Phosphatidylinositol 3-kinase encoded by yeast VPS34 gene essential for protein sorting

Inhibition of endosome fusion by wortmannin persists in the presence of activated Rab5

Rab5-dependent endosome fusion is sensitive to the phosphoinositide 3-kinase inhibitor, wortmannin. It has been proposed that phosphoinositide 3-kinase activity may be required for activation of rab5 by influencing its nucleotide cycle such as to promote its active GTP state. In this report we demonstrate that endosome fusion remains sensitive to wortmannin despite preloading of endosomes with stimulatory levels of a GTPase-defective mutant rab5(Q79L) or of a xanthosine triphosphate-binding mutant, rab5(D136N), in the presence of the nonhydrolysable analogue XTPgammaS. These results suggest that activation of rab5 cannot be the principal function of the wortmannin-sensitive factor on the endosome fusion pathway. This result is extrapolated to all GTPases by demonstrating that endosome fusion remains wortmannin sensitive despite prior incubation with the nonhydrolysable nucleotide analogue GTPgammaS. Consistent with these results, direct measurement of clathrin-coated vesicle-stimulated nucleotide dissociation from exogenous rab5 was insensitive to the presence of wortmannin. A large excess of rab5(Q79L), beyond levels required for maximal stimulation of the fusion assay, afforded protection against wortmannin inhibition, and partial protection was also observed with an excess of wild-type rab5 independent of GTPgammaS.

The p85 and p110 Subunits of Phosphatidylinositol 3-Kinase-α Are Substrates, In Vitro, for a Constitutively Associated Protein Tyrosine Kinase in Platelets

Phosphatidylinositol 3-kinase (PI3K) is a heterodimer lipid kinase consisting of an 85-kD subunit bound to a 110-kD catalytic subunit that also possesses intrinsic, Mn2+-dependent protein serine kinase activity capable of phosphorylating the 85-kD subunit. Here, we examine the Mn2+-dependent protein kinase activity of PI3Kα immunoprecipitated from normal resting or thrombin-stimulated platelets, and characterize p85/p110 phosphorylation, in vitro. Phosphoamino acid analysis of phosphorylated PI3Kα showed p85 and p110 were phosphorylated on serine, but in contrast to previous results, were also phosphorylated on threonine and tyrosine. Wortmannin and LY294002 inhibited p85 phosphorylation; however, p110 phosphorylation was also inhibited suggesting p110 autophosphorylation on serine/threonine. The protein tyrosine kinase inhibitor, erbstatin analog, partially inhibited p85 and p110 phosphorylation but did not appear to affect PI3K lipid kinase activity. The in vitro phosphorylation of p85α or p110α derived from thrombin-stimulated platelets was no different than that of resting platelets, but we confirm that in thrombin receptor-stimulated platelets enhanced levels of p85α and PI3K lipid kinase activity were recovered in antiphosphotyrosine antibody immunoprecipitates. These results suggest PI3Kα can autophosphorylate on serine and threonine, and both p85α and p110α are substrates for a constitutively-associated protein tyrosine kinase in platelets.

The Golgi apparatus: 100 years of progress and controversy

Research on the Golgi apparatus has resulted in major advances in understanding its structure and functions, but many important questions remain unanswered. The history of the Golgi apparatus has been marked by arguments and controversies, some of which have been resolved, whereas others are still ongoing. This article charts progress in understanding the role of the Golgi apparatus during the 100 years since it was discovered, highlighting major milestones and discoveries that have led to the concepts of the organization and functions of this organelle that we have today.

Cooperativity of phosphatidylinositol transfer protein and phospholipase D in secretory vesicle formation from the TGN--phosphoinositides as a common denominator?

Phosphatidylinositol transfer protein (PITP) and phospholipase D (PLD) stimulate the formation of constitutive secretory vesicles (CSVs) and immature secretory granules (ISGs) from the trans-Golgi network (TGN) in a cell-free system. The stimulatory effects of PITP and PLD are additive. Stimulation by either PITP or PLD is blocked by geneticin, a member of the aminoglycoside antibiotics known to bind to phosphoinositides. Since the PLD we used is insensitive to geneticin, our results suggest that phosphoinositides promote secretory vesicle formation as downstream effectors of both PITP and PLD, possibly via the recruitment of proteins mediating membrane budding and fission.

Phosphoinositide 3-kinase activity is necessary for insulin-dependent inhibition of apolipoprotein B secretion by rat hepatocytes and localizes to the endoplasmic reticulum

Insulin inhibits apolipoprotein B (apoB) secretion by primary rat hepatocytes through activation of phosphoinositide 3-kinase (PI 3-K). Current studies demonstrate that the PI 3-K inhibitor wortmannin inhibits both basal and insulin-stimulated PI 3-K activities. Wortmannin and LY 294002, two structurally distinct PI 3-K inhibitors, prevent insulin-dependent inhibition of apoB secretion in a dose-dependent manner. To link PI 3-K activation to insulin action on apoB, we investigated whether insulin induced localization of activated PI 3-K to the endoplasmic reticulum (ER), where apoB biogenesis is initiated. Insulin action results in a significant redistribution of PI 3-K to a low density microsome (LDM) fraction containing apoB protein and apoB mRNA. Insulin stimulates a significant increase in PI 3-K activity associated with insulin receptor substrate-1 as well as an increase in insulin receptor substrate-1/PI 3-K mass in LDM. Subfractionation of LDM on sucrose density gradients shows that insulin significantly increases the amount of PI 3-K present in an ER fraction containing apoB. Insulin stimulates PI 3-K activity in smooth and rough microsomes isolated from rat hepatocytes, the latter of which contain rough ER as demonstrated by electron microscopy. Studies indicate that 1) PI 3-K activity is necessary for insulin-dependent inhibition of apoB secretion by rat hepatocytes; 2) insulin action leads to the activation and localization of PI 3-K in an ER fraction containing apoB; and 3) insulin stimulates PI 3-K activity in the rough ER.

Phosphatidylinositol 3-kinase and Ca2+ influx dependence for ligand-stimulated internalization of the c-Kit receptor

We have evaluated the role of phosphatidylinositol 3-kinase (PI3-kinase) and Ca2+ influx in ligand-stimulated internalization of the c-Kit receptor. The wild type (wt) c-Kit receptor and YF719, a mutant receptor in which the SH2-mediated binding site for the p85 subunit of PI3-kinase is disrupted, were expressed in DA-1 cells. YF719 internalized with similar kinetics as wt c-Kit although the receptor remained localized close to the plasma membrane. However, in the absence of extracellular Ca2+, or in the presence of the competitive Ca2+ influx blocker Ni2+, the YF719 mutant failed to internalize. Failure to internalize in the absence of Ca2+ was also observed for the wt c-Kit receptor in cells that were pretreated with the PI3-kinase inhibitor, wortmannin. Following stimulation with ligand, clathrin heavy chains were found to co-immunoprecipitate with c-Kit. However, under conditions in which PI3-kinase activity is inhibited and Ca2+ influx is blocked, clathrin failed to co-immunoprecipitate with c-Kit. Our results demonstrate that both Ca2+ influx and PI3-kinase activity influence c-Kit endocytosis, and inhibition of these two signals disrupts the earliest stages of ligand-mediated internalization.

Osmotic stress activates phosphatidylinositol-3,5-bisphosphate synthesis

Inositol phospholipids play multiple roles in cell signalling systems. Two widespread eukaryotic phosphoinositide-based signal transduction mechanisms, phosphoinositidase C-catalysed phosphatidylinositol-4,5-bisphosphate (PtdIns(4,5)P2) hydrolysis and 3-OH kinase-catalysed PtdIns(4,5)P2 phosphorylation, make the second messengers inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) sn-1,2-diacylglycerol and PtdIns(3,4,5)P3. In addition, PtdIns(4,5)P2 and PtdIns3P have been implicated in exocytosis and membrane trafficking. We now show that when the yeasts Saccharomyces cerevisiae and Schizosaccharomyces pombe are hyperosmotically stressed, they rapidly synthesize phosphatidylinositol-3,5-bisphosphate (PtdIns(3,5)P2) by a process that involves activation of a PtdIns3P 5-OH kinase. This PtdIns(3,5)P2 accumulation only occurs in yeasts that have an active vps34-encoded PtdIns 3-OH kinase, showing that this latter kinase makes the PtdIns3P needed for PtdIns(3,5)P2 synthesis and indicating that PtdIns(3,5)P2 may have a role in sorting vesicular proteins. PtdIns(3,5)P2 is also present in mammalian and plant cells: in monkey Cos-7 cells, its labelling is inversely related to the external osmotic pressure. The stimulation of a PtdIns3P 5-OH kinase-catalysed synthesis of PtdIns(3,5)P2, a molecule that might be a new type of phosphoinositide 'second messenger, thus appears to be central to a widespread and previously uncharacterized regulatory pathway.

Phosphatidylinositol 3-kinase is required for the formation of constitutive transport vesicles from the TGN

An 85-kD cytosolic complex (p62(cplx)), consisting of a 62-kD phosphoprotein (p62) and a 25-kD GTPase, has been shown to be essential for the cell-free reconstitution of polymeric IgA receptor (pIgA-R)-containing exocytic transport vesicle formation from the TGN (Jones, S.M., J.R. Crosby, J. Salamero, and K.E. Howell. 1993. J. Cell Biol. 122:775-788). Here the p62(cplx) is identified as a regulatory subunit of a novel phosphatidylinositol 3-kinase (PI3-kinase). This p62(cplx)-associated PI3-kinase activity is stimulated by activation of the p62(cplx)-associated GTPase, and is specific for phosphatidylinositol (PI) as substrate, and is sensitive to wortmannin at micromolar concentrations. The direct role of this p62(cplx)-associated PI3-kinase activity in TGN-derived vesicle formation is indicated by the finding that both lipid kinase activity and the formation of pIgA-R-containing exocytic vesicles from the TGN are inhibited by wortmannin with similar dose-response curves and 50% inhibitory concentrations (3.5 microM). These findings indicate that phosphatidylinositol-3-phosphate (PI[3]P) is required for the formation of TGN-derived exocytic transport vesicles, and that the p62(cplx)-associated PI3-kinase and an activated GTPase are the essential molecules that drive production of this PI(3)P.

Regulation of eIF-4E BP1 phosphorylation by mTOR

The proteins eIF-4E BP1 and p70 S6 kinase each undergo an insulin/mitogen-stimulated phosphorylation in situ that is partially inhibited by rapamycin. Previous work has established that the protein known as mTOR/RAFT-1/FRAP is the target through which the rapamycin.FKBP12 complex acts to dephosphorylate/deactivate the p70 S6 kinase; thus, some mTOR mutants that have lost the ability to bind to the rapamycin.FKBP12 complex in vitro can protect the p70 S6 kinase against rapamycin-induced dephosphorylation/deactivation in situ. We show herein that such mTOR mutants also protect eIF-4E BP1 against rapamycin-induced dephosphorylation, and for both p70 S6 kinase and eIF-4E BP1, such protection requires that the rapamycin-resistant mTOR variant retains an active catalytic domain. In contrast, mutants of p70 S6 kinase rendered intrinsically resistant to inhibition by rapamycin in situ are not able to protect coexpressed eIF-4E BP1 from rapamycin-induced dephosphorylation. We conclude that mTOR is an upstream regulator of eIF-4E BP1 as well as the p70 S6 kinase; moreover, these two mTOR targets are regulated in a parallel rather than sequential manner.

Isolation and characterization of the droPIK57 gene encoding a new regulatory subunit of phosphatidylinositol 3-kinase from Drosophila melanogaster

Mammalian phosphatidylinositol 3-kinase (PI 3-kinase) plays an important role in the regulation of various cellular, receptor tyrosine kinase-mediated processes, such as mitogenesis and transformation. PI 3-kinase is composed of a 110-kDa catalytic subunit and a regulatory subunit of 85 kDa or 55 kDa. We have cloned a gene for a regulatory subunit from Drosophila melanogaster, named droPIK57, from head-specific cDNA libraries. The droPIK57 gene encodes a protein containing two SH2 domains with significant sequence homology to those in p85 and p55. Like the p55 subunits, DroPIK57 is missing the SH3 domain and the bcr homology region of the p85 subunit. The short N-terminus as well as the C-terminus of the DroPIK57 protein show no identity to the known PI 3-kinase subunits, suggesting that it is a new member in the family of regulatory subunits. In-situ hybridization and Northern blot analysis indicate a widespread function of this gene during embryogenesis and in the CNS.

Phosphatidylinositol 3-kinase mediates epidermal growth factor-induced activation of the c-Jun N-terminal kinase signaling pathway

The signaling events which mediate activation of c-Jun N-terminal kinase (JNK) are not yet well characterized. To broaden our understanding of upstream mediators which link extracellular signals to the JNK pathway, we investigated the role of phosphatidylinositol (PI) 3-kinase in epidermal growth factor (EGF)-mediated JNK activation. In this report we demonstrate that a dominant negative form of PI 3-kinase as well as the inhibitor wortmannin blocks EGF-induced JNK activation dramatically. However, wortmannin does not have an effect on JNK activation induced by UV irradiation or osmotic shock. In addition, a membrane-targeted, constitutively active PI 3-kinase (p110beta) was shown to produce in vivo products and to activate JNK, while a kinase-mutated form of this protein showed no activation. On the basis of these experiments, we propose that PI 3-kinase activity plays a role in EGF-induced JNK activation in these cells.

Inhibition of calcium-independent mannose 6-phosphate receptor incorporation into trans-Golgi network-derived clathrin-coated vesicles by wortmannin

The transport of pro-cathepsin D from the trans-Golgi network (TGN) to the endosomal pathway is dependent on binding to the calcium-independent mannose 6-phosphate receptor (ci-M6PR), which is incorporated into TGN-derived clathrin-coated transport vesicles (CCVs). Inhibition of this transport step by wortmannin has led to the proposal that it is dependent upon a phosphoinositide 3-kinase activity necessary for ci-M6PR recruitment into TGN-derived CCVs or in the formation of those vesicles (Brown, W. J., DeWald, D. B., Emr, S. D., Plutner, H., and Balch, W. E. (1995) J. Cell Biol. 130, 781-796; Davidson, H. W. (1995) J. Cell Biol. 130, 797-806). In this study we have addressed the effect of wortmannin on the TGN step of the ci-M6PR cycle. CCVs from K562 cells, pretreated or not with 250 nM wortmannin, were purified on equilibrium density gradients. Quantification of TGN-derived CCVs, assessed by gamma-adaptin content in purified vesicle fractions, showed that the formation of the vesicles was only marginally decreased after 20 min of treatment with the drug, while for the same wortmannin treatment, the amount of ci-M6PR recruited into those vesicles was decreased by 70% compared with control. At a later time point (2 h), a reduction in the amount of gamma-adaptin in CCV fractions was also observed. These findings demonstrate that inhibition of ci-M6PR recruitment into CCVs but not of vesicle formation is the primary reason for the observed defect in cathepsin D transport following wortmannin treatment.

Cloning of a human phosphoinositide 3-kinase with a C2 domain that displays reduced sensitivity to the inhibitor wortmannin

The generation of phosphatidylinositide 3-phosphates has been observed in a variety of cellular responses. The enzymes that mediate synthesis are the phosphoinositide 3-kinases (PI3-Ks) that form a family of structurally diverse enzymes with distinct substrate specificities. In this paper, we describe the cloning of a novel human PI3-K, namely PI3-K-C2 alpha, which contains a C-terminal C2 domain. This enzyme can be assigned to the class II PI3-Ks, which was defined by characterization of the Drosophila 68D enzyme and includes the recently described murine enzymes m-cpk and p170. Despite the overall similarity in the amino acid sequence of the murine and human enzymes, which suggests that they are encoded by closely related genes, these molecules show marked sequence heterogeneity at their N-termini. Biochemical analysis of recombinant PI3-K-C2 alpha demonstrates a restricted lipid substrate specificity. As reported for other members of this class, the enzyme only phosphorylates PtdIns and PtdIns4P when the lipids are presented alone. However, when lipids were presented together with phosphatidylserine acting as a carrier, phosphorylation of PtdIns(4,5)P2 was also observed. The catalytic activity of PI3-K-C2 alpha is refractory to concentrations of wortmannin and LY294002 which inhibit the PI3-K activity of other family members. The comparative insensitivity of PI3-K-C2 alpha to these inhibitors suggests that their use should be reevaluated in the study of PI3-Ks.

Phospholipase D stimulates release of nascent secretory vesicles from the trans-Golgi network

Phospholipase D (PLD) is a phospholipid hydrolyzing enzyme whose activation has been implicated in mediating signal transduction pathways, cell growth, and membrane trafficking in mammalian cells. Several laboratories have demonstrated that small GTP-binding proteins including ADP-ribosylation factor (ARF) can stimulate PLD activity in vitro and an ARF-activated PLD activity has been found in Golgi membranes. Since ARF-1 has also been shown to enhance release of nascent secretory vesicles from the TGN of endocrine cells, we hypothesized that this reaction occurred via PLD activation. Using a permeabilized cell system derived from growth hormone and prolactin-secreting pituitary GH3 cells, we demonstrate that immunoaffinity-purified human PLD1 stimulated nascent secretory vesicle budding from the TGN approximately twofold. In contrast, a similarly purified but enzymatically inactive mutant form of PLD1, designated Lys898Arg, had no effect on vesicle budding when added to the permeabilized cells. The release of nascent secretory vesicles from the TGN was sensitive to 1% 1-butanol, a concentration that inhibited PLD-catalyzed formation of phosphatidic acid. Furthermore, ARF-1 stimulated endogenous PLD activity in Golgi membranes approximately threefold and this activation correlated with its enhancement of vesicle budding. Our results suggest that ARF regulation of PLD activity plays an important role in the release of nascent secretory vesicles from the TGN.

Phosphatidylinositol-3 kinase is involved in ruffled border formation in osteoclasts

Phosphatidylinositol (PI)-3 kinase has been implicated in several aspects of intracellular membrane trafficking, although the detailed mechanism is yet to be established. We previously reported that wortmannin (WT), a selective inhibitor of PI-3 kinase, inhibited the bone-resorbing activity of osteoclasts (Nakamura et al., 1995, FEBS Lett., 361:79-84). In this study, we examined how PI-3 kinase was involved in membrane trafficking in osteoclasts which are primary bone-resorbing cells. Osteoclasts exhibit a highly polarized cytoplasmic organization, the ruffled border. Ruffled borders are formed by numerous deep membrane invaginations, on which vacuolar H(+)-ATPase (V-ATPase) is localized in a high density. Immunoelectron microscopic analyses revealed that PI-3 kinase was specifically present along ruffled border membranes and the limiting membranes of associated intracellular vacuoles in rat authentic osteoclasts. WT and LY294002, another inhibitor of PI-3 kinase, caused the accumulation of numerous acidic vacuoles which were stained with acridine orange in murine osteoclast-like multinucleated cells formed in vitro. An electron microscopic examination showed that these vacuoles contained V-ATPase along their limiting membranes and appeared to be derived from the Golgi apparatus as ruffled border precursors. A time course study revealed that WT-induced vacuoles began to accumulate in the region close to the apical membrane and were finally distributed throughout the cytoplasm. Removal of WT from the culture medium resulted in the disappearance of vacuoles in the cytoplasm, leading to the formation of ruffled borders. During the culture period, some vacuoles were observed to fuse with the ruffled border membrane. A pit formation assay on dentine slices also showed that the pit-forming activity of osteoclast-like cells was recovered by the removal of WT from the assay. These results suggest that PI-3 kinase plays an important role in ruffled border formation in osteoclasts, probably in the fusion of membrane vacuoles with the plasma membrane.

The role of lipid signaling in constitutive membrane traffic

Several lines of evidence indicate that enzymes that modify membrane lipids function in the regulation of constitutive membrane traffic. Recent evidence suggests that specific phosphatidylinositides may regulate the activity of proteins with diverse functions in membrane transport, such as dynamin, the clathrin-associated AP-2 complex, and proteins that stimulate guanine nucleotide exchange on ADP-ribosylation factors (ARFs). ARF proteins activate a phospholipase D that produces phosphatidic acid from phosphatidylcholine, and this may be essential for the formation of certain types of transport vesicles or may be constitutive vesicular transport to signal transduction pathways.

Isolation and molecular cloning of wortmannin-sensitive bovine type III phosphatidylinositol 4-kinases

Agonist-sensitive phosphoinositide pools are maintained by recently-identified wortmannin (WT)-sensitive phosphatidylinositol (PI) 4-kinase(s) (Nakanishi, S., Catt, K. J., and Balla, T. (1995) Proc. Natl. Acad. Sci. U.S.A. 92, 5317-5321). Two loosely membrane-associated WT-sensitive type III PI 4-kinases were isolated from bovine adrenal cortex as [3H]WT-labeled 110- and 210-kDa proteins. Based on peptide sequences from the smaller enzyme, a 3. 9-kilobase pair (kb) cDNA with an open reading frame encoding a 90-kDa protein was isolated from a bovine brain cDNA library. Expression of this cDNA in COS-7 cells yielded a 110-kDa protein with WT-sensitive PI 4-kinase activity. Northern blot analysis of a human mRNA panel showed a single approximately 3.8-kb transcript. Peptide sequences obtained from the 210-kDa enzyme corresponded to those of a recently described rat 230-kDa PI 4-kinase. A 6.5-kb cDNA containing an open reading frame of 6129 nucleotides that encoded a 230-kDa protein, was isolated from brain cDNA. Northern blot analysis of human mRNA revealed a major 7.5-kb transcript. The molecular cloning of these novel WT-sensitive type III PI 4-kinases will allow detailed analysis of their signaling and other regulatory functions in mammalian cells.

Vacuolar protein sorting in fission yeast: cloning, biosynthesis, transport, and processing of carboxypeptidase Y from Schizosaccharomyces pombe

PCR was used to isolate a carboxypeptidase Y (CPY) homolog gene from the fission yeast Schizosaccharomyces pombe. The cloned S. pombe cpy1+ gene has a single open reading frame, which encodes 950 amino acids with one potential N-glycosylation site. It appears to be synthesized as an inactive pre-pro protein that likely undergoes processing following translocation into appropriate intracellular organelles. The C-terminal mature region is highly conserved in other serine carboxypeptidases. In contrast, the N-terminal pro region containing the vacuolar sorting signal in CPY from Saccharomyces cerevisiae shows fewer identical residues. The pro region contains two unusual repeating sequences; repeating sequence I consists of seven contiguous repeating segments of 13 amino acids each, and repeating sequence II consists of seven contiguous repeating segments of 9 amino acids each. Pulse-chase radiolabeling analysis revealed that Cpy1p was initially synthesized in a 110-kDa pro-precursor form and via the 51-kDa single-polypeptide-chain intermediate form which has had its pro segment removed is finally converted to a heterodimer, the mature form, which is detected as a 32-kDa protein on sodium dodecyl sulfate-polyacrylamide gel electrophoresis under reducing conditions. Like S. cerevisiae CPY, S. pombe Cpy1p does not require the N-linked oligosaccharide moiety for vacuolar delivery. To investigate the vacuolar sorting signal of S. pombe Cpy1p, we have constructed cpy1+-SUC2 gene fusions that direct the synthesis of hybrid proteins consisting of N-terminal segments of various lengths of S. pombe Cpy1p fused to the secreted enzyme S. cerevisiae invertase. The N-terminal 478 amino acids of Cpy1 are sufficient to direct delivery of a Cpy1-Inv hybrid protein to the vacuole. These results showed that the pro peptide of Cpy1 contains the putative vacuolar sorting signal.

Signalling through the lipid products of phosphoinositide-3-OH kinase

When a stimulatory agonist molecule binds at the exterior of the cell membrane, a second messenger transduces the signal to the interior of the cell. Second messengers can be derived from phospholipids in the membrane by the action of the enzymes phospholipase C or phosphoinositide-3-OH kinase (PI(3)K). PI(3)K is a key player in many cellular responses, including the movement of organelle membranes, shape alteration through rearrangement of cytoskeletal actin, transformation and chemotaxis. But how PI(3)K mediates these responses is only now becoming clear.

Identification of four novel human phosphoinositide 3-kinases defines a multi-isoform subfamily

Phosphoinositide (PI) 3-kinases have critical roles in diverse cellular signalling processes and in protein trafficking. This suggests that like other intracellular signalling molecules, e.g., phospholipase C and protein kinase C, there might be a large family of PI 3-kinase isoforms with the individual members having discrete signalling roles. Reverse transcription-polymerase chain reaction methods, using degenerate oligonucleotide primers against the lipid kinase consensus region, revealed eight sequences from human cDNA containing a high degree of identity to the family of PI 3-kinases. The sequences obtained included the previously described p110 alpha, p110 beta, and p110 gamma isoforms and HsVps34. Additionally, we have identified four novel sequences which are related to PI 3-kinases. Three of the novel sequences would appear to form a distinct sub-family of PI 3-kinases. We report the expression of these novel PI 3-kinases in human tissues and in cells derived from normal breast.

Subcellular locations of phosphatidylinositol 4-kinase isoforms

Phosphatidylinositol (PtdIns) 4-kinase catalyzes the synthesis of PtdIns-4-P, the precursor of an array of lipid second messengers generated by additional phosphorylation by PtdIns-4-P 5-kinase and PtdIns 3-kinase. PtdIns 4-kinase activity is conserved from yeast to higher eukaryotes. Multiple isoforms of mammalian PtdIns 4-kinase have been purified, and the activities have been detected in almost all subcellular locations. We previously reported the cloning and characterization of the first mammalian PtdIns 4-kinase named PI4Kalpha (Wong, K., and Cantley, L. C. (1994) J. Biol. Chem. 269, 28878-28884). Alternatively spliced forms of PI4Kalpha have also been identified from several sources including bovine brain (Gehrmann, T., Vereb, G., Schmidt, M., Klix, D., Meyer, H. E., Varsanyi, M., and Heilmeyer, L. M., Jr. (1996) Biochim. Biophys. Acta 1311, 53-63). Recently we isolated a distinct human PtdIns 4-kinase gene, named PI4Kbeta, that encodes an enzyme that is wortmannin sensitive (Meyers, R., and Cantley, L. C. (1997) J. Biol. Chem. 272, 4384-4390). Here we report the locations of these enzymes and provide evidence for other yet unidentified isoforms present in specific organelles. PI4Kalpha is mostly membrane-bound and located at the endoplasmic reticulum; whereas PI4Kbeta is in the cytosol and also present in the Golgi region. Neither of these isoforms accounts for the major type II PtdIns 4-kinase activity detected in the lysosomes and plasma membrane fraction.

Rab7 regulates transport from early to late endocytic compartments in Xenopus oocytes

Rab7 has been shown to localize to late endosomes and to mediate transport from early to late endosome/lysosome in mammalian cells and in yeast. We developed a novel assay to quantify transport from early to late endosomes using the Xenopus oocyte. Oocytes were pulsed with avidin after which the oocytes were incubated to allow avidin transport to a late compartment. The oocytes were then allowed to internalize biotin-horseradish peroxidase (HRP). The oocytes were then injected with test proteins and incubated further to allow transport of biotin-HRP from early endosomes to late endosomal/lysosomal compartments. Transport was quantified by assessing the formation of HRP-biotin-avidin complexes. Injection of Rab7:wild-type (WT) and Rab7:Q67L, a GTPase defective mutant, stimulated transport. Rab5:WT had no effect. Rab7:WT-stimulated transport was inhibited by nocodazole, suggesting a role for intact microtubules. Wortmannin, a phosphatidylinositol 3-kinase inhibitor, blocked Rab7:WT-stimulated transport, but Rab7:Q67L-stimulated transport was unaffected by the drug. Rab7:Q67L is constitutively activated and may not require phosphatidylinositol 3-kinase activity for activation. Rab7-stimulated transport requires N-ethylmaleimide-sensitive factor (NSF) activity as transport was blocked by N-ethylmaleimide and ATPase defective NSF mutants. Our results indicate that sequentially acting endocytic Rab GTPases utilize similar factors although their modes of action may be different.

Insulin receptor substrate (IRS)-2 is dephosphorylated more rapidly than IRS-1 via its association with phosphatidylinositol 3-kinase in skeletal muscle cells

Insulin receptor substrate (IRS)-2 is structurally and functionally similar to IRS-1. Indeed, stimulation with insulin or insulin-like growth factor I led to the rapid tyrosine phosphorylation of both IRS-1 and IRS-2, which in turn activated phosphatidylinositol (PI) 3-kinase in L6 cells and rat skeletal muscle. However, IRS-2 was rapidly dephosphorylated (3-10 min after the addition of insulin/insulin-like growth factor I), whereas IRS-1 phosphorylation continued for at least 60 min. The time courses of the PI 3-kinase activity associated with IRS-1 and IRS-2 paralleled the tyrosine phosphorylation of these proteins. Preincubation with sodium orthovanadate, an inhibitor of protein tyrosine phosphatase, blocked the rapid dephosphorylation of IRS-2, suggesting the involvement of tyrosine phosphatase. The activation of PI 3-kinase apparently plays an important role in the rapid dephosphorylation of IRS-2, as IRS-2 dephosphorylation was inhibited markedly by suppressing PI 3-kinase activity with wortmannin or overexpression of the dominant negative p85 subunit of PI 3-kinase, which cannot bind the p110 catalytic subunit. In addition, platelet-derived growth factor stimulation prior to insulin stimulation decreased IRS-associated PI 3-kinase and significantly inhibited the dephosphorylation of IRS-2. Taken together, these observations suggest that IRS-2 plays a unique role in mediating the signals from the insulin receptor to downstream molecules and that this effect is more transient than that of IRS-1. Tyrosine phosphatase and IRS-associated PI 3-kinase activity thus contribute to the rapid dephosphorylation of IRS-2.

Autophagic proteolysis: control and specificity

The rate of proteolysis is an important determinant of the intracellular protein content. Part of the degradation of intracellular proteins occurs in the lysosomes and is mediated by macroautophagy. In liver, macroautophagy is very active and almost completely accounts for starvation-induced proteolysis. Factors inhibiting this process include amino acids, cell swelling and insulin. In the mechanisms controlling macroautophagy, protein phosphorylation plays an important role. Activation of a signal transduction pathway, ultimately leading to phosphorylation of ribosomal protein S6, accompanies inhibition of macroautophagy. Components of this pathway may include a heterotrimeric Gi3-protein, phosphatidylinositol 3-kinase and p70S6 kinase. Recent evidence indicates that lysosomal protein degradation can be selective and occurs via ubiquitin-dependent and -independent pathways.

Identification and cDNA cloning of a novel mammalian C2 domain-containing phosphoinositide 3-kinase, HsC2-PI3K

Phosphoinositide (PI) 3-kinases have been shown to have critical roles in signal transduction, cell transformation and intracellular protein trafficking. Reverse-transcription polymerase chain reaction methods, using degenerate primers derived from the lipid kinase consensus region, were utilised to identify PI 3-kinases in the normal human breast. Here we report the cDNA cloning of a novel human PI 3-kinase isoform, HsC2-PI3K. This PI 3-kinase is most closely related to the recently described C2 domain-containing family of PI 3-kinases which includes Drosophila PI3K_68D/cpk and murine cpk-m/p170. Sequence analysis suggests that HsC2-PI3K is a second distinct mammalian member of the C2 domain-containing PI 3-kinase family. Northern blot analysis of human tissues indicates that HsC2-PI3K is widely expressed. Fluorescence in situ hybridisation has mapped HsC2-PI3K to chromosome 1q32.

Synaptojanin is the major constitutively active phosphatidylinositol-3,4,5-trisphosphate 5-phosphatase in rodent brain

The major constitutive phosphatidylinositol-3,4,5-P3 (PtdIns) 5-phosphatase activity was purified and subjected to peptide sequence analysis providing extensive amino acid sequence which was subsequently used for cloning the cDNA. Peptide and cDNA sequences revealed that the purified PtdIns(3,4,5)P3 5-phosphatase was identical to a splice variant of a recently cloned inositol polyphosphate 5-phosphatase termed synaptojanin. Since synaptojanin is not known to possess PtdIns(3,4,5)P3 5-phosphatase activity, we verified that the purified PtdIns(3,4,5)P3 5-phosphatase activity and synaptojanin are identical by Western blot using specific antibodies raised against synaptojanin sequences. Immunoprecipitation from crude lysates of rat brain tissue showed that synaptojanin accounts for the major part of the active PtdIns(3, 4,5)P3 5-phosphatase activity. It is also shown that the protein is localized to the soluble fraction. Expression of a truncated recombinant protein demonstrates that the conserved 5-phosphatase region of the synaptojanin gene expresses PtdIns(3,4,5)P3 5-phosphatase activity. However, immunological analysis demonstrates that the PtdIns(3,4,5)P3 5-phosphatase activity expressed from the synaptojanin gene in brain is due to a particular splice variant which contains a 16-amino acid insert as shown by immunoprecipitation using a specific antibody raised against this particular splice variant.

Inactivation of two Dictyostelium discoideum genes, DdPIK1 and DdPIK2, encoding proteins related to mammalian phosphatidylinositide 3-kinases, results in defects in endocytosis, lysosome to postlysosome transport, and actin cytoskeleton organization

Phosphatidylinositide 3-kinases (PI3-kinases) have been implicated in controlling cell proliferation, actin cytoskeleton organization, and the regulation of vesicle trafficking between intracellular organelles. There are at least three genes in Dictyostelium discoideum. DdPIK1, DdPIK2, and DdPIK3, encoding proteins most closely related to the mammalian 110-kD PI-3 kinase in amino acid sequence within the kinase domain. A mutant disrupted in DdPIK1 and DdPIK2 (delta ddpik1/ddpik2) grows slowly in liquid medium. Using FITC-dextran (FD) as a fluid phase marker, we determined that the mutant strain was impaired in pinocytosis but normal in phagocytosis of beads or bacteria. Microscopic and biochemical approaches indicated that the transport rate of fluid-phase from acidic lysosomes to non-acidic postlysosomal vacuoles was reduced in mutant cells resulting in a reduction in efflux of fluid phase. Mutant cells were also almost completely devoid of large postlysosomal vacuoles as determined by transmission EM. However, delta ddpik1/ddpik2 cells functioned normally in the regulation of other membrane traffic. For instance, radiolabel pulse-chase experiments indicated that the transport rates along the secretory pathway and the sorting efficiency of the lysosomal enzyme alpha-mannosidase were normal in the mutant strain. Furthermore, the contractile vacuole network of membranes (probably connected to the endosomal pathway by membrane traffic) was functionally and morphologically normal in mutant cells. Light microscopy revealed that delta ddpik1/ddpik2 cells appeared smaller and more irregularly shaped than wild-type cells; 1-3% of the mutant cells were also connected by a thin cytoplasmic bridge. Scanning EM indicated that the mutant cells contained numerous filopodia projecting laterally and vertically from the cell surface, and fluorescent microscopy indicated that these filopodia were enriched in F-actin which accumulated in a cortical pattern in control cells. Finally, delta ddpik1/ddpik2 cells responded and moved more rapidly towards cAMP. Together, these results suggest that Dictyostelium DdPIK1 and DdPIK2 gene products regulate multiple steps in the endosomal pathway, and function in the regulation of cell shape and movement perhaps through changes in actin organization.

p85alpha gene generates three isoforms of regulatory subunit for phosphatidylinositol 3-kinase (PI 3-Kinase), p50alpha, p55alpha, and p85alpha, with different PI 3-kinase activity elevating responses to insulin

Phosphatidylinositol 3-kinase (PI 3-kinase) is stimulated by association with a variety of tyrosine kinase receptors and intracellular tyrosine-phosphorylated substrates. We isolated a cDNA that encodes a 50-kDa regulatory subunit of PI 3-kinase with an expression cloning method using 32P-labeled insulin receptor substrate-1 (IRS-1). This 50-kDa protein contains two SH2 domains and an inter-SH2 domain of p85alpha, but the SH3 and bcr homology domains of p85alpha were replaced by a unique 6-amino acid sequence. Thus, this protein appears to be generated by alternative splicing of the p85alpha gene product. We suggest that this protein be called p50alpha. Northern blotting using a specific DNA probe corresponding to p50alpha revealed 6.0- and 2.8-kb bands in hepatic, brain, and renal tissues. The expression of p50alpha protein and its associated PI 3-kinase were detected in lysates prepared from the liver, brain, and muscle using a specific antibody against p50alpha. Taken together, these observations indicate that the p85alpha gene actually generates three protein products of 85, 55, and 50 kDa. The distributions of the three proteins (p85alpha, p55alpha, and p50alpha), in various rat tissues and also in various brain compartments, were found to be different. Interestingly, p50alpha forms a heterodimer with p110 that can as well as cannot be labeled with wortmannin, whereas p85alpha and p55alpha associate only with p110 that can be wortmannin-labeled. Furthermore, p50alpha exhibits a markedly higher capacity for activation of associated PI 3-kinase via insulin stimulation and has a higher affinity for tyrosine-phosphorylated IRS-1 than the other isoforms. Considering the high level of p50alpha expression in the liver and its marked responsiveness to insulin, p50alpha appears to play an important role in the activation of hepatic PI 3-kinase. Each of the three alpha isoforms has a different function and may have specific roles in various tissues.

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.

Growth factor-dependent phosphoinositide signalling

A wide variety of messages, in the form of diffusible growth factors, hormones and cytokines, are carried throughout multicellular organisms to coordinate important physiological properties of target cells, such as proliferation, differentiation, migration, apoptosis and metabolism. Most messengers bind to cognate receptors on target cells, which initiate a characteristic cascade of reactions within the cell, ultimately leading to the desired response. The cellular response is defined by the combination of signalling components whose individual activity depends upon the number and type of surface receptors. Consequently the responses of different cell types to one or more stimuli can be quite disparate. A molecular understanding of the signalling pathways employed by each type of receptor therefore underlies the ability to rationalize many cellular functions and to correct disfunctions. As a well studied example of the primary signalling events that take place on the cytoplasmic leaflet of the plasma membrane following receptor activation, we will discuss how the widely expressed receptor for epidermal growth factor (EGF) causes the phosphorylation and hydrolysis of a signalling precursor, the membrane lipid phosphatidylinositol. This paradigm will be used to illustrate certain general principles of signalling, including formation of multienzyme complexes, compartmentation of second messengers and intermediates, and cross-talk between different signalling pathways.

Cloning and characterization of a wortmannin-sensitive human phosphatidylinositol 4-kinase

Phosphatidylinositol (PtdIns) 4-kinases catalyze the synthesis of PtdIns-4-P, the immediate precursor of PtdIns-4,5-P2. Here we report the cloning of a novel, ubiquitously expressed PtdIns 4-kinase (PI4Kbeta). The 2.4-kilobase pair cDNA encodes a putative translation product of 801 amino acids which shows greatest homology to the yeast PIK1 gene. The recombinant protein exhibits lipid kinase activity when expressed in Escherichia coli, and specific antibodies recognize a 110-kDa PtdIns 4-kinase in cell lysates. The biochemical properties of PI4Kbeta are characteristic of a type III enzyme. Interestingly, both recombinant PI4Kbeta and the endogenous protein are inhibited by 150 nM wortmannin, suggesting that we have cloned the previously described PtdIns 4-kinase that is responsible for regulating the synthesis of agonist-sensitive pools of polyphosphoinositides (Nakanishi, S., Catt, J. K., and Balla, T. (1995) Proc. Natl. Acad. Sci. U. S. A. 92, 5317-5321).

Molecular cloning and biochemical characterization of a Drosophila phosphatidylinositol-specific phosphoinositide 3-kinase

Molecular, biochemical and genetic characterization of phosphoinositide 3-kinases (PI3Ks) have identified distinct classes of enzymes involved in processes mediated by activation of cell-surface receptors and in constitutive intracellular protein trafficking events. The latter process appears to involve a PtdIns-specific PI3K first described in yeast as a mutant, vps34, defective in the sorting of newly synthesized proteins from the Golgi to the vacuole. We have identified a representative member of each class of PI3Ks in Drosophila using a PCR-based approach. In the present paper we describe the molecular cloning of a PI3K from Drosophila, P13K_59F, that shows sequence similarity to Vps34. PI3K_59F encodes a protein of 108 kDa co-linear with Vps34 homologues, and with three regions of sequence similarity to other PI3Ks. Biochemical characterization of the enzyme, by expression of the complete coding sequence as a glutathione S-transferase fusion protein in Sf9 cells, demonstrates that PI3K_59F is a PtdIns-specific PI3K that can utilize either Mg2+ or Mn2+. This activity is sensitive to inhibition both by non-ionic detergent (Nonidet P40) and by wortmannin (IC50 10 nM). PI3K_59F, therefore, conserves both the structural and biochemical properties of the Vps34 class of enzymes.

Characterization of p150, an adaptor protein for the human phosphatidylinositol (PtdIns) 3-kinase. Substrate presentation by phosphatidylinositol transfer protein to the p150.Ptdins 3-kinase complex

Genetic and biochemical studies have shown that the phosphatidylinositol (PtdIns) 3-kinase encoded by the yeast VPS34 gene is required for the efficient sorting and delivery of proteins to the vacuole. A human homologue of the yeast VPS34 gene product has recently been characterized as part of a complex with a cellular protein of 150 kDa (Volinia, S., Dhand, R., Vanhaesebroeck, B., MacDougall, L. K., Stein, R., Zvelebil, M. J., Domin, J., Panaretou, C., and Waterfield, M. D. (1995) EMBO J. 14, 3339-3348). Here, cDNA cloning is used to show that the amino acid sequence of this protein, termed p150, is 29.6% identical and 53% similar to the yeast Vps15p protein, an established regulator of Vps34p. Northern blot analysis showed a ubiquitous tissue distribution for p150 similar to that previously observed with PtdIns 3-kinase. Recombinant p150 associated with PtdIns 3-kinase in vitro in a stable manner, resulting in a 2-fold increase in lipid kinase activity. Addition of phosphatidylinositol transfer protein (PI-TP) further stimulated the lipid kinase activity of the p150.PtdIns 3-kinase complex 3-fold. A PtdIns 3-kinase activity could also be co-immunoprecipitated from human cell lysates using anti-PI-TP antisera. This observation demonstrates that an interaction between a PtdIns 3-kinase and PI-TP occurs in vivo, which further implicates lipid transfer proteins in the regulation of PtdIns 3-kinase activity. These results suggest that the Vps15p.Vps34p complex has been conserved from yeast to man and in both species is involved in protein trafficking.

Prolactin activates tyrosyl phosphorylation of insulin receptor substrate 1 and phosphatidylinositol-3-OH kinase

Prolactin (PRL) has been demonstrated to induce tyrosine phosphorylation and activation of the cytoplasmic tyrosine kinase JAK2. The present study represents an initial effort to identify the phosphorylation repertoire of the PRL receptor (PRLR). For this purpose we have modified the rat PRLR cDNA to encode an additional N-terminal epitope specifically designed to allow the rapid purification of the PRLR and associated proteins from transfected cells. The Flag-tagged PRLR was stably expressed in the human 293 cell line. PRL induced tyrosine phosphorylation of proteins of 85, 95, and 185 kDa from 10 to 30 min after PRL stimulation. Immunoblot analysis of immunoprecipitation indicates that p85 corresponds to the 85-kDa regulatory subunit of phosphatidylinositol (PI)-3' kinase, p95 to PRLR, and p185 to insulin receptor substrate 1 (IRS-1). Both PI-3' kinase and IRS-1 appear to associate with PRLR in a PRL-dependent manner. These results thus indicate that kinases other than JAK2, namely PI-3' kinase, are activated by PRL.

The phosphatidylinositol 3-kinase inhibitors wortmannin and LY294002 inhibit autophagy in isolated rat hepatocytes

Recent studies indicate that phosphatidylinositol 3-kinase is essential in the regulation of many processes dependent on membrane flow. Autophagy is a complex pathway in which cell material, including proteins, can be degraded. Membrane flow plays a pivotal role in this process. To find out whether phosphatidylinositol 3-kinase is also required for autophagy, we tested the effects on autophagy of two structurally unrelated phosphatidylinositol 3-kinase inhibitors, wortmannin and 2-(4-morpholinyl)-8-phenylchromone (LY294002). The addition of low concentrations of each of these inhibitors to incubations of hepatocytes in the absence of amino acids resulted in a strong inhibition of proteolysis. The antiproteolytic effect of wortmannin (IC50 30 nM) and LY294002 (IC50 10 microM) was accompanied by inhibition of autophagic sequestration and not by an increase in lysosomal pH or a decrease in intracellular ATP. No further inhibition of proteolysis by the two compounds was observed when autophagy was already maximally inhibited by high concentrations of amino acids. 3-Methyladenine, which is commonly used as a specific inhibitor of autophagic sequestration, was an inhibitor of phosphatidylinositol 3-kinase, thus providing a target for its action. It is proposed that phosphatidylinositol 3-kinase activity is required for autophagy. 3-Methyladenine inhibits autophagy by inhibition of this enzyme.

Cloning and mutagenesis of the p110 alpha subunit of human phosphoinositide 3'-hydroxykinase

Activation of phosphoinositide 3'-hydroxykinase (P13K) is required for mitogenic signal transduction by several growth factors and oncogenes. P13K is a heterodimer consisting of a p85 regulatory subunit and a p110 catalytic subunit. In the current study, we report the cloning and characterization of the p110 alpha catalytic subunit of human P13K. This clone is highly homologous (> 99% amino acid identity) to bovine brain p110 alpha, but contains 10 amino acid differences from the human p110 alpha sequence previously reported. Comparison of this sequence with known Ser/Thr kinases and p110 homologs highlighted several conserved residues within the putative kinase domain. Mutational analysis of these residues (Asp915, (Asp933 + Phe934)) yielded P13K mutants with virtually complete loss of phosphoinositide phosphorylating activity. Expression of the wild-type p110 alpha protein in CHO cells is sufficient to activate the serum response element derived from the promoter of c-fos, an immediate early gene product. In contrast, the catalytically impaired p110 alpha mutants as well as the p85 alpha subunit of P13K were inactive in the fos assay. These studies suggest that the mitogenic signal transduction pathway mediated by P13K is dependent upon the enzymatic activity of the p110 alpha subunit of P13K.

Transport through the yeast endocytic pathway occurs through morphologically distinct compartments and requires an active secretory pathway and Sec18p/N-ethylmaleimide-sensitive fusion protein

Molecules travel through the yeast endocytic pathway from the cell surface to the lysosome-like vacuole by passing through two sequential intermediates. Immunofluorescent detection of an endocytosed pheromone receptor was used to morphologically identify these intermediates, the early and late endosomes. The early endosome is a peripheral organelle that is heterogeneous in appearance, whereas the late endosome is a large perivacuolar compartment that corresponds to the prevacuolar compartment previously shown to be an endocytic intermediate. We demonstrate that inhibiting transport through the early secretory pathway in sec mutants quickly impedes transport from the early endosome. Treatment of sensitive cells with brefeldin A also blocks transport from this compartment. We provide evidence that Sec18p/N-ethylmaleimide-sensitive fusion protein, a protein required for membrane fusion, is directly required in vivo for forward transport early in the endocytic pathway. Inhibiting protein synthesis does not affect transport from the early endosome but causes endocytosed proteins to accumulate in the late endosome. As newly synthesized proteins and the late steps of secretion are not required for early to late endosome transport, but endoplasmic reticulum through Golgi traffic is, we propose that efficient forward transport in the early endocytic pathway requires delivery of lipid from secretory organelles to endosomes.

Actin-, myosin- and ubiquitin-dependent endocytosis

Endocytosis is a general term that is used to describe the internalization of external and plasma membrane molecules into the cell interior. In fact, several different mechanisms exist for the internalization step of this process. In this review we emphasize the work on the actin-dependent pathways, in particular in the yeast Saccharomyces cerevisiae, because several components of the molecular machinery are identified. In this yeast, the analysis of endocytosis in various mutants reveals a requirement for actin, calmodulin, a type I myosin, as well as a number of other proteins that affect actin dynamics. Some of these proteins have homology to proteins in animal cells that are believed to be involved in endocytosis. In addition, the demonstration that ubiquitination of some cell surface molecules is required for their efficient internalization is described. We compare the actin, myosin and ubiquitin requirements for endocytosis with recent results found studying these processes using Dictyostelium discoideum and animal cells.

Translocation of Src kinase to the cell periphery is mediated by the actin cytoskeleton under the control of the Rho family of small G proteins

We have isolated Swiss 3T3 subclones that are resistant to the mitogenic and morphological transforming effects of v-Src as a consequence of aberrant translocation of the oncoprotein under low serum conditions. In chicken embryo and NIH 3T3 fibroblasts under similar conditions, v-Src rapidly translocates from the perinuclear region to the focal adhesions upon activation of the tyrosine kinase, resulting in downstream activation of activator protein-1 and mitogen-activated protein kinase, which are required for the mitogenic and transforming activity of the oncoprotein. Since serum deprivation induces cytoskeletal disorganization in Swiss 3T3, we examined whether regulators of the cytoskeleton play a role in the translocation of v-Src, and also c-Src, in response to biological stimuli. Actin stress fibers and translocation of active v-Src to focal adhesions in quiescent Swiss 3T3 cells were restored by microinjection of activated Rho A and by serum. Double labeling with anti-Src and phalloidin demonstrated that v-Src localized along the reformed actin filaments in a pattern that would be consistent with trafficking in complexes along the stress fibers to focal adhesions. Furthermore, treatment with the actin-disrupting drug cytochalasin D, but not the microtubule-disrupting drug nocodazole, prevented v-Src translocation. In addition to v-Src, we observed that PDGF-induced, Rac-mediated membrane ruffling was accompanied by translocation of c-Src from the cytoplasm to the plasma membrane, an effect that was also blocked by cytochalasin D. Thus, we conclude that translocation of Src from its site of synthesis to its site of action at the cell membrane requires an intact cytoskeletal network and that the small G proteins of the Rho family may specify the peripheral localization in focal adhesions or along the membrane, mediated by their effects on the cytoskeleton.

A role for phosphoinositide 3-kinase in the completion of macropinocytosis and phagocytosis by macrophages

Phosphoinositide 3-kinase (PI 3-kinase) has been implicated in growth factor signal transduction and vesicular membrane traffic. It is thought to mediate the earliest steps leading from ligation of cell surface receptors to increased cell surface ruffling. We show here that inhibitors of PI 3-kinase inhibit endocytosis in macrophages, not by interfering with the initiation of the process but rather by preventing its completion. Consistent with earlier studies, the inhibitors wortmannin and LY294002 inhibited fluid-phase pinocytosis and Fc receptor-mediated phagocytosis, but they had little effect on the receptor-mediated endocytosis of diI-labeled, acetylated, low density lipoprotein. Large solute probes of endocytosis reported greater inhibition by wortmannin than smaller probes did, indicating that macropinocytosis was affected more than micropinocytosis. Since macropinocytosis and phagocytosis are actin-mediated processes, we expected that their inhibition by wortmannin resulted from deficient signaling from macrophage colony-stimulating factor (M-CSF) receptors or Fc receptors to the actin cytoskeleton. However, video microscopy showed cell surface ruffling in wortmannin-treated cells, and increased ruffling after addition of M-CSF or phorbol myristate acetate. Quantitative measurements of video data reported slightly diminished ruffling in wortmannin-treated cells. Remarkably, the ruffles that formed in wortmannin-treated macrophages all receded into the cytoplasm without closing into macropinosomes. Similarly, wortmannin and LY294002 did not inhibit the extension of actin-rich pseudopodia along IgG-opsonized sheep erythrocytes, but instead prevented them from closing into phagosomes. These findings indicate that PI 3-kinase is not necessary for receptor-mediated stimulation of pseudopod extension, but rather functions in the closure of macropinosomes and phagosomes into intracellular organelles.

TOR2 is required for organization of the actin cytoskeleton in yeast

The Saccharomyces cerevisiae gene TOR2 encodes a putative phosphatidylinositol kinase that has two essential functions. One function is redundant with TOR1, a TOR2 homolog, and is required for signaling translation initiation and early G1 progression. The second essential function is unique to TOR2. Here we report that loss of the TOR2-unique function disrupts polarized distribution of the actin cytoskeleton. A screen for dosage suppressors of a dominant negative TOR2 allele identified TCP20/CCT6, encoding a subunit of the cytosolic chaperonin TCP-1 that is involved in the biogenesis of actin structures. Overexpression of TCP20 restores growth and polarized distribution of the actin cytoskeleton in a tor2 mutant. TCP20 overexpression does not restore growth in a tor1 tor2 double mutant. We suggest that the unique function of the phosphatidylinositol kinase homolog TOR2 is required for signaling organization of the actin cytoskeleton during the cell cycle. TOR2, via its two functions, may thus integrate temporal and spatial control of cell growth.

Phosphatidylinositol 3-kinase-independent signal transduction pathway for platelet-derived growth factor-induced chemotaxis

Platelet-derived growth factor (PDGF)-BB is a potent chemoattractant for mesenchymal cells. Intracellular signal transduction for PDGF-induced chemotactic response has been reported to be dependent on phosphatidylinositol 3-kinase (PI3K) activation. Here, we report a PI3K-independent pathway operating for PDGF-induced chemotaxis in vascular smooth muscle cells and other cell types. Two different PI3K inhibitors, wortmannin (WT, 1 nM-1 microM) and LY294002 (100 nM-10 microM), did not inhibit PDGF-induced chemotaxis in smooth muscle cells and Swiss 3T3 cells, whereas WT inhibited activity of PI3K that were immunopurified from PDGF-stimulated cells as well as PI3K purified from cells that were stimulated with PDGF in the presence of the same concentrations of WT. Similarly, WT (100 nM) abolished the increase in intracellular phosphatidylinositol 3,4,5-triphosphate after PDGF stimulation. Furthermore, Chinese hamster ovary/Deltap85 cells overexpressing a dominant negative p85 subunit of PI3K showed a chemotactic response comparable to that of parental cells while showing a remarkable decrease in PI3K activity. Rapamycin, a specific inhibitor of pp70 S6 kinase, which is one of the well characterized downstreams of PI3K, did not inhibit PDGF-induced chemotaxis. Both WT and LY294002 inhibited PDGF-induced amino acid uptake and actin-stress fiber reorganization and partly inhibited PDGF-induced glucose incorporation in Swiss 3T3 cells. Our findings indicate that, in vascular smooth muscle cells and other cell types, the signal transduction for PDGF-induced chemotaxis is independent of PI3K activity while the signal transduction for PDGF-induced amino acid uptake, glucose incorporation, and cytoskeletal reorganization is dependent on PI3K.

Association between phosphatidylinositol-3 kinase, Cbl and other tyrosine phosphorylated proteins in colony-stimulating factor-1-stimulated macrophages

Colony stimulating factor-1 (CSF-1) stimulation of the macrophage cell line BAC1.2F5 and murine bone marrow-derived macrophages resulted in tyrosine phosphorylation of phosphatidylinositol-3 kinase (PI-3 kinase) p85 alpha and its stable association with several tyrosine phosphorylated proteins, including CSF-1 receptor (p165), p120, p95 and p55-p60. p120 co-migrated with the product of the protooncogene c-cb1 in anti-p85 alpha immunoprecipitates, and associated with p85 alpha in a rapid and transient manner. Reciprocal experiments confirmed the presence of p85 alpha in anti-Cb1 immunoprecipitates on CSF-1 stimulation of macrophages. PI-3 kinase immunoprecipitates from the myeloid FDC-P1 cell line expressing mutant CSF-1 receptor (Y721F), which does not associate with PI-3 kinase, still contained Cbl. The identity of the tyrosine phosphorylated protein p95 remains unknown. The interaction between p85 alpha and the tyrosine phosphorylated proteins survived anion-exchange chromatography, suggesting perhaps the presence of a stable complex; furthermore, in CSF-1-treated BAC1.2F5 cell extracts, only one of the two pools of PI-3 kinase separated by chromatography was present in this putative complex. The association did not appear to correlate with proliferation, since a similar interaction between p85 alpha and tyrosine phosphorylated proteins was also observed in poorly proliferating resident peritoneal macrophages stimulated with CSF-1. The possible significance of these findings for CSF-1-regulated macrophage functions is discussed.

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.

Wortmannin inhibits both insulin- and contraction-stimulated glucose uptake and transport in rat skeletal muscle

The role of phosphatidylinositol (PI) 3-kinase for insulin- and contraction-stimulated muscle glucose transport was investigated in rat skeletal muscle perfused with a cell-free perfusate. The insulin receptor substrate-1-associated PI 3-kinase activity was increased sixfold upon insulin stimulation but was unaffected by contractions. In addition, the insulin-stimulated PI 3-kinase activity and muscle glucose uptake and transport in individual muscles were dose-dependently inhibited by wortmannin with one-half maximal inhibition values of approximately 10 nM and total inhibition at 1 microM. This concentration of wortmannin also decreased the contraction-stimulated glucose transport and uptake by approximately 30-70% without confounding effects on contractility or on muscle ATP and phosphocreatine concentrations. At higher concentrations (3 and 10 microM), wortmannin completely blocked the contraction-stimulated glucose uptake but also decreased the contractility. In conclusion, inhibition of PI 3-kinase with wortmannin in skeletal muscle coincides with inhibition of insulin-stimulated glucose uptake and transport. Furthermore, in contrast to recent findings in incubated muscle, wortmannin also inhibited contraction-stimulated glucose uptake and transport. The inhibitory effect of wortmannin on contraction-stimulated glucose uptake may be independent of PI 3-kinase activity or due to inhibition of a subfraction of PI 3-kinase with low sensitivity to wortmannin.

Binding of ATP and ATP analogues to the uncoating ATPase Hsc70 (70 kDa heat-shock cognate protein)

Nucleotide binding to the 70 kDa heat-shock cognate protein (Hsc70) from mung bean seeds and pig brain was investigated, as well as the clathrin uncoating activity of Hsc70 in the presence of these nucleotides. The two enzymes were found to behave identically. ATP bound to two different forms of Hsc70, with dissociation constants of 1.1 +/- 0.1 microM and 1.4 +/- 0.7 mM respectively at 25 degrees C. This corresponds to delta G0' = -34 and -16 kJ/mol respectively. From the temperature-dependence of the dissociation constant of the high-affinity site, delta H0' was calculated to -36 +/- 2 kJ/mol. This gives delta S0' = 6.7 J/mol per K. Adenosine 5'-[gamma-thio]triphosphate, ADP, adenosine 5'-[beta, gamma-imino]triphosphate and adenosine 5'-[beta, gamma-methylene]triphosphate showed dissociation constants of 2.3, 11, 31 and 284 microM respectively. The order of affinities corresponded to the order of effectiveness in uncoating of pig brain coated vesicles. The implications of these findings for the mechanism of Hsc70 action are discussed.