Ibeta, Igamma, types IIalpha, IIbeta, IIgamma, and type III-was examined using in situ hybridization histochemistry, in the mouse brain during normal development. In the embryonic mouse brain, positive expression signals were detected only for the genes encoding PIPK Igamma and PIPK IIbeta in both the cerebral ventricular and mantle zones, with weaker signals in the former zone. On the other hand, the genes encoding all PIPKs were essentially detected in the external granule cell layer which represents the germinal zone for the neuronal granule cells. In the postnatal brain, among the seven PIPKs, the expression for genes encoding PIPK Igamma and IIbeta is evident in most gray matter, while the expression for the other five types was weak in the cortical gray matter and negligible in most non-cortical gray matter such as the diencephalon and brain stem nuclei. While the expression for most PIPKs in the mature hippocampus was distinct, the expression in the CA3 and the dentate gyrus was less definite for the genes encoding PIPK Ialpha and IIgamma, respectively. The distinct expression for the gene encoding PIPK IIalpha was detected in the postnatal white matter such as the cerebellar medulla, the corpus callosum, the hippocampal fimbriae, and the internal capsule.

Phosphatidylinositol 3-kinase and the control of endosome dynamics: new players defined by structural motifs

Phosphatidylinositol (PtdIns) 3-kinase (PI 3-kinase) activity has been implicated in fundamental cellular functions such as endosomal trafficking, growth-factor receptor signal transduction, and cell survival. This multiplicity of actions can be attributed to the existence of three classes of PI 3-kinases in mammalian cells, which can together lead to the production of four known distinct end products: PtdIns(3)P, PtdIns(3,4)P2, PtdIns(3,4,5)P3 and PtdIns(3,5)P2. The challenge of deciphering the connection between PI 3-kinase activity, the production of specific phosphoinositides and the control of specific cellular events is being met with the discovery of novel structural motifs that interact specifically with distinct PI 3-kinase products.

Wortmannin-sensitive trafficking steps in the endocytic pathway in rat liver endothelial cells

Liver endothelial cells (LECs) play an important homoeostatic role by removing potentially harmful macromolecules from blood. The extremely efficient endocytosis in LECs makes these cells an interesting model for the study of the involvement of phosphoinositides in the different steps of the endocytic process. In the present investigation we have studied the effect of wortmannin, an inhibitor of phosphatidylinositol kinases, on uptake, recycling and intracellular transport of (125)I-labelled ovalbumin, which is taken up in LECs via mannose-receptor-mediated endocytosis. Wortmannin was found to inhibit both uptake and degradation of ovalbumin. Further studies indicated that the reduced uptake via the mannose receptor was due both to a reduction of the number of surface receptors and a reduction in the rate of receptor-ligand internalization. Transport of ligand from endosomes to lysosomes was prevented, leading to increased recycling of internalized ligand. Wortmannin treatment released the Rab5 effector EEA1 from the endosomes and caused reduced size of early endosomes.

Mammalian cell morphology and endocytic membrane homeostasis require enzymatically active phosphoinositide 5-kinase PIKfyve

The dual specificity mammalian enzyme PIKfyve phosphorylates in vitro position d-5 in phosphatidylinositol (PtdIns) and PtdIns 3-P, itself or exogenous protein substrates. Here we have addressed the crucial questions for the identity of the lipid products and the role of PIKfyve enzymatic activity in mammalian cells. CHO, HEK293, and COS cells expressing PIKfyve(WT) at high levels and >90% efficiencies increased selectively the intracellular PtdIns 3,5-P(2) production by 30--55%. In these cell types PtdIns 5-P was undetectable. A kinase-deficient point mutant, PIKfyve(K1831E), transiently transfected into these or other cells elicited a dramatic dominant phenotype. Subsequent to a dilation of the PIKfyve-containing vesicles, PIKfyve(K1831E)-expressing cells progressively accumulated multiple swollen lucent vacuoles of endosomal origin, first in the perinuclear cytoplasm and then toward the cell periphery. Despite their drastically altered morphology, the PIKfyve(K1831E)-expressing cells were viable and functionally active, evidenced by several criteria. This phenotype was completely reversed by introducing PIKfyve(WT) into the PIKfyve(K1831E)-transfected cells. Disruptions of the localization signal in the PIKfyve kinase-deficient mutant yielded a PIKfyve(K1831E Delta fyve) protein, incompetent to vacuolate cells, implying that an active PIKfyve enzyme at distinct late endocytic membranes is crucial for normal cell morphology. This was further substantiated by examining the vacuolation-induced potency of several pharmacological stimuli in cells expressing high PIKfyve(WT) levels. Together, the results indicate that PIKfyve enzymatic activity, possibly through the generation of PtdIns 3,5-P(2), and/or yet to be identified endogenous phosphoproteins, is critical for cell morphology and endomembrane homeostasis.

Selective insulin-induced activation of class I(A) phosphoinositide 3-kinase in PIKfyve immune complexes from 3T3-L1 adipocytes

A diverse range of insulin-regulated cellular processes are dependent on class I(A) phosphatidylinositol 3-kinases (PI 3-Ks) and their association with and activation by up-stream signaling molecules. Here we report on the identification of the phosphoinositide 5'-kinase PIKfyve as a partner of class I(A) PI 3-K. Thus, both p85 and p110 subunits (class I(A)) of PI 3-Ks co-precipitated with anti-PIKfyve antibodies from lysates of resting 3T3-L1 adipocytes and, vice versa, PIKfyve co-precipitated with anti-p85 PI 3-K antibodies. Assignment to class I(A) PI 3-K enzymatic activity was further substantiated by the inhibition of PtdIns 3-P production in PIKfyve immune complexes by low concentrations of wortmannin and Triton X-100, and its preferences for Mg(2+) versus Mn(2+). Insulin but not PDGF or EGF stimulation of 3T3-L1 adipocytes markedly increased the PtdIns 3-P production (4.2-fold) in PIKfyve immune complexes, primarily as a result of increased PI 3-K intrinsic enzymatic activity. Intriguingly, while both insulin and PDGF caused an increase of class I(A) PI 3-K activity co-immunoprecipitated with tyrosine phosphorylated proteins, only insulin treatment yielded an activation of class I(A) PI 3-K in PIKfyve immune complexes. Studies aiming at identifying the underlying mechanism revealed that PIKfyve-class I(A) PI 3-K association and the insulin-induced activation likely operate independently of tyrosine phosphorylated insulin receptor substrate proteins. Together, these results establish PIKfyve as a novel source of activated class I(A) PI 3-K molecules that may be relevant in the insulin-signal transduction pathway.

Phosphatidylinositol 4,5-bisphosphate mediates Ca2+-induced platelet alpha-granule secretion: evidence for type II phosphatidylinositol 5-phosphate 4-kinase function

To understand the molecular basis of granule release from platelets, we examined the role of phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P(2)) in alpha-granule secretion. Streptolysin O-permeabilized platelets synthesized PtdIns(4,5)P(2) when incubated in the presence of ATP. Incubation of streptolysin O-permeabilized platelets with phosphatidylinositol-specific phospholipase C reduced PtdIns(4,5)P(2) levels and resulted in a dose- and time-dependent inhibition of Ca(2+)-induced alpha-granule secretion. Exogenously added PtdIns(4,5)P(2) inhibited alpha-granule secretion, with 80% inhibition at 50 microm PtdIns(4,5)P(2). Nanomolar concentrations of wortmannin, 33.3 microm LY294002, and antibodies directed against PtdIns 3-kinase did not inhibit Ca(2+)-induced alpha-granule secretion, suggesting that PtdIns 3-kinase is not involved in alpha-granule secretion. However, micromolar concentrations of wortmannin inhibited both PtdIns(4,5)P(2) synthesis and alpha-granule secretion by approximately 50%. Antibodies directed against type II phosphatidylinositol-phosphate kinase (phosphatidylinositol 5-phosphate 4-kinase) also inhibited both PtdIns(4,5)P(2) synthesis and Ca(2+)-induced alpha-granule secretion by approximately 50%. These antibodies inhibited alpha-granule secretion only when added prior to ATP exposure and not when added following ATP exposure, prior to Ca(2+)-mediated triggering. The inhibitory effects of micromolar wortmannin and anti-type II phosphatidylinositol-phosphate kinase antibodies were additive. These results show that PtdIns(4,5)P(2) mediates platelet alpha-granule secretion and that PtdIns(4,5)P(2) synthesis required for Ca(2+)-induced alpha-granule secretion involves the type II phosphatidylinositol 5-phosphate 4-kinase-dependent pathway.

Endocytosis and signaling cascades: a close encounter

Internalization of receptors and other cell surface components is well known to occur via clathrin-mediated endocytosis, although other less well characterized pathways are also involved. Internalized receptors are then delivered to early endosomes, where they are sorted to be recycled back to the plasma membrane for reutilization or transported to late endosomes/lysosomes for degradation. Endocytosis has long been considered as a constitutive, housekeeping function of animal cells that occurs independently of the cellular environment in contrast to regulated secretion. Here, we will discuss recent studies that are uncovering the existence of cross-talk between signaling molecules and components of the transport machinery, indicating that endocytosis can be modulated by signaling pathways.

Phosphoinositides: key players in cell signalling, in time and space

Over the last few years, many reports have extended our knowledge of the inositol lipid metabolism and brought out some exciting information about the location, the variety and the role of phosphoinositides (PIs). Besides the so-called "canonical PI pathway" leading to the production of phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2), the precursor of the intracellular second messengers inositol 1,4,5-trisphosphate and diacylglycerol (DAG), many other metabolic pathways have been identified to produce seven different polyphosphoinositides. Several of these quantitatively minor lipid molecules appear to be specifically involved in the control of cellular events, such as the spatial and temporal organisation of key signalling pathways, the rearrangement of the actin cytoskeleton or the intracellular vesicle trafficking. This is consistent with the fact that many of the enzymes, such as kinases and phosphatases, involved in the tight control of the intracellular level of polyphosphoinositides, are regulated and/or relocated through cell surface receptors for extracellular ligands. The remarkable feature of PIs, which can be rapidly synthesised and degraded in discrete membrane domains or even subnuclear structures, places them as ideal regulators and integrators of very dynamic mechanisms of cell regulation. In this review, we will summarise recent studies on the potential location, the metabolic pathways and the role of the different PIs. Some aspects of the temporal synthesis of D3 PIs will also be discussed.

Localization and insulin-regulated relocation of phosphoinositide 5-kinase PIKfyve in 3T3-L1 adipocytes

The mammalian phosphoinositide kinase PIKfyve catalyzes the synthesis of phosphatidylinositol 5-P and phosphatidylinositol 3,5-P(2), thought essential in cellular functions, including membrane trafficking. To discern the intracellular loci of PIKfyve products' formation, we have examined the localization of PIKfyve protein versus enzymatic activity and a possible acutely regulated redistribution in 3T3-L1 adipocytes. Subcellular fractions of resting cells that were positive for immunoreactive PIKfyve, such as cytosol ( approximately 76%), internal structures (low density microsomal fraction (LDM), composed of recycling endosomes, GLUT4 storage compartment, Golgi, and cytoskeletal elements) ( approximately 20%), and plasma membrane (( approximately )4%), expressed enzymatically active PIKfyve. While the presence of a FYVE finger in PIKfyve predicts early endosome targeting, density gradient sedimentation, immunoadsorption, and fluorescence microscopy analyses segregated the LDM-associated PIKfyve from the membranes of the recycling endosomes and GLUT4. PIKfyve fluorescence staining largely coincided with trans-Golgi network/multivesicular body markers, indicating PIKfyve's role in the late endocytic/biosynthetic pathways. A subfraction of particulate PIKfyve resisted nonionic detergent treatment, implying association with cytoskeletal structures, previously found positive for key members of the insulin signaling cascade. Upon acute stimulation of 3T3-L1 adipocytes with insulin or pervanadate, a portion of the cytosolic PIKfyve was recruited onto LDM, which was coupled with a commensurate increase of PIKfyve lipid kinase activity and an electrophoretic mobility shift. We suggest the recruited PIKfyve specifies the site and timing of phosphoinositide signals that are relevant to the acute insulin action.

Characterization and chromosomal mapping of a novel human gene, ANKHZN

Ankhzn (ankyrin repeats hooked to a zinc finger motif) was originally isolated by means of the gene trap method, as a novel cytoplasmic protein on mouse embryonic stem cells. The Ankhzn protein is ubiquitously expressed in a spatiotemporal-specific manner and is located on endosomes. In the present study, we have cloned human ANKHZN cDNA by PCR using candidate EST clones exhibiting a high homology to mouse Ankhzn cDNA. The human ANKHZN cDNA encoded a 1166aa protein exhibiting 84.9% identity to the mouse one. The size of the transcript was found to be about 7kb on a Northern blot analysis, and ANKHZN mRNA was found to be ubiquitously expressed in human tissues on RT-PCR analysis. Western blot analysis showed that a 130kDa protein was detected at various levels in human tissues and also present in both membrane and soluble fractions obtained on subcellular fractionation. Human ANKHZN is a single copy gene consisting of predicted 25 exons in the human genome, and has been mapped to human chromosome 17p13 by radiation hybrid panel and fluorescence in-situ hybridization.

AKT/PKB and other D3 phosphoinositide-regulated kinases: kinase activation by phosphoinositide-dependent phosphorylation

The protein kinase Akt/PKB is activated via a multistep process by a variety of signals. In the early steps of this process, PI-3 kinase-generated D3-phosphorylated phosphoinositides bind the Akt PH domain and induce the translocation of the kinase to the plasma membrane where it co-localizes with phosphoinositide-dependent kinase-1. By binding to the PH domains of both Akt and phosphoinositide-dependent kinase-1, D3-phosphorylated phosphoinositides appear to also induce conformational changes that permit phosphoinositide-dependent kinase-1 to phosphorylate the activation loop of Akt. The paradigm of Akt activation via phosphoinositide-dependent phosphorylation provided a framework for research into the mechanism of activation of other members of the AGC kinase group (p70S6K, PKC, and PKA) and members of the Tec tyrosine kinase family (TecI, TecII, Btk/Atk, Itk/Tsk/Emt, Txk/Rlk, and Bm/Etk). The result was the discovery that these kinases and Akt are activated by overlapping pathways. In this review, we present our current understanding of the regulation and function of the Akt kinase and we discuss the common and unique features of the activation processes of Akt and the AGC and Tec kinase families. In addition, we present an overview of the biosynthesis of phosphoinositides that contribute to the regulation of these kinases.

Autophosphorylation of type I phosphatidylinositol phosphate kinase regulates its lipid kinase activity

Phosphatidylinositol phosphate kinases (PIPKs) have important roles in the production of various phosphoinositides. For type I PIP5Ks (PIP5KI), a broad substrate specificity is known. They phosphorylate phosphatidylinositol 4-phosphate most effectively but also phosphorylate phosphatidylinositol (PI), phosphatidylinositol 3-phosphate, and phosphatidylinositol (3,4)-bisphosphate (PI(3, 4)P(2)), resulting in the production of phosphatidylinositol (4, 5)-bisphosphate (PI(4,5)P(2)), phosphatidylinositol 3-phosphate, phosphatidylinositol (3,4)-bisphosphate (PI(3,4)P(2)), phosphatidylinositol (3,5)-bisphosphate (PI(3,5)P(2)), and phosphatidylinositol (3,4,5)-trisphosphate. We show here that PIP5KIs have also protein kinase activities. When each isozyme of PIP5KI (PIP5KIalpha, -beta, and -gamma) was subjected to in vitro kinase assay, autophosphorylation occurred. The lipid kinase-negative mutant of PIP5KIalpha (K138A) lost the protein kinase activity, suggesting the same catalytic mechanism for the lipid and the protein kinase activities. PIP5KIbeta expressed in Escherichia coli also retains this protein kinase activity, thus confirming that no co-immunoprecipitated protein kinase is involved. In addition, the autophosphorylation of PIP5KI is markedly enhanced by the addition of PI. No other phosphoinositides such as phosphatidylinositol phosphate, phosphatidylinositol bisphosphate, or phosphatidylinositol trisphosphate have such an effect. We also found that the PI-dependent autophosphorylation strongly suppresses the lipid kinase activity of PIP5KI. The lipid kinase activity of PIP5KI was decreased to one-tenth upon PI-dependent autophosphorylation. All these results indicate that the lipid kinase activity of PIP5KI that acts predominantly for PI(4,5)P(2) synthesis is regulated by PI-dependent autophosphorylation in vivo.

Signal transduction: stuck with FYVE domains

The FYVE domain is a protein motif that allows the interaction of cytosolic proteins with membranes containing the lipid phosphatidylinositol 3-phosphate. Structural information about FYVE domains has come from two crystal structures and NMR analysis. Corvera discusses how these structures differ and what they tell us about how proteins with FYVE domains interact with biological membranes. The Perspective also addresses how proteins with FYVE domains and protein internalization are involved in signal transduction.

FYVE-DSP1, a dual-specificity protein phosphatase containing an FYVE domain

Dual-specificity protein phosphatases (DSPs) dephosphorylate proteins at Ser/Thr and Tyr. FYVE domain is a double zinc finger motif which specifically binds phosphatidylinositol(3)-phosphate. Here, we report a novel dual specificity phosphatase that contains a FYVE domain at the C-terminus. We designate the protein FYVE-DSP1. Molecular cloning yielded three isoforms of the enzyme presumably derived from alternate RNA splicing. Sequence alignment revealed that the catalytic phosphatase domain of FYVE-DSP1 closely resembled that of myotubularin, while its FYVE domain has all the conserved amino acid residues found in other proteins of the same family. Recombinant FYVE-DSP1 is partitioned in both cytosolic and membrane fractions. It dephosphorylates proteins phosphorylated on Ser, Thr, and Tyr residues and low molecular weight phosphatase substrate para-nitrophenylphosphate. It shows typical characteristics of other DSPs and protein tyrosine phosphatases (PTPs). These include inhibition by sodium vanadate and pervanadate, pH dependency, and inactivation by mutation of the key cysteinyl residue at the phosphatase signature motif. Finally, PCR analyses demonstrated that FYVE-DSP1 is widely distributed in human tissues but different spliced forms expressed differently.

FYVE-finger proteins--effectors of an inositol lipid

The binding of cytosolic proteins to specific intracellular membranes containing phosphorylated derivatives of phosphatidylinositol (PtdIns) is a common theme in vital cellular processes, such as cytoskeletal function, receptor signalling and membrane trafficking. Recently, several potential effectors of the phosphoinositide 3-kinase product PtdIns 3-phosphate (PtdIns(3)P) have emerged through the observation that a conserved zinc-finger-like domain, the FYVE-finger, binds specifically to this lipid. Here we review current knowledge about the structural basis for the FYVE-PtdIns(3)P interaction, its role in membrane recruitment of proteins and the functions of FYVE-finger proteins in membrane trafficking and other cellular processes.

Complementation analysis in PtdInsP kinase-deficient yeast mutants demonstrates that Schizosaccharomyces pombe and murine Fab1p homologues are phosphatidylinositol 3-phosphate 5-kinases

Phosphatidylinositol 3,5-bisphosphate (PtdIns(3,5)P(2)) is widespread in eukaryotic cells. In Saccharomyces cerevisiae, PtdIns(3,5)P(2) synthesis is catalyzed by the PtdIns3P 5-kinase Fab1p, and loss of this activity results in vacuolar morphological defects, indicating that PtdIns(3,5)P(2) is essential for vacuole homeostasis. We have therefore suggested that all Fab1p homologues may be PtdIns3P 5-kinases involved in membrane trafficking. It is unclear which phosphatidylinositol phosphate kinases (PIPkins) are responsible for PtdIns(3,5)P(2) synthesis in higher eukaryotes. To clarify how PtdIns(3,5)P(2) is synthesized in mammalian and other cells, we determined whether yeast and mammalian Fab1p homologues or mammalian Type I PIPkins (PtdIns4P 5-kinases) make PtdIns(3,5)P(2) in vivo. The recently cloned murine (p235) and Schizosaccharomyces pombe FAB1 homologues both restored basal PtdIns(3,5)P(2) synthesis in Deltafab1 cells and made PtdIns(3,5)P(2) in vitro. Only p235 corrected the growth and vacuolar defects of fab1 S. cerevisiae. A mammalian Type I PIPkin supported no PtdIns(3,5)P(2) synthesis. Thus, FAB1 and its homologues constitute a distinct class of Type III PIPkins dedicated to PtdIns(3,5)P(2) synthesis. The differential abilities of p235 and of SpFab1p to complement the phenotypic defects of Deltafab1 cells suggests that interaction(s) with other protein factors may be important for spatial and/or temporal regulation of PtdIns(3,5)P(2) synthesis. These results also suggest that p235 may regulate a step in membrane trafficking in mammalian cells that is analogous to its function in yeast.

Phosphoinositides in membrane traffic

Phosphoinositides serve as direct local modulators or recruiters of the protein machineries that control membrane trafficking. In the past year, examples of phosphoinositide effectors include regulators of small GTPases in coat assembly, dynamin in clathrin coated vesicle formation and FYVE finger proteins in endocytic membrane traffic. A novel phosphoinositide appears to regulate effectors involved in the formation of multivesicular endosomes.

PIKfyve, a mammalian ortholog of yeast Fab1p lipid kinase, synthesizes 5-phosphoinositides. Effect of insulin

One or more free hydroxyls of the phosphatidylinositol (PtdIns) head group undergo enzymatic phosphorylation, yielding phosphoinositides (PIs) with key functions in eukaryotic cellular regulation. Two such species, PtdIns 5-P and PtdIns 3,5-P(2), have now been identified in mammalian cells, but their biosynthesis remains unclear. We have isolated a novel mammalian PI kinase, p235, whose exact substrate specificity remained to be determined (Shisheva, A., Sbrissa, D., and Ikonomov, O. (1999) Mol. Cell. Biol. 19, 623-634). Here we report that recombinant p235 expressed in COS cells, like the authentic p235 in adipocytes, displays striking specificity for PtdIns over PI substrates and generates two products identified as PtdIns 5-P and PtdIns 3,5-P(2) by HPLC analyses. Synthetic PtdIns 3-P substrates were also converted to PtdIns 3,5-P(2) but to a substantially lesser extent than PtdIns isolated from natural sources. Important properties of the p235 PI 5-kinase include high sensitivity to nonionic detergents and relative resistance to wortmannin and adenosine. By analyzing deletion mutants in a heterologous cell system, we determined that in addition to the predicted catalytic domain other regions of the molecule are critical for the p235 enzymatic activity. HPLC resolution of monophosphoinositide products, generated by p235 immune complexes derived from lysates of 3T3-L1 adipocytes acutely stimulated with insulin, revealed essentially the same PtdIns 5-P levels as the corresponding p235 immune complexes of resting cells. However, the acute insulin action resulted in an increase of a wortmannin-sensitive PtdIns 3-P peak, suggestive of a plausible recruitment of wortmannin-sensitive PI 3-kinase(s) to p235. In conclusion, mouse p235 (renamed here PIKfyve) displays a strong in vitro activity for PtdIns 5-P and PtdIns 3,5-P(2) generation, implying PIKfyve has a key role in their biosynthesis.

Phosphatidylinositol 3-phosphate recognition by the FYVE domain

Recognition of phosphatidylinositol 3-phosphate (Ptdlns(3)P) is crucial for a broad range of cellular signaling and membrane trafficking events regulated by phosphoinositide (PI) 3-kinases. PtdIns(3)P binding by the FYVE domain of human early endosome autoantigen 1 (EEA1), a protein implicated in endosome fusion, involves two beta hairpins and an alpha helix. Specific amino acids, including those of the FYVE domain's conserved RRHHCRQCGNIF motif, contact soluble and micelle-embedded lipid and provide specificity for Ptdlns(3)P over Ptdlns(5)P and Ptdlns, as shown by heteronuclear magnetic resonance spectroscopy. Although the FYVE domain relies on a zinc-binding motif reminiscent of RING fingers, it is distinguished by ovel structural features and its ptdlns(3)P-binding site.

Crystal structure of a phosphatidylinositol 3-phosphate-specific membrane-targeting motif, the FYVE domain of Vps27p

Phosphatidylinositol 3-phosphate regulates membrane trafficking and signaling pathways by interacting with the FYVE domains of target proteins. The 1.15 A structure of the Vps27p FYVE domain reveals two antiparallel beta sheets and an alpha helix stabilized by two Zn2+-binding clusters. The core secondary structures are similar to a rabphilin-3A Zn2+-binding domain and to the C1 and LIM domains. Phosphatidylinositol 3-phosphate binds to a pocket formed by the (R/K)(R/K)HHCR motif. A lattice contact shows how anionic ligands can interact with the phosphatidylinositol 3-phosphate-binding site. The tip of the FYVE domain has basic and hydrophobic surfaces positioned so that nonspecific interactions with the phospholipid bilayer can abet specific binding to phosphatidylinositol 3-phosphate.