Sec18p (NSF)-driven release of Sec17p (alpha-SNAP) can precede docking and fusion of yeast vacuoles

S. cerevisiae inherits its vacuole by projecting vacuole-derived membrane vesicles and tubules into the bud, where they fuse to establish the daughter vacuole. This homotypic fusion event can be assayed in vitro. It requires Sec17p and Sec18p, the homologs of the mammalian alpha-SNAP and NSF, which cooperate in multiple steps of membrane trafficking. We now report that Sec17p, Sec18p, and ATP are only needed for an early stage of the reaction that results in Sec17p release. Sec17p and Sec18p actions precede, and are needed for, the step employing the Ras-like GTPase Ypt7p. Sec18p-driven release of Sec17p can even precede vacuole docking, as it can occur prior to mixing of vacuoles and is insensitive to vacuole concentration. Sec17p and Sec18p thus may function in a predocking stage of the reaction, rather than in bilayer fusion per se.

Trans-complex formation by proteolipid channels in the terminal phase of membrane fusion

SNAREs (soluble N-ethylmaleimide-sensitive factor attachment protein receptors) and Rab-GTPases, together with their cofactors, mediate the attachment step in the membrane fusion of vesicles. But how bilayer mixing--the subsequent core process of fusion--is catalysed remains unclear. Ca2+/calmodulin controls this terminal process in many intracellular fusion events. Here we identify V0, the membrane-integral sector of the vacuolar H+-ATPase, as a target of calmodulin on yeast vacuoles. Between docking and bilayer fusion, V0 sectors from opposing membranes form complexes. V0 trans-complex formation occurs downstream from trans-SNARE pairing, and depends on both the Rab-GTPase Ypt7 and calmodulin. The maintenance of existing complexes and completion of fusion are independent of trans-SNARE pairs. Reconstituted proteolipids form sealed channels, which can expand to form aqueous pores in a Ca2+/calmodulin-dependent fashion. V0 trans-complexes may therefore form a continuous, proteolipid-lined channel at the fusion site. We propose that radial expansion of such a protein pore may be a mechanism for intracellular membrane fusion.

Ca2+/calmodulin signals the completion of docking and triggers a late step of vacuole fusion

The basic reaction mechanisms for membrane fusion in the trafficking of intracellular membranes and in exocytosis are probably identical. But in contrast to regulated exocytosis, intracellular fusion reactions are referred to as 'constitutive' as no final Ca2+-dependent triggering step has been observed. Although transport from the endoplasmic reticulum to the Golgi apparatus in the cell depends on Ca2+, as does endosome fusion and assembly of the nuclear envelope, it is unclear whether Ca2+ triggers these events. Membrane fusion involves several subreactions: priming, tethering and docking. Proteins that are needed for fusion include p115, SNAPs, NSF, SNAREs and small GTPases, which operate in these early reactions, but the machinery that catalyses the final mixing of biological membranes is still unknown. Here we show that Ca2+ is released from the vacuolar lumen following completion of the docking step. We have identified calmodulin as the putative Ca2+ sensor and as the first component required in the post-docking phase of vacuole fusion. Calmodulin binds tightly to vacuoles upon Ca2+ release. Unlike synaptotagmin or syncollin in exocytosis, calmodulin does not act as a fusion clamp but actively promotes bilayer mixing. Hence, activation of SNAREs is not sufficient to drive bilayer mixing between physiological membranes. We propose that Ca2+ control of the latest phase of membrane fusion may be a conserved feature, relevant not only for exocytosis, but also for intracellular, 'constitutive' fusion reactions. However, the origin of the Ca2+ signal, its receptor and its mode of processing differ.

The prognostic significance of proliferating cell nuclear antigen, epidermal growth factor receptor, and mdr gene expression in colorectal cancer

BACKGROUND

Proliferating cell nuclear antigen (PCNA), a proliferation marker, epidermal growth factor receptor (EGFR), a glycoprotein that plays a role in tumorigenesis by binding the mitogenic epidermal growth factor, and P-glycoprotein, the mdr gene product, are considered to be of prognostic relevance in different tumor types. Parameters that allow prediction of the course of disease in colorectal cancer would aid the development of improved treatment strategies.

METHODS

Immunohistochemical staining was performed on paraffin-embedded sections of 82 colorectal adenocarcinomas and 18 lymph node metastases. EGFR and P glycoprotein expression was evaluated semiquantitatively; PCNA expression was analyzed quantitatively.

RESULTS

An inverse relationship between the percentage of PCNA-positive cells and survival times could be demonstrated, survival differed significantly among the quartiles (P < 0.02). The median and range of the percentage of PCNA-positive cells in primary tumors and lymph node metastases were similar. The extent of EGFR expression also revealed significant differences concerning survival times; patients with more than 50% stained tumor cells had a poorer prognosis than those with less than 50% stained cells. P-glycoprotein expression was found to have no influence on survival.

CONCLUSIONS

Knowledge of the percentage of PCNA-positive cells could be especially helpful in deciding whether to treat patients with localized disease further because adjuvant chemotherapy affects mainly dividing cells and should, therefore, be more successful in tumors with high proliferative activity.

Classification of Non-Bacterial Osteitis: Retrospective study of clinical, immunological and genetic aspects in 89 patients

OBJECTIVE

To define non-bacterial osteitis (NBO) as a clinical entity possibly associated with autoimmune manifestations. Patients with sterile osteitis were analysed to develop diagnostic criteria.

METHODS

A total of 89 patients with non-bacterial inflammatory bone lesions were observed for a median of 49 months. History, diagnostic imaging, laboratory and histological data were obtained. Mutation analysis in the genes PSTPIP1 and PSTPIP2 was performed.

RESULTS

Patients had an onset of disease at a median age of 10 yrs [interquartile range (IQR) 7.5-12] and suffered a median period of 21 (IQR 9-52) months with a median of three foci per patient. Twenty percent of all the patients demonstrated associated autoimmune disorders, particularly of the skin and bowel. The majority of bone lesions were located in the vertebrae and metaphyses. Slight-to-moderate elevation of inflammation values were found in all the patients and antinuclear antibodies were elevated in 30%. Non-steroidal anti-inflammatory drugs (NSAIDs) were effective in 85% of the patients. HLA-B27 and Human Leukocyte Antigen-DR (HLA-DR)-classification did not differ from the general population. Autoimmune diseases in 40% of all the families, multiply affected family members, linkage to 18q21 and mouse models strongly indicate a genetic basis for NBO. We observed three different courses of disease regarding the duration of complaints, rate of complications and associated autoimmune manifestations leading to a new classification of NBO.

CONCLUSIONS

Clinical analysis of our cohort leads us to define NBO as a distinct disease entity with three clinical presentations: acute NBO, chronic recurrent multifocal osteomyelitis or persistent chronic NBO. Diagnostic criteria were proposed to differentiate NBO from diseases with similar clinical presentation.

Lithium induces rapid relief of depression in tricyclic antidepressant drug non-responders

Eight patients suffering from a major unipolar depression and having field to respond to treatment for three weeks or more with tricyclic antidepressants were given lithium. All eight patients experienced a remarkable relief of their depression within 48 hours. This rapid antidepressant effect of lithium in "treatment-resistant' patients might be due to the enhancement of the efficacy of the central serotoninergic system, unveiling the tricyclic antidepressant-induced sensitization of the serotoninergic postsynaptic receptors.

Metabolic effects of a low-glycemic-index diet

Six healthy male volunteers underwent 2-wk metabolically controlled high-glycemic-index (GI) and low-GI diets in random order. Over the low-GI diet significant reductions were seen in serum fructosamine (7.0 +/- 1.0%, p less than 0.01), 12-h blood glucose profile (37 +/- 7%, p less than 0.01), and total serum cholesterol (15 +/- 3%, p less than 0.01). As a measure of insulin secretion, 24-h urinary C-peptide levels were 32 +/- 10% lower (p less than 0.05) after the low-GI than after the high-GI diet. Lower C-peptide levels were maintained after a standard carbohydrate challenge after the low-GI diet despite higher blood glucose levels. Differences in blood glucose were not seen after a 5-g intravenous glucose challenge. These results are of interest with respect to the effect that prolonged postprandial reductions in nutrient fluxes and insulin secretion may have on carbohydrate and lipid metabolism and renal function.

Ubiquitin-dependent degradation of certain protein substrates in vitro requires the molecular chaperone Hsc70

Degradation of a protein via the ubiquitin system involves two discrete steps, signaling by covalent conjugation of multiple moieties of ubiquitin and degradation of the tagged substrate. Conjugation is catalyzed via a three-step mechanism that involves three distinct enzymes that act successively: E1, E2, and E3. The first two enzymes catalyze activation of ubiquitin and transfer of the activated moiety to E3, respectively. E3, to which the substrate is specifically bound, catalyzes formation of a polyubiquitin chain that is anchored to the targeted protein. The polyubiquitin-tagged protein is degraded by the 26 S proteasome, and free and reutilizable ubiquitin is released. In addition to the three conjugating enzymes, targeting of certain proteins requires association with ancillary proteins and/or post-translational modification(s). Using a specific antibody to deplete cell extract from the molecular chaperone Hsc70, we demonstrate that this protein is required for the degradation of actin, alpha-crystallin, glyceraldehyde-3-phosphate dehydrogenase, alpha-lactalbumin, and histone H2A. In contrast, the degradation of bovine serum albumin, lysozyme, and oxidized RNase A is Hsc70-independent. Mechanistic analysis revealed that the chaperone is required for the conjugation reaction; however, it does not substitute for E3. Involvement of the chaperone in the proteolytic process requires complex formation with the substrate. Formation of this complex appears to be essential in the proteolytic process. In addition, the proper function of the chaperone in the proteolytic process requires the presence of K+, which allows rapid cycles of dissociation and association of the complex. The chaperone may act by binding to the substrate and unfolding it to expose a ubiquitin ligase-binding site. In addition, it can also act directly on the ubiquitination machinery.

Docking of yeast vacuoles is catalyzed by the Ras-like GTPase Ypt7p after symmetric priming by Sec18p (NSF)

Vacuole inheritance in yeast involves the formation of tubular and vesicular "segregation structures" which migrate into the bud and fuse there to establish the daughter cell vacuole. Vacuole fusion has been reconstituted in vitro and may be used as a model for an NSF-dependent reaction of priming, docking, and fusion. We have developed biochemical and microscopic assays for the docking step of in vitro vacuole fusion and characterized its requirements. The vacuoles must be primed for docking by the action of Sec17p (alpha-SNAP) and Sec18p (NSF). Priming is necessary for both fusion partners. It produces a labile state which requires rapid docking in order to lead productively to fusion. In addition to Sec17p/Sec18p, docking requires the activity of the Ras-like GTPase Ypt7p. Unlike Sec17p/Sec18p, which must act before docking, Ypt7p is directly involved in the docking process itself.

Viruses transfer the antiviral second messenger cGAMP between cells

Cyclic GMP-AMP synthase (cGAS) detects cytosolic DNA during virus infection and induces an antiviral state. cGAS signals by synthesis of a second messenger, cyclic GMP-AMP (cGAMP), which activates stimulator of interferon genes (STING). We show that cGAMP is incorporated into viral particles, including lentivirus and herpesvirus virions, when these are produced in cGAS-expressing cells. Virions transferred cGAMP to newly infected cells and triggered a STING-dependent antiviral program. These effects were independent of exosomes and viral nucleic acids. Our results reveal a way by which a signal for innate immunity is transferred between cells, potentially accelerating and broadening antiviral responses. Moreover, infection of dendritic cells with cGAMP-loaded lentiviruses enhanced their activation. Loading viral vectors with cGAMP therefore holds promise for vaccine development.

Phospholipid composition of highly purified mitochondrial outer membranes of rat liver and Neurospora crassa. Is cardiolipin present in the mitochondrial outer membrane?

Isolated mitochondrial outer membrane vesicles (OMV) are a suitable system for studying various functions of the mitochondrial outer membrane. For studies on mitochondrial lipid import as well as for studies on the role of lipids in processes occurring in the outer membrane, knowledge of the phospholipid composition of the outer membrane is indispensable. Recently, a mild subfractionation procedure was described for the isolation of highly purified OMV from mitochondria of Neurospora crassa (Mayer, A., Lill, R. and Neupert, W. (1993) J. Cell Biol. 121, 1233-1243). This procedure, which consists of swelling and mechanical disruption of mitochondria followed by two steps of sucrose density gradient centrifugation, was adapted for the isolation of OMV from rat liver mitochondria. Using the appropriate enzyme markers it is shown that the resulting OMV are obtained in a yield of 25%, and that their purity is superior to that of previous OMV preparations. Analysis of the phospholipid composition of the OMV showed that phosphatidylcholine, phosphatidylethanolamine and phosphatidylinositol are the major phospholipid constituents, and that cardiolipin is only present in trace amounts. The phospholipid composition is very similar to that of the highly purified OMV from mitochondria of Neurospora crassa, although the latter still contain a small amount of cardiolipin.

A cholera toxoid-insulin conjugate as an oral vaccine against spontaneous autoimmune diabetes

Mucosally induced immunological tolerance is an attractive strategy for preventing or treating illnesses resulting from untoward inflammatory immune reactions against self- or non-self-antigens. Oral administration of relevant autoantigens and allergens has been reported to delay or suppress onset of clinical disease in a number of experimental autoimmune and allergic disorders. However, the approach often requires repeated feeding of large amounts of tolerogens over long periods and is only partly effective in animals already systemically sensitized to the ingested antigen such as in animals already harboring autoreactive T cells, and thus presumably also in humans with an autoimmune disease. We have recently shown that oral administration of microgram amounts of antigen coupled to cholera toxin B subunit (CTB), can effectively suppress systemic T cell reactivity in naive as well as in immune animals. We now report that feeding small amounts (2-20 microg) of human insulin conjugated to CTB can effectively suppress beta cell destruction and clinical diabetes in adult nonobese diabetic (NOD) mice. The protective effect could be transferred by T cells from CTB-insulin-treated animals and was associated with reduced lesions of insulitis. Furthermore, adoptive co-transfer experiments involving injection of Thy-1,2 recipients with diabetogenic T cells from syngeneic mice and T cells from congenic Thy-1,1 mice fed with CTB-insulin demonstrated a selective recruitment of Thy-1,1 donor cells in the peripancreatic lymph nodes concomitant with reduced islet cell infiltration. These results suggest that protection against autoimmune diabetes can be achieved by feeding minute amounts of a pancreas islet cell autoantigen linked to CTB and appears to involve the selective migration and retention of protective T cells into lymphoid tissues draining the site of organ injury.

Identification of human immunodeficiency virus type 1 envelope genes recombinant between subtypes B and F in two epidemiologically linked individuals from Brazil

Sequence analysis of a human immunodeficiency virus type 1 env gene PCR amplified from a Brazilian woman's peripheral blood mononuclear cell DNA (sample RJIO1) showed that it was likely to have been derived from a double recombination event between human immunodeficiency virus type 1 subtypes B and F. The major portion of the gp120 coding sequence belonged to the B lineage, but a segment of the C2 to V3 region (approximately 135 nucleotides) clearly associated with sequences of the F lineage. The subtype F-like segment had 15 noncontiguous signature nucleotides in common with Brazilian subtype F sequences that were not found, or were rare, in subtype B sequences. In contrast, this same segment had only 3 signature nucleotides shared with subtype B sequences and not present in the Brazilian subtype F sequences. Phylogenetic analysis, amino acid signature pattern analysis, and the pattern of synonymous mutations all supported the hypothesis of a recombinational origin of the RJIO1 sequence. Related recombinant genes were also detected in peripheral blood mononuclear cell DNA obtained from the woman's recent sexual partner, indicating that the recombination event probably occurred at some previous time in the chain of virus transmission. Divergent viral sequences in the V3 region were found in the male sexual partner, while a relatively homogeneous viral population was detected in the woman, consistent with her recent infection.

Mitochondrial protein import: reversible binding of the presequence at the trans side of the outer membrane drives partial translocation and unfolding

The mechanism of translocation of matrix-targeted, cleavable preproteins across the mitochondrial outer membrane was studied using purified outer membrane vesicles. The N-terminal presequence interacts in a sequential and reversible fashion with two specific binding sites. The first one is provided by protease-sensitive receptors on the surface of the membrane (cis site); the second one is located at the inner face of the outer membrane (trans site). Binding to the trans site drives translocation of the N-terminal portion of the preprotein and, at the same time, unfolding of its mature part. We suggest that the outer membrane protein import machinery forms a translocation channel that permits reversible sliding of preproteins and prevents their lateral aggregation in the membrane. Although translocation can be initiated by the outer membrane, its completion requires coupling to the energetic system of the inner membrane. Our data assign an essential role to the presequence, not only for efficient targeting, but also for the translocation step.

A molecular approach to leukemogenesis: mouse lymphomas contain an activated c-ras oncogene

By inducing mouse thymomas with carcinogens and gamma-radiation, we have studied the potential of tumor DNA to induce foci in rodent fibroblasts. A high percentage of the tumors used transformed the cultured cells, and the oncogenic phenotype segregated with extra copies of the c-ras gene family. There appears to be selectivity in the activated gene because so far all analyzed tumors induced by carcinogen have activated the N-ras gene, and those induced by radiation have activated the K-ras gene. The K-ras gene is the cellular counterpart of the viral ras oncogene in Kirsten murine sarcoma virus, but the N-ras has not yet been found in a retrovirus. The transformed cells have a marked increase in expression of the oncogene at the RNA and protein level. This model system might be a powerful tool in the study of leukemogenesis.

Autophagic tubes: vacuolar invaginations involved in lateral membrane sorting and inverse vesicle budding

Many intracellular compartments of eukaryotic cells do not adopt a spherical shape, which would be expected in the absence of mechanisms organizing their structure. However, little is known about the principles determining the shape of organelles. We have observed very defined structural changes of vacuoles, the lysosome equivalents of yeast. The vacuolar membrane can form a large tubular invagination from which vesicles bud off into the lumen of the organelle. Formation of the tube is regulated via the Apg/Aut pathway. Its lumen is continuous with the cytosol, making this inverse budding reaction equivalent to microautophagocytosis. The tube is highly dynamic, often branched, and defined by a sharp kink of the vacuolar membrane at the site of invagination. The tube is formed by vacuoles in an autonomous fashion. It persists after vacuole isolation and, therefore, is independent of surrounding cytoskeleton. There is a striking lateral heterogeneity along the tube, with a high density of transmembrane particles at the base and a smooth zone devoid of transmembrane particles at the tip where budding occurs. We postulate a lateral sorting mechanism along the tube that mediates a depletion of large transmembrane proteins at the tip and results in the inverse budding of lipid-rich vesicles into the lumen of the organelle.

Control of the terminal step of intracellular membrane fusion by protein phosphatase 1

Intracellular membrane fusion is crucial for the biogenesis and maintenance of cellular compartments, for vesicular traffic between them, and for exo- and endocytosis. Parts of the molecular machinery underlying this process have been identified, but most of these components operate in mutual recognition of the membranes. Here it is shown that protein phosphatase 1 (PP1) is essential for bilayer mixing, the last step of membrane fusion. PP1 was also identified in a complex that contained calmodulin, the second known factor implicated in the regulation of bilayer mixing. The PP1-calmodulin complex was required at multiple sites of intracellular trafficking; hence, PP1 may be a general factor controlling membrane bilayer mixing.

Latency versus persistence or intermittent recurrences: evidence for a latent state of murine cytomegalovirus in the lungs

The state of cytomegalovirus (CMV) after the resolution of acute infection is an unsolved problem in CMV research. While the term "latency" is in general use to indicate the maintenance of the viral genome, a formal exclusion of low-level persistent productive infection depends on the sensitivity of the assay for detecting infectious virus. We have improved the method for detecting infectivity by combining centrifugal infection of permissive indicator cells in culture, expansion to an infectious focus, and sensitive detection of immediate-early RNA in the infected cells by reverse transcriptase PCR. A limiting-dilution approach defined the sensitivity of this assay. Infectivity was thereby found to require as few as 2 to 9 virion DNA molecules of murine CMV, whereas the standard measure of infectivity, the PFU, is the equivalent of ca. 500 viral genomes. Since murine CMV forms multicapsid virions in most infected tissues, the genome-to-infectivity ratio is necessarily >1. This assay thus sets a new standard for investigating CMV latency. In mice in which acute infection was resolved, the viral DNA load in the lungs, a known organ site of CMV latency and recurrence, was found to be 1 genome per 40 lung cells, or a total of ca. 1 million genomes. Despite this high load of CMV DNA, infectious virus was not detected with the improved assay, but recurrence was inducible. These data provide evidence against a low-level persistent productive infection and also imply that intermittent spontaneous recurrence is not a frequent event in latently infected lungs.

Primary thyroid lymphoma is a heterogeneous disease

We retrospectively analyzed 26 patients with thyroid lymphoma (TL). Patients were mostly females, with a median age of 59 yr, presenting a rapidly growing nodular goiter with or without cervical adenopathy, without symptoms related to lymphoma for 81% and hypothyroidism in 61%. A previous history of Hashimoto thyroiditis was observed in 11 patients. Six different subtypes of lymphoma were observed: 13 of 26 (50%) had diffuse large B cell lymphoma, 6 (23%) mucosa- associated lymphoid tissue (MALT) lymphoma, 3 (12%) had follicular lymphoma, 2 (7%) had Hodgkin's disease, 1 (4%) had small lymphocytic lymphoma, and 1 (4%) had Burkitt's lymphoma. Diffuse large B cell lymphoma patients presented a compressive multinodular goiter, cervical adenopathy (66%), disseminated disease (50%), and poor performance status, with a poor prognosis (5-yr survival at 44%) despite a treatment based on a multidrug regimen. MALT lymphoma arose in patients with previous history of Hashimoto disease, was localized in all but 1, and was biologically associated with hypothyroidism and a high level of serum antithyroid antibodies. With total thyroidectomy, prognosis was good (5-yr survival at 100%). We did not find any routine clinical or biological parameters that could predict the evolution from Hashimoto's thyroiditis to MALT lymphoma. In conclusion, we confirmed the histological heterogeneity of TL corresponding to different clinical presentations and different prognoses.

Translocation and insertion of precursor proteins into isolated outer membranes of mitochondria

Nuclear-encoded proteins destined for mitochondria must cross the outer or both outer and inner membranes to reach their final sub-mitochondrial locations. While the inner membrane can translocate preproteins by itself, it is not known whether the outer membrane also contains an endogenous protein translocation activity which can function independently of the inner membrane. To selectively study the protein transport into and across the outer membrane of Neurospora crassa mitochondria, outer membrane vesicles were isolated which were sealed, in a right-side-out orientation, and virtually free of inner membranes. The vesicles were functional in the insertion and assembly of various outer membrane proteins such as porin, MOM19, and MOM22. Like with intact mitochondria, import into isolated outer membranes was dependent on protease-sensitive surface receptors and led to correct folding and membrane integration. The vesicles were also capable of importing a peripheral component of the inner membrane, cytochrome c heme lyase (CCHL), in a receptor-dependent fashion. Thus, the protein translocation machinery of the outer mitochondrial membrane can function as an independent entity which recognizes, inserts, and translocates mitochondrial preproteins of the outer membrane and the intermembrane space. In contrast, proteins which have to be translocated into or across the inner membrane were only specifically bound to the vesicles, but not imported. This suggests that transport of such proteins involves the participation of components of the intermembrane space and/or the inner membrane, and that in these cases the outer membrane translocation machinery has to act in concert with that of the inner membrane.

MOM22 is a receptor for mitochondrial targeting sequences and cooperates with MOM19

Recognition of targeting signals is a crucial step in protein sorting within the cell. So far, only a few components capable of deciphering targeting signals have been identified, and insights into the chemical nature of the interaction between the signals and their receptors are scarce. Using highly purified mitochondrial outer membrane vesicles, we demonstrate that MOM22 and MOM19, components of the protein import complex of the outer membrane, bind preproteins at the mitochondrial surface in a reversible fashion. Interaction specifically and directly occurs with the N-terminal presequence and is abolished after inactivation of either MOM22 or MOM19. Binding is salt sensitive, suggesting that recognition involves electrostatic forces between the positive charges of the presequence and the acidic cytosolic domain of MOM22. MOM19 and MOM22 can be cross-linked with high efficiency. We propose that the two proteins form a complex which functions as the presequence receptor at the mitochondrial surface and facilitates the movement of preproteins into the translocation pore.

Phosphatidylinositol 4,5-bisphosphate regulates two steps of homotypic vacuole fusion

Yeast vacuoles undergo cycles of fragmentation and fusion as part of their transmission to the daughter cell and in response to changes of nutrients and the environment. Vacuole fusion can be reconstituted in a cell free system. We now show that the vacuoles synthesize phosphoinositides during in vitro fusion. Of these phosphoinositides, phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate (PI(4,5)P(2)) are important for fusion. Monoclonal antibodies to PI(4,5)P(2), neomycin (a phosphoinositide ligand), and phosphatidylinositol-specific phospholipase C interfere with the reaction. Readdition of PI(4, 5)P(2) restores fusion in each case. Phosphatidylinositol 3-phosphate and PI(3,5)P(2) synthesis are not required. PI(4,5)P(2) is necessary for priming, i.e., for the Sec18p (NSF)-driven release of Sec17p (alpha-SNAP), which activates the vacuoles for subsequent tethering and docking. Therefore, it represents the kinetically earliest requirement identified for vacuole fusion so far. Furthermore, PI(4,5)P(2) is required at a step that can only occur after docking but before the BAPTA sensitive step in the latest stage of the reaction. We hence propose that PI(4,5)P(2) controls two steps of vacuole fusion.