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

Lysophosphatidic acid induces exocytic trafficking of Na(+)/H(+) exchanger 3 by E3KARP-dependent activation of phospholipase C

Lysophosphatidic acid (LPA) stimulates Na(+)/H(+) exchanger 3 (NHE3) activity in opossum kidney proximal tubule (OK) cells by increasing the apical membrane amount of NHE3. This occurs by stimulation of exocytic trafficking of NHE3 to the apical plasma membrane by an E3KARP-dependent mechanism. However, it is still unclear how E3KARP leads to the LPA-induced exocytosis of NHE3. In the current study, we demonstrate that stable expression of exogenous E3KARP increases LPA-induced phospholipase C (PLC) activation and subsequent elevation of intracellular Ca(2+) in opossum kidney proximal tubule (OK) cells. Pretreatment with U73122, a PLC inhibitor, prevented the LPA-induced NHE3 activation and the exocytic trafficking of NHE3. To understand how the elevation of intracellular Ca(2+) leads to the stimulation of NHE3, we pretreated OK cells with BAPTA-AM, an intracellular Ca(2+) chelator. BAPTA-AM completely blocked the LPA-induced increase of NHE3 activity and surface NHE3 amount by decreasing the LPA-induced exocytic trafficking of NHE3. Pretreatment with GF109203X, a PKC inhibitor, did not affect the percent of LPA-induced NHE3 activation and increase of surface NHE3 amount. From these results, we suggest that E3KARP plays a necessary role in LPA-induced PLC activation, and that PLC-dependent elevation of intracellular Ca(2+) but not PKC activation is necessary for the LPA-induced increase of NHE3 exocytosis.

Resolution of organelle docking and fusion kinetics in a cell-free assay

In vitro assays of compartment mixing have been key tools in the biochemical dissection of organelle docking and fusion. Many such assays measure compartment mixing through the enzymatic modification of reporter proteins. Homotypic fusion of yeast vacuoles is measured with a coupled assay of proteolytic maturation of pro-alkaline phosphatase (pro-ALP). A kinetic lag is observed between the end of docking, marked by the acquisition of resistance to anti-SNARE reagents, and ALP maturation. We therefore asked whether the time taken for pro-ALP maturation adds a kinetic lag to the measured fusion signal. Prb1p promotes ALP maturation; overproduction of Prb1p accelerates ALP activation in detergent lysates but does not alter the measured kinetics of docking or fusion. Thus, the lag between docking and ALP activation reflects a lag between docking and fusion. Many vacuoles in the population undergo multiple rounds of fusion; methods are presented for distinguishing the first round of fusion from ongoing rounds of fusion. A simple kinetic model distinguishes between two rates, the rate of fusion and the rate at which fusion competence is lost, and allows estimation of the number of rounds of fusion completed.

A soluble SNARE drives rapid docking, bypassing ATP and Sec17/18p for vacuole fusion

Membrane fusion requires priming, the disassembly of cis-SNARE complexes by the ATP-driven chaperones Sec18/17p. Yeast vacuole priming releases Vam7p, a soluble SNARE. Vam7p reassociation during docking allows trans-SNARE pairing and fusion. We now report that recombinant Vam7p (rVam7p) enters into complex with other SNAREs in vitro and bypasses the need for Sec17p, Sec18p, and ATP. Thus, the sole essential function of vacuole priming in vitro is the release of Vam7p from cis-SNARE complexes. In 'bypass fusion', without ATP but with added rVam7p, there are sufficient unpaired vacuolar SNAREs Vam3p, Vti1p, and Nyv1p to interact with Vam7p and support fusion. However, active SNARE proteins are not sufficient for bypass fusion. rVam7p does not bypass requirements for Rho GTPases,Vps33p, Vps39p, Vps41p, calmodulin, specific lipids, or Vph1p, a subunit of the V-ATPase. With excess rVam7p, reduced levels of PI(3)P or functional Ypt7p suffice for bypass fusion. High concentrations of rVam7p allow the R-SNARE Ykt6p to substitute for Nyv1p for fusion; this functional redundancy among vacuole SNAREs may explain why nyv1delta strains lack the vacuole fragmentation seen with mutants in other fusion catalysts.

Ca2+ dependency of 'Ca2+-independent' exocytosis in SPOC1 airway goblet cells

SPOC1 airway goblet cells secrete mucin in response to P2Y2 receptor agonists and to secretagogues, phorbol 12-myristate 13-acetate (PMA) and ionomycin, which mobilize elements of the phospholipase C pathway, PKC and Ca2+, respectively. Previous studies demonstrated that mucin secretion from SLO-permeabilized, EGTA-buffered SPOC1 cells was stimulated by PMA at low Ca2+ levels (< 0.1 microm), consistent with the notion that regulated exocytosis may occur by Ca2+-independent pathways. We tested the alternative hypothesis that PMA-induced mucin secretion is, in fact, a Ca2+-dependent process under the conditions of low bulk Ca2+, one that is permitted in the typical SLO-permeabilized cell model by the slow binding kinetics of EGTA. Both IP3 and elevated bulk Ca2+ activated mucin secretion in SPOC1 cells buffered by EGTA, suggesting that IP3 generates a local Ca2+ gradient in the vicinity of the secretory granules to the degree necessary to trigger exocytosis. BAPTA, which binds Ca2+ approximately 100-fold faster than EGTA, diminished IP3-induced mucin release over a range of concentrations by > or = 69%, yet maintained an essentially normal mucin secretory response to elevated bulk Ca2+ in permeabilized SPOC1 cells. BAPTA also diminished the mucin secretory response of permeabilized cells to PMA, relative to the EGTA-buffered control: at PMA below 30 nm, BAPTA abolished the secretory response, and at higher concentrations it was reduced significantly relative to the EGTA-buffered controls. PMA-induced secretion in EGTA was insensitive to heparin. These results suggest that Ca2+ is released locally during PMA-induced exocytosis, by an IP3-independent mechanism.

Principles of exocytosis and membrane fusion

Exocytosis is a ubiquitous process occurring in every eukaryotic cell including processes as diverse as membrane expansion during growth and the highly regulated release of neurotransmitter from neurons. Work during the past decade has established that exocytotic membrane fusion is mediated by members of conserved protein families including Rab proteins and SNAREs. SNAREs are probably catalyzing membrane fusion, and major progress has been made in unraveling their molecular mechanism. In contrast, less is known about regulatory mechanisms. Here, a brief overview is given about the current state of knowledge, focusing on SNAREs involved in neuronal exocytosis.

Spatio-temporal organization of Vam6P and SNAP on mouse spermatozoa and their involvement in sperm-zona pellucida interactions

Acrosomal assembly during spermatogenesis and acrosome reaction during sperm-oocyte interaction are unique events of vesicle synthesis, transport, and fusion leading to fertilization. SNARE complex formation is essential for membrane fusion, and vesicle-associated (v-) SNARE intertwines with target membrane (t-) SNARE to form a coiled coil that bridges two membranes and facilitates fusion. We detected messages of Vam6P and SNAP in mammalian testis and epididymis. Vam6P and SNAP were detected in a temporally organized fashion on the spermatozoa from testis and epididymis, which showed accumulation on the principal acrosomal domains during capacitation. Vam6P and SNAP were shed off from the principal acrosomal domain after acrosome reaction, but the equatorial and the post-acrosomal domains retained these proteins. Antibodies to VAMP and SNAP inhibited sperm-zona pellucida interaction, suggesting their possible involvement in sperm membrane vesiculation.

A link of Ca2+ to cAMP oscillations in Dictyostelium: the calmodulin antagonist W-7 potentiates cAMP relay and transiently inhibits the acidic Ca2+-store

Background

During early differentiation of Dictyostelium the attractant cAMP is released periodically to induce aggregation of the cells. Here we pursue the question whether pulsatile cAMP signaling is coupled to a basic Ca²⁺-oscillation.

Results

We found that the calmodulin antagonist W-7 transiently enhanced cAMP spikes. We show that W-7 acts on an acidic Ca²⁺-store: it abolished ATP-dependent vesicular acidification, inhibited V-type H⁺ATPase activity more potently than the weaker antagonist W-5 and caused vesicular Ca²⁺-leakage. Concanamycin A, an inhibitor of the V-type H⁺-pump, blocked the Ca²⁺-leakage elicited by W-7 as well as cAMP-oscillations in the presence of W-7. Concanamycin A caused an increase of the cytosolic Ca²⁺-concentration whereas W-7 did not. In case of the latter, Ca²⁺ was secreted by the cells. In accord with our hypothesis that the link from Ca²⁺ to cAMP synthesis is mediated by a Ca²⁺-dependent phospholipase C we found that W-7 was not active in the phospholipase C knockout mutant.

Conclusion

We conclude that the potentiation of cAMP relay by W-7 is due to a transient inhibition of the acidic Ca²⁺-store. The inhibition of the proton pump by W-7 causes a leakage of Ca²⁺ that indirectly stimulates adenylyl cyclase activity via phospholipase C.

N-myc downregulated gene 1 is a phosphorylated protein in mast cells

We previously demonstrated that the in vitro maturation of mouse immature bone marrow-derived mast cells into a mature connective tissue mast cell-like phenotype is accompanied by a marked induction of N-myc downregulated gene (NDRG) 1, a cytosolic protein with unknown function. Here we show that NDRG1 undergoes phosphorylation in mast cells. Recombinant NDRG1 was phosphorylated by calmodulin kinase-II, protein kinase (PK) A and PKC in vitro. Deletion of the C-terminal tandem repeats of NDRG1 resulted in increased phosphorylation by PKA and PKC, but not by calmodulin kinase-II. Furthermore, NDRG1 was phosphorylated on serine and threonine residues in mast cells, a process that was accelerated transiently following cell activation. Pharmacologic studies using kinase-specific inhibitors demonstrated that this NDRG1 phosphorylation in mast cells depended on calmodulin kinase-II and PKA, but not PKC. Collectively, our results indicate that NDRG1 is a multiphosphorylated protein in mast cells, and that the kinetics of increased NDRG1 phosphorylation parallels signaling events leading to exocytosis.

Ca2+ and N-Ethylmaleimide-sensitive Factor Differentially Regulate Disassembly of SNARE Complexes on Early Endosomes

The endosome-associated protein Hrs inhibits the homotypic fusion of early endosomes. A helical region of Hrs containing a Q-SNARE motif mediates this effect as well as its endosomal membrane association via SNAP-25, an endosomal receptor for Hrs. Hrs inhibits formation of an early endosomal SNARE complex by displacing VAMP-2 from the complex, suggesting a mechanism by which Hrs inhibits early endosome fusion. We examined the regulation of endosomal SNARE complexes to probe how Hrs may function as a negative regulator. We show that although NSF dissociates the VAMP-2.SNAP-25.syntaxin 13 complex, it has no effect on the Hrs-containing complex. Whereas Ca(2+) dissociates the Hrs-containing complex but not the VAMP-2-containing SNARE complex. This is the first demonstration of differential regulation of R/Q-SNARE and all Q-SNARE-containing SNARE complexes. Ca(2+) also reverses the Hrs-induced inhibition of early endosome fusion in a tetanus toxin-sensitive manner and removes Hrs from early endosomal membranes. Moreover, Hrs inhibition of endosome fusion and its endosomal localization are sensitive to bafilomycin, implying a role for luminal Ca(2+). Thus, Hrs may bind a SNARE protein on early endosomal membranes negatively regulating trans-SNARE pairing and endosomal fusion. The release of Ca(2+) from the endosome lumen dissociates Hrs, allowing a VAMP-2-containing complex to form enabling fusion.

Ca2+/calmodulin transfers the membrane-proximal lipid-binding domain of the v-SNARE synaptobrevin from cis to trans bilayers

Soluble N-ethylmaleimide-sensitive fusion protein attachment protein receptor (SNARE) protein interactions at the synaptic vesicle/plasma membrane interface play an essential role in neurotransmitter release. The membrane-proximal region (amino acids 77-90) of the v-SNARE vesicle-associated membrane protein 2 (VAMP 2, synaptobrevin) binds acidic phospholipids or Ca(2+)/calmodulin in a mutually exclusive manner, processes that are required for Ca(2+)-dependent exocytosis. To address the mechanisms involved, we asked whether this region of VAMP can interact with cis (outer vesicle leaflet) and/or trans (inner plasma membrane leaflet) lipids. To evaluate cis lipid binding, recombinant VAMP was reconstituted into liposomes and accessibility to site-directed antibodies was probed by surface plasmon resonance. Data indicated that the membrane-proximal domain of VAMP dips into the cis lipid bilayer, sequestering epitopes between the tetanus toxin cleavage site and the membrane anchor. These epitopes were unmasked by VAMP double mutation W89A, W90A, which abolishes lipid interactions. To evaluate trans lipid binding, VAMP was reconstituted in cis liposomes, which were then immobilized on beads. The ability of VAMP to capture protein-free (3)H-labeled trans liposomes was then measured. When cis lipid interactions were eliminated by omitting negatively charged lipids, trans lipid binding to VAMP was revealed. In contrast, when cis and trans liposomes both contained acidic headgroups (i.e., approximating physiological conditions), cis lipid interactions totally occluded trans lipid binding. In these conditions Ca(2+)/calmodulin displaced cis inhibition, transferring the lipid-binding domain of VAMP from the cis to the trans bilayer. Our results suggest that calmodulin acts as a unidirectional Ca(2+)-activated shuttle that docks the juxtamembrane portion of the v-SNARE in the target membrane to prepare fusion.

Mycobacterium tuberculosis phagosome maturation arrest: mycobacterial phosphatidylinositol analog phosphatidylinositol mannoside stimulates early endosomal fusion

Mycobacterium tuberculosis is a facultative intracellular pathogen that parasitizes macrophages by modulating properties of the Mycobacterium-containing phagosome. Mycobacterial phagosomes do not fuse with late endosomal/lysosomal organelles but retain access to early endosomal contents by an unknown mechanism. We have previously reported that mycobacterial phosphatidylinositol analog lipoarabinomannan (LAM) blocks a trans-Golgi network-to-phagosome phosphatidylinositol 3-kinase-dependent pathway. In this work, we extend our investigations of the effects of mycobacterial phosphoinositides on host membrane trafficking. We present data demonstrating that phosphatidylinositol mannoside (PIM) specifically stimulated homotypic fusion of early endosomes in an ATP-, cytosol-, and N-ethylmaleimide sensitive factor-dependent manner. The fusion showed absolute requirement for small Rab GTPases, and the stimulatory effect of PIM increased upon partial depletion of membrane Rabs with RabGDI. We found that stimulation of early endosomal fusion by PIM was higher when phosphatidylinositol 3-kinase was inhibited by wortmannin. PIM also stimulated in vitro fusion between model phagosomes and early endosomes. Finally, PIM displayed in vivo effects in macrophages by increasing accumulation of plasma membrane-endosomal syntaxin 4 and transferrin receptor on PIM-coated latex bead phagosomes. In addition, inhibition of phagosomal acidification was detected with PIM-coated beads. The effects of PIM, along with the previously reported action of LAM, suggest that M. tuberculosis has evolved a two-prong strategy to modify its intracellular niche: its products block acquisition of late endosomal/lysosomal constituents, while facilitating fusion with early endosomal compartments.

Phagocytosis by neutrophils

Phagocytosis is central to the microbicidal function of neutrophils. Pathogens are initially engulfed into a plasma membrane-derived vacuole, the phagosome, which proceeds to acquire degradative properties by a complex process termed maturation. In this chapter, we discuss the current knowledge of the molecular mechanisms underlying phagosome formation and maturation in neutrophils.

Loss of inositol 1,4,5-trisphosphate receptors from bile duct epithelia is a common event in cholestasis

BACKGROUND AND AIMS

Cholestasis is one of the principal manifestations of liver disease and often results from disorders involving bile duct epithelia rather than hepatocytes. A range of disorders affects biliary epithelia, and no unifying pathophysiologic event in these cells has been identified as the cause of cholestasis. Here we examined the role of the inositol 1,4,5-trisphosphate receptor (InsP3R)/Ca(2+) release channel in Ca(2+) signaling and ductular secretion in animal models of cholestasis and in patients with cholestatic disorders.

METHODS

The expression and distribution of the InsP3R and related proteins were examined in rat cholangiocytes before and after bile duct ligation or treatment with endotoxin. Ca(2+) signaling was examined in isolated bile ducts from these animals, whereas ductular bicarbonate secretion was examined in isolated perfused livers. Confocal immunofluorescence was used to examine cholangiocyte InsP3R expression in human liver biopsy specimens.

RESULTS

Expression of the InsP3R was selectively lost from biliary epithelia after bile duct ligation or endotoxin treatment. As a result, Ca(2+) signaling and Ca(2+)-mediated bicarbonate secretion were lost as well, although other components of the Ca(2+) signaling pathway and adenosine 3',5'-cyclic monophosphate (cAMP)-mediated bicarbonate secretion both were preserved. Examination of human liver biopsy specimens showed that InsP3Rs also were lost from bile duct epithelia in a range of human cholestatic disorders, although InsP3R expression was intact in noncholestatic liver disease.

CONCLUSIONS

InsP3-mediated Ca(2+) signaling in bile duct epithelia appears to be important for normal bile secretion in the liver, and loss of InsP3Rs may be a final common pathway for cholestasis.

Mycobacterium tuberculosis phagosome maturation arrest: selective targeting of PI3P-dependent membrane trafficking

The ability of Mycobacterium tuberculosis to enter host macrophages, and reside in a phagosome, which does not mature into a phagolysosome, is central to the spread of tuberculosis and the associated pandemic involving billions of people worldwide. Tuberculosis can be viewed as a disease with a significant intracellular trafficking and organellar biogenesis component. Current understanding of the block in M. tuberculosis phagosome maturation also sheds light on fundamental aspects of phagolysosome biogenesis. The maturation block involves interference with the recruitment and function of rabs, rab effectors (phosphatidylinositol 3-kinases and tethering molecules such as EEA1), SNAREs (Syntaxin 6 and cellubrevin) and Ca2+/calmodulin signaling. M. tuberculosis analogs of mammalian phosphatidylinositols interfere with these systems and associated processes.

Involvement of LMA1 and GATE-16 family members in intracellular membrane dynamics

Intracellular membrane fusion is conserved from yeast to man as well as among different intracellular trafficking pathways. This process can be generally divided into several well-defined biochemical reactions. First, an early recognition (or tethering) takes place between donor and acceptor membranes, mediated by ypt/rab GTPases and complexes of tethering factors. Subsequently, a closer association between the two membranes is achieved by a docking process, which involves tight association between membrane proteins termed SNAREs. The formation of such a trans-SNARE complex leads to the final membrane fusion, resulting in an accumulation of cis-SNARE complexes on the acceptor membrane. Thus, multiple rounds of transport and delivery of the donor SNARE back to its original membrane require dissociation of the SNARE complexes. SNARE dissociation, termed priming, is mediated by the AAA ATPase, N-ethylmaleimide-sensitive factor (NSF) and its partner, soluble NSF attachment protein (SNAP), in a reaction that requires ATP hydrolysis. In the present review we focus on LMA1 and GATE-16, two low-molecular-weight proteins, which assist in priming SNARE molecules in the vacuole in yeast and the Golgi complex in mammals, respectively. LMA1 and GATE-16 are suggested to keep the dissociated cis-SNAREs apart from each other, allowing multiple fusion processes to take place. GATE-16 belongs to a novel family of ubiquitin-like proteins conserved from yeast to man. We discuss here the involvement of this family in multiple intracellular trafficking pathways.

Dense-core secretory vesicle docking and exocytotic membrane fusion in Paramecium cells

Work with Paramecium has contributed to the actual understanding of certain aspects of exocytosis regulation, including membrane fusion. The system is faster and more synchronous than any other dense-core vesicle system described and its highly regular design facilitates correlation of functional and ultrastructural (freeze-fracture) features. From early times on, several crucial aspects of exocytosis regulation have been found in Paramecium cells, e.g. genetically controlled microdomains (with distinct ultrastructure) for organelle docking and membrane fusion, involvement of calmodulin in establishing such microdomains, priming by ATP, occurrence of focal fusion with active participation of integral and peripheral proteins, decay of a population of integral proteins ("rosettes", mandatory for fusion capacity) into subunits and their lateral dispersal during fusion, etc. The size of rosette particles and their dispersal upon focal fusion would be directly compatible with proteolipid V(0) subunits of a V-ATPase, much better than the size predicted for oligomeric SNARE pins (SCAMPs are unknown from Paramecium at this time). However, there are some restrictions for a straightforward interpretation of ultrastructural results. The rather pointed, nipple-like tip of the trichocyst membrane could accommodate only one (or very few) potential V(0) counterpart(s), while the overlaying domain of the cell membrane contains numerous rosette particles. Particle size is compatible with V(0), but larger than that assumed for the SNARE complexes. When membrane fusion is induced in the presence of antibodies against cell surface components, focal fusion is seen to occur with dispersing rosette particles but without dispersal of their subunits and without pore expansion. Clearly, this is required for completing fusion and pore expansion. After cloning SNARE and V(0) components in Paramecium (with increasing details becoming rapidly available), we may soon be able to address the question more directly, whether any of these components or some new ones to be detected, serve exocytotic and/or any other membrane fusions in Paramecium.

Calcium and calmodulin in membrane fusion

Regulated exocytosis was the first intracellular membrane fusion step that was suggested to involve both Ca(2+) and calmodulin. In recent years, it has become clear that calmodulin is not an essential Ca(2+) sensor for exocytosis but that it is likely to have a more regulatory role. A requirement for cytosolic Ca(2+) in other vesicle fusion events within cells has become apparent and in certain cases, such as homotypic fusion of early endosomes and yeast vacuoles, calmodulin may be the primary Ca(2+) sensor. A number of distinct targets for calmodulin have been identified including SNARE proteins and subunits of the vacuolar ATPase. The extent to which calmodulin regulates different intracellular fusion events through conserved SNARE-dependent or other mechanisms remains to be resolved.

What is the role of SNARE proteins in membrane fusion?

Soluble N-ethylmaleimide-sensitive factor activating protein receptor (SNARE) proteins have been at the fore-front of research on biological membrane fusion for some time. The subcellular localization of SNAREs and their ability to form the so-called SNARE complex may be integral to determining the specificity of intracellular fusion (the SNARE hypothesis) and/or serving as the minimal fusion machinery. Both the SNARE hypothesis and the idea of the minimal fusion machinery have been challenged by a number of experimental observations in various model systems, suggesting that SNAREs may have other functions. Considering recent advances in the SNARE literature, it appears that SNAREs may actually function as part of a complex fusion "machine." Their role in the machinery could be any one or a combination of roles, including establishing tight membrane contact, formation of a scaffolding on which to build the machine, binding of lipid surfaces, and many others. It is also possible that complexations other than the classic SNARE complex participate in membrane fusion.

ATP-independent luminal oscillations and release of Ca2+ and H+ from mast cell secretory granules: implications for signal transduction

InsP(3) is an important link in the intracellular information network. Previous observations show that activation of InsP(3)-receptor channels on the granular membrane can turn secretory granules into Ca(2+) oscillators that deliver periodic trains of Ca(2+) release to the cytosol (T. Nguyen, W. C. Chin, and P. Verdugo, 1998, Nature, 395:908-912; I. Quesada, W. C. Chin, J. Steed, P. Campos-Bedolla, and P. Verdugo, 2001, BIOPHYS: J. 80:2133-2139). Here we show that InsP(3) can also turn mast cell granules into proton oscillators. InsP(3)-induced intralumenal [H(+)] oscillations are ATP-independent, result from H(+)/K(+) exchange in the heparin matrix, and produce perigranular pH oscillations with the same frequency. These perigranular pH oscillations are in-phase with intralumenal [H(+)] but out-of-phase with the corresponding perigranular [Ca(2+)] oscillations. The low pH of the secretory compartment has critical implications in a broad range of intracellular processes. However, the association of proton release with InsP(3)-induced Ca(2+) signals, their similar periodic nature, and the sensitivity of important exocytic proteins to the joint action of Ca(2+) and pH strongly suggests that granules might encode a combined Ca(2+)/H(+) intracellular signal. A H(+)/Ca(2+) signal could significantly increase the specificity of the information sent by the granule by transmitting two frequency encoded messages targeted exclusively to proteins like calmodulin, annexins, or syncollin that are crucial for exocytosis and require specific combinations of [Ca(2+)] "and" pH for their action.

Vacuole membrane fusion: V0 functions after trans-SNARE pairing and is coupled to the Ca2+-releasing channel

Pore models of membrane fusion postulate that cylinders of integral membrane proteins can initiate a fusion pore after conformational rearrangement of pore subunits. In the fusion of yeast vacuoles, V-ATPase V0 sectors, which contain a central cylinder of membrane integral proteolipid subunits, associate to form a transcomplex that might resemble an intermediate postulated in some pore models. We tested the role of V0 sectors in vacuole fusion. V0 functions in fusion and proton translocation could be experimentally separated via the differential effects of mutations and inhibitory antibodies. Inactivation of the V0 subunit Vph1p blocked fusion in the terminal reaction stage that is independent of a proton gradient. Deltavph1 mutants were capable of docking and trans-SNARE pairing and of subsequent release of lumenal Ca2+, but they did not fuse. The Ca2+-releasing channel appears to be tightly coupled to V0 because inactivation of Vph1p by antibodies blocked Ca2+ release. Vph1 deletion on only one fusion partner sufficed to severely reduce fusion activity. The functional requirement for Vph1p correlates to V0 transcomplex formation in that both occur after docking and Ca2+ release. These observations establish V0 as a crucial factor in vacuole fusion acting downstream of trans-SNARE pairing.

The role of mVps18p in clustering, fusion, and intracellular localization of late endocytic organelles

Delivery of endocytosed macromolecules to mammalian cell lysosomes occurs by direct fusion of late endosomes with lysosomes, resulting in the formation of hybrid organelles from which lysosomes are reformed. The molecular mechanisms of this fusion are analogous to those of homotypic vacuole fusion in Saccharomyces cerevisiae. We report herein the major roles of the mammalian homolog of yeast Vps18p (mVps18p), a member of the homotypic fusion and vacuole protein sorting complex. When overexpressed, mVps18p caused the clustering of late endosomes/lysosomes and the recruitment of other mammalian homologs of the homotypic fusion and vacuole protein sorting complex, plus Rab7-interacting lysosomal protein. The clusters were surrounded by components of the actin cytoskeleton, including actin, ezrin, and specific unconventional myosins. Overexpression of mVps18p also overcame the effect of wortmannin treatment, which inhibits membrane traffic out of late endocytic organelles and causes their swelling. Reduction of mVps18p by RNA interference caused lysosomes to disperse away from their juxtanuclear location. Thus, mVps18p plays a critical role in endosome/lysosome tethering, fusion, intracellular localization and in the reformation of lysosomes from hybrid organelles.

Identification of a novel binding site for calmodulin in ammodytoxin A, a neurotoxic group IIA phospholipase A2

The molecular mechanism of the presynaptic neurotoxicity of snake venom phospholipases A2 (PLA2s) is not yet fully elucidated. Recently, new high-affinity binding proteins for PLA2 toxins have been discovered, including the important intracellular Ca2+ sensor, calmodulin (CaM). In the present study, the mode of interaction of group IIA PLA2s with the Ca2+-bound form of CaM was investigated by mutational analysis of ammodytoxin A (AtxA) from the long-nosed viper (Vipera ammodytes ammodytes). Several residues in the C-terminal part of AtxA were found to be important in this interaction, particularly those in the region 115-119. In support of this finding, introduction of Y115, I116, R118 and N119, present in AtxA, into a weakly neurotoxic PLA2 from Russell's viper (Daboia russellii russellii) increased by sevenfold its binding affinity for CaM. Furthermore, two out of four peptides deduced from different regions of AtxA were able to compete with the toxin in binding to CaM. The nonapeptide showing the strongest inhibition was that comprising the AtxA region 115-119. This stretch contributes to a distinct hydrophobic patch within the region 107-125 in the C-terminal part of the molecule. This lacks any substantial helical structure and is surrounded by several basic residues, which may form a novel binding motif for CaM on the molecular surface of the PLA2 toxin.

Essential role of Ca2+/calmodulin in Early Endosome Antigen-1 localization

Ca2+ is an essential requirement in membrane fusion, acting through binding proteins such as calmodulin (CaM). Ca2+/CaM is required for early endosome fusion in vitro, however, the molecular basis for this requirement is unknown. An additional requirement for endosome fusion is the protein Early Endosome Antigen 1 (EEA1), and its recruitment to the endosome depends on phosphatidylinositol 3-phosphate [PI(3)P] and the Rab5 GTPase. Herein, we demonstrate that inhibition of Ca2+/CaM, by using either chemical inhibitors or specific antibodies directed to CaM, results in a profound inhibition of EEA1 binding to endosomal membranes both in live cells and in vitro. The concentration of Ca2+/CaM inhibitors required for a full dissociation of EEA1 from endosomal membranes had no effect on the activity of phosphatidylinositol 3-kinases or on endogenous levels of PI(3)P. However, the interaction of EEA1 with liposomes containing PI(3)P was decreased by Ca2+/CaM inhibitors. Thus, Ca2+/CaM seems to be required for the stable interaction of EEA1 with endosomal PI(3)P, perhaps by directly or indirectly stabilizing the quaternary organization of the C-terminal FYVE domain of EEA1. This requirement is likely to underlie at least in part the essential role of Ca2+/CaM in endosome fusion.

A fuzzy mitochondrial fusion apparatus comes into focus

Membrane fusion is fundamental to eukaryotic life. Unlike the predominant intracellular fusion machineries that fuse compartments bounded by a single membrane, the mitochondrial fusion machinery must sequentially fuse the outer and inner mitochondrial membranes. These coordinated fusion events rely on a transmembrane GTPase that is known as fuzzy onions or Fzo. Recent studies have revealed that Fzo has an evolutionarily conserved role in mitochondrial fusion, and they take the first strides in determining the molecular nature of such a role.

Membrane fusion in eukaryotic cells

Membrane fusion is a fundamental biochemical reaction and the final step in all vesicular trafficking events. It is crucial for the transfer of proteins and lipids between different compartments and for exo- and endocytic traffic of signaling molecules and receptors. It leads to the reconstruction of organelles such as the Golgi or the nuclear envelope, which decay into fragments during mitosis. Hence, controlled membrane fusion reactions are indispensible for the compartmental organization of eukaryotic cells; for their communication with the environment via hormones, neurotransmitters, growth factors, and receptors; and for the integration of cells into multicellular organisms. Intracellular pathogenic bacteria, such as Mycobacteria or Salmonellae, have developed means to control fusion reactions in their host cells. They persist in phagosomes whose fusion with lysosomes they actively suppress-a means to ensure survival inside host cells. The past decade has witnessed rapid progress in the elucidation of parts of the molecular machinery involved in these membrane fusion reactions. Whereas some elements of the fusion apparatus are remarkably similar in several compartments, there is an equally striking divergence of others. The purpose of this review is to highlight common features of different fusion reactions and the concepts that emerged from them but also to stress the differences and challenge parts of the current hypotheses. This review covers only the endoplasmic fusion reactions mentioned above, i.e., reactions initiated by contacts of membranes with their cytoplasmic faces. Ectoplasmic fusion events, which depend on an initial contact of the fusion partners via the membrane surfaces exposed to the surrounding medium are not discussed, nor are topics such as the entry of enveloped viruses, formation of syncytia, gamete fusion, or vesicle scission (a fusion reaction that leads to the fission of, e.g., transport vesicles).

Cloning and expression of calglandulin, a new EF-hand protein from the venom glands of Bothrops insularis snake in E. coli

The EF-hand protein family is comprised of many proteins with conserved Ca(2+)-binding motifs with important biological roles in intracellular communication. During the generation of Expressed Sequence Tags (ESTs) from the venom glands of the Viperidae snake Bothrops insularis, we identified a cDNA coding for a putative Ca(2+) binding protein with four EF-hand motifs, named here calglandulin. The deduced amino acid sequence displayed the greatest sequence similarity with calmodulin (59%), followed by troponin-C (52%). The encoded polypeptide was first expressed in Escherichia coli as a 6XHis-tagged fusion protein. The expressed protein was purified by Ni(2+)-charged affinity chromatography and circular dichroism (CD) spectroscopy confirmed the prevalence of alpha-helix as observed in calmodulin/calmodulin-like proteins. A polyclonal antiserum was generated in mice using this recombinant calglandulin. To investigate the tissue-specific biological occurrence of this protein, this antiserum was used in Western blot experiments, which revealed an immunoreactive band in samples of venom gland extracts from different snakes, but not in the crude venom or in brain, heart and other tissues. This exclusive occurrence suggests a specialized function of calglandulin in snake venom glands.

The Yip1p.Yif1p complex is required for the fusion competence of endoplasmic reticulum-derived vesicles

Here we report that Yip1p and Yif1p, two members of an integral membrane protein complex that bind to the Rab Ypt1p, are required for membrane fusion with the Golgi in vitro. To block fusion, anti-Yip1p or anti-Yif1p antibodies must be added before vesicles bud from the endoplasmic reticulum (ER). These antibodies do not block the packaging of Yip1p, Yif1p, or the soluble NSF attachment protein receptor (SNAREs) into vesicles. We propose that Yip1p and Yif1p perform a critical role in establishing the fusion competence of ER to Golgi vesicles at the time of budding. Consistent with this proposal, we observe that the Yip1p.Yif1p complex binds to the ER to Golgi SNAREs Bos1p and Sec22p, two components of the membrane fusion machinery.

Vam10p defines a Sec18p-independent step of priming that allows yeast vacuole tethering

YOR068c, termed VAM10 (altered vacuole morphology), lies within the VPS5 gene on the opposite DNA strand. VAM10 deletion causes vacuole fragmentation in vivo. The in vitro fusion of purified yeast vacuoles is stimulated by recombinant Vam10p and blocked by antibody to Vam10p. Vam10p acts early in the priming stage of fusion, independent of Sec18p. After priming, recombinant Vam10p will not stimulate fusion and anti-Vam10p antibodies will not inhibit; Vam10p provides a functional marker for this Sec18p-independent priming step. Pure Vam10p restores normal, Ypt7p-dependent tethering to vacuoles from a vam10Delta strain.

Controlling the intracellular localization of fluorescent polyamide analogues in cultured cells

The intracellular distribution of fluorescent-labeled polyamides was examined in live cells. We showed that BODIPY-labeled polyamides accumulate in acidic vesicles, mainly lysosomes, in the cytoplasm of HCT116 colon cancer cells and human rheumatoid synovial fibroblasts (RSF). Verapamil blocked vesicular accumulation and led to nuclear accumulation of the BODIPY-labeled polyamide in RSFs. We infer that the basic amine group commonly found at the end of synthetic polyamide chains is responsible for their accumulation in cytoplasmic vesicles in mammalian cells. Modifying the charge on a polyamide by replacing the BODIPY moiety with a fluorescein moiety on the amine tail allowed the polyamide to localize in the nucleus of the cell and bypass the cytoplasmic vesicles in HCT116 cells.

Secretory granule exocytosis

Regulated exocytosis of secretory granules or dense-core granules has been examined in many well-characterized cell types including neurons, neuroendocrine, endocrine, exocrine, and hemopoietic cells and also in other less well-studied cell types. Secretory granule exocytosis occurs through mechanisms with many aspects in common with synaptic vesicle exocytosis and most likely uses the same basic protein components. Despite the widespread expression and conservation of a core exocytotic machinery, many variations occur in the control of secretory granule exocytosis that are related to the specialized physiological role of particular cell types. In this review we describe the wide range of cell types in which regulated secretory granule exocytosis occurs and assess the evidence for the expression of the conserved fusion machinery in these cells. The signals that trigger and regulate exocytosis are reviewed. Aspects of the control of exocytosis that are specific for secretory granules compared with synaptic vesicles or for particular cell types are described and compared to define the range of accessory control mechanisms that exert their effects on the core exocytotic machinery.

Control of membrane fusion during spermiogenesis and the acrosome reaction

Membrane fusion is important to reproduction because it occurs in several steps during the process of fertilization. Many events of intracellular trafficking occur during both spermiogenesis and oogenesis. The acrosome reaction, a key feature during mammalian fertilization, is a secretory event involving the specific fusion of the outer acrosomal membrane and the sperm plasma membrane overlaying the principal piece of the acrosome. Once the sperm has crossed the zona pellucida, the gametes fuse, but in the case of the sperm this process takes place through a specific membrane domain in the head, the equatorial segment. The cortical reaction, a process that prevents polyspermy, involves the exocytosis of the cortical granules to the extracellular milieu. In lower vertebrates, the formation of the zygotic nucleus involves the fusion (syngamia) of the male pronucleus with the female pronucleus. Other undiscovered membrane trafficking processes may also be relevant for the formation of the zygotic centrosome or other zygotic structures. In this review, we focus on the recent discovery of molecular machinery components involved in intracellular trafficking during mammalian spermiogenesis, notably related to acrosome biogenesis. We also extend our discussion to the molecular mechanism of membrane fusion during the acrosome reaction. The data available so far suggest that proteins participating in the intracellular trafficking events leading to the formation of the acrosome during mammalian spermiogenesis are also involved in controlling the acrosome reaction during fertilization.

Cloning and characterization of human CADPS and CADPS2, new members of the Ca2+-dependent activator for secretion protein family

The recent identification of some of the components involved in regulated and constitutive exocytotic pathways has yielded important insights into the mechanisms of membrane trafficking and vesicle secretion. To understand precisely the molecular events taking place during vesicle exocytosis, we must identify all of the proteins implicated in these pathways. In this paper we describe the full-length cloning and characterization of human CADPS and CADPS2, two new homologs of the mouse Cadps protein involved in large dense-core vesicle (LDCV)-regulated exocytosis. We show that these two genes have disparate RNA expression patterns, with CADPS restricted to neural and endocrine tissues and CADPS2 expressed ubiquitously. We also identify a C2 domain, a known protein motif involved in calcium and phospholipid interactions, in both CADPS and CADPS2. We propose that CADPS functions as a calcium sensor in regulated exocytosis, whereas CADPS2 acts as a calcium sensor in constitutive vesicle trafficking and secretion. CADPS and CADPS2 were determined to span 475 kb and 561 kb on human chromosomes 3p21.1 and 7q31.3, respectively. The q31-q34 of human chromosome 7 has recently been identified to contain a putative susceptibility locus for autism (AUTS1). The function, expression profile, and location of CADPS2 make it a candidate gene for autism, and thus we conducted mutation screening for all 28 exons in 90 unrelated autistic individuals. We identified several nucleotide substitutions, including only one that would affect the amino acid sequence. No disease-specific variants were identified.

Evolution of chloroplast vesicle transport

Vesicle traffic plays a central role in eukaryotic transport. The presence of a vesicle transport system inside chloroplasts of spermatophytes raises the question of its phylogenetic origin. To elucidate the evolution of this transport system we analyzed organisms belonging to different lineages that arose from the first photosynthetic eukaryote, i.e. glaucocystophytes, chlorophytes, rhodophytes, and charophytes/embryophytes. Intriguingly, vesicle transport is not apparent in any group other than embryophytes. The transfer of this eukaryotic-type vesicle transport system from the cytosol into the chloroplast thus seems a late evolutionary development that was acquired by land plants in order to adapt to new environmental challenges.

A gonadotropin-releasing hormone insensitive, thapsigargin-sensitive Ca2+ store reduces basal gonadotropin exocytosis and gene expression: comparison with agonist-sensitive Ca2+ stores

We examined whether distinct Ca2+ stores differentially control basal and gonadotropin (GTH-II)-releasing hormone (GnRH)-evoked GTH-II release, long-term GTH-II secretion and contents, and GTH-II-beta mRNA expression in goldfish. Thapsigargin (Tg)-sensitive Ca2+ stores mediated neither caffeine-evoked GTH-II release, nor salmon (s)GnRH- and chicken (c)GnRH-II-stimulated secretion; the latter responses were previously shown to involve ryanodine (Ry)-sensitive Ca2+ stores. Surprisingly, Tg decreased basal GTH-II release. This response was attenuated by prior exposure to sGnRH and caffeine, but was insensitive to the phosphatase inhibitor okadaic acid, the inhibitor of constitutive release brefeldin A and cGnRH-II. GTH-II-beta mRNA expression was decreased at 24 h by 2 microm Tg, and by inhibiting (10 microm Ry) and stimulating (1 nm Ry) Ry receptors. Transient increases in GTH-II-beta mRNA were observed at 2 h and 12 h following 10 microm and 1 nm Ry treatment, respectively. Effects of Tg, Ry and GnRH on long-term GTH-II secretion, contents and apparent production differed from one another, and these changes were not well correlated with changes in GTH-II-beta mRNA expression. Our data show that GTH-II secretion, storage and transcription can be independently controlled by distinct Ca2+ stores.

The transmembrane domain of Vam3 affects the composition of cis- and trans-SNARE complexes to promote homotypic vacuole fusion

It is presently not clear how the function of SNARE proteins is affected by their transmembrane domains. Here, we analyzed the role of the transmembrane domain of the vacuolar SNARE Vam3 by replacing it by a lipid anchor. Vacuoles with mutant Vam3 fuse poorly and have increased amounts of cis-SNARE complexes, indicating that they are more stable. As a consequence efficient cis-SNARE complex disassembly that occurs at priming as a prerequisite of fusion requires addition of exogenous Sec18. trans-SNARE complexes in this mutant accumulate up to 4-fold over wild type, suggesting that the transmembrane domain of Vam3 is required to transit through this step. Finally, palmitoylation of Vac8, a reaction that also occurs early during priming is reduced by almost one-half. Since palmitoylated Vac8 is required beyond trans-SNARE complex formation, this may partially explain the fusion deficiency.

Molecular Aspects of Membrane Trafficking in Paramecium

Results achieved in the molecular biology of Paramecium have shed new light on its elaborate membrane trafficking system. Paramecium disposes not only of the standard routes (endoplasmic reticulum --> Golgi --> lysosomes or secretory vesicles; endo- and phagosomes --> lysosomes/digesting vacuoles), but also of some unique features, e.g. and elaborate phagocytic route with the cytoproct and membrane recycling to the cytopharynx, as well as the osmoregulatory system with multiple membrane fusion sites. Exocytosis sites for trichocysts (dense-core secretory vesicles), parasomal sacs (coated pits), and terminal cisternae (early endosomes) display additional regularly arranged predetermined fusion/fission sites, which now can be discussed on a molecular basis. Considering the regular, repetitive arrangements of membrane components, availability of mutants for complementation studies, sensitivity to gene silencing, and so on, Paramecium continues to be a valuable model system for analyzing membrane interactions. This review intends to set a new baseline for ongoing work along these lines.

In vitro fusion of plant Golgi membranes can be influenced by divalent cations

The fusogenic activity of plant Golgi membranes was studied in a cell-free system by assaying lipid mixing and content leakages of fluorescence probes. Golgi membranes from mung bean (Vigna radiata L.) hypocotyl cells fused to liposomes in the absence of any cytosolic proteins and nucleotides. It was demonstrated that the fusion was mediated by integral membrane protein(s), and was influenced by divalent cations (mm). Mg(2+), Ca(2+), and Mn(2+) ions enhanced the lipid mixing by reducing repulsive forces between membranes. In the content leakage assay, Mg(2+) ions also showed a stimulative effect. However, other divalent cations were inhibitory. It is suggested that the fusion system of Golgi membranes comprises at least two components: one that mediates the formation of fusion intermediates prior to pore opening, and one that mediates the subsequent processes. The latter must be sensitive to divalent cations at millimolar concentrations. The fusion of Golgi and biological membranes was induced by divalent cations. We speculated about the biological role of the fusion system studied here.

Identification and characterization of the single channel function of human mucolipin-1 implicated in mucolipidosis type IV, a disorder affecting the lysosomal pathway

Mucolipin-1 (MLN1) is a membrane protein with homology to the transient receptor potential channels and other non-selective cation channels. It is encoded by the MCOLN1 gene, which is mutated in patients with mucolipidosis type IV (MLIV), an autosomal recessive disease that is characterized by severe abnormalities in neurological development as well as by ophthalmologic defects. At the cellular level, MLIV is associated with abnormal lysosomal sorting and trafficking. Here we identify the channel function of human MLN1 and characterize its properties. MLN1 represents a novel Ca(2+)-permeable channel that is transiently modulated by changes in [Ca(2+)]. It is also permeable to Na(+) and K(+). Large unitary conductances were measured in the presence of these cations. With its Ca(2+) permeability and modulation by [Ca(2+)], MLN1 could play a major role in Ca(2+) transport regulating lysosomal exocytosis and potentially other phenomena related to the trafficking of late endosomes and lysosomes.

Drosophila syntaxin 16 is a Q-SNARE implicated in Golgi dynamics

SNARE isoforms appear to regulate specific intracellular membrane trafficking steps. To identify new SNARE proteins in Drosophila melanogaster we used a yeast two-hybrid screen to search for proteins that interact with SNAP. Here we report the identification of the Drosophila homologue of syntaxin 16. dsyntaxin 16 binds SNAP in a concentration-dependent fashion and genetically interacts with NSF2. Like its mammalian homologue, dsyntaxin 16 is ubiquitously expressed and appears to be localized to the Golgi apparatus. In addition, membranes containing dsyntaxin 16 become aggregated upon Brefeldin A treatment and are dispersed during meiosis. Inhibition of dsyntaxin 16 function by overexpression of truncated forms in cultured Schneider cells indicates that dsyntaxin 16 may selectively regulate Golgi dynamics.

Selective effects of calcium chelators on anterograde and retrograde protein transport in the cell

Calcium plays a regulatory role in several aspects of protein trafficking in the cell. Both vesicle fusion and vesicle formation can be inhibited by the addition of calcium chelators. Because the effects of calcium chelators have been studied predominantly in cell-free systems, it is not clear exactly which transport steps in the secretory pathway are sensitive to calcium levels. In this regard, we have studied the effects of calcium chelators on both anterograde and retrograde protein transport in whole cells. Using both cytochemical and biochemical analyses, we find that the anterograde-directed exit of vesicular stomatitis virus G protein and the retrograde-directed exit of Shiga toxin from the Golgi apparatus are both inhibited by calcium chelation. The exit of vesicular stomatitis virus G from a pre-Golgi compartment and the exit of Shiga toxin from an endosomal compartment are sensitive to the membrane-permeant calcium chelator 1,2-bis(2-amino phenoxy)ethane-N,N,N',N'-tetraacetic acid-tetrakis (acetoxymethyl ester) (BAPTA-AM). By contrast, endoplasmic reticulum exit and endocytic internalization from the plasma membrane are not affected by BAPTA. Together, our data show that some, but not all, trafficking steps in the cell may be regulated by calcium. These studies provide a framework for a more detailed analysis of the role of calcium as a regulatory agent during protein transport.

Perturbation of a very late step of regulated exocytosis by a secretory carrier membrane protein (SCAMP2)-derived peptide

Secretory carrier membrane proteins (SCAMPs) are conserved four transmembrane-spanning proteins associated with recycling vesicular carriers. In mast cells, as in other cell types, SCAMPs 1 and 2 are present in secretory granule membranes and other intracellular membranes. We now demonstrate a population of these SCAMPs in plasma membranes. Although small, this population partially colocalizes with SNARE proteins SNAP-23 and syntaxin 4. A fraction of SCAMPs 1 and 2 also coimmunoprecipitates with SNAP-23. An oligopeptide, E peptide, within the cytoplasmic segment linking the second and third transmembrane spans, particularly of SCAMP2, potently inhibits exocytosis in streptolysin O-permeabilized mast cells. The E peptide is unique to SCAMPs and highly conserved among SCAMP isoforms, and minor changes in its sequence abrogate inhibition. It blocks fusion beyond the putative docking step where granules contact the cell surface and each other during compound exocytosis. Blockade is also beyond Ca(2+)/ATP-dependent relocation of SNAP-23, which regulates compound exocytosis, and beyond ATP-dependent priming of fusion. Kinetic ordering of exocytotic inhibitors has shown that E peptide acts later than other perturbants at a stage closely associated with membrane fusion. These findings identify a new reagent for analyzing the final stage of exocytosis and point to the likely action of SCAMP2 in this process.

Phagosome maturation: aging gracefully

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

Selective regulation of the Rab9-independent transport of ricin to the Golgi apparatus by calcium

Transport of ricin from endosomes to the Golgi apparatus occurs, in contrast to the transport of the mannose 6-phosphate receptor, by a Rab9-independent process. To characterize the pathway of ricin transport to the Golgi apparatus, we investigated whether it was regulated by calcium. As shown here, our data indicate that calcium is selectively involved in the regulation of ricin transport to the Golgi apparatus. Thapsigargin, which inhibits calcium transport into the ER, and the calcium ionophore A23187 both increased the transport of ricin to the Golgi apparatus by a factor of 20. By contrast, transport of the mannose 6-phosphate receptor to the Golgi apparatus was unaffected. Ricin and mannose 6-phosphate receptor transport were measured by quantifying the sulfation of modified forms of ricin and the mannose 6-phosphate receptor. The increased transport of ricin was reduced by wortmannin and LY294002, suggesting that phosphoinositide 3-kinase might be involved in transport of ricin to the Golgi apparatus. Together, these findings indicate that the different pathways to the Golgi apparatus utilized by ricin and the mannose 6-phosphate receptor are regulated by different mechanisms.

Remodeling of organelle-bound actin is required for yeast vacuole fusion

Actin participates in several intracellular trafficking pathways. We now find that actin, bound to the surface of purified yeast vacuoles in the absence of cytosol or cytoskeleton, regulates the last compartment mixing stage of homotypic vacuole fusion. The Cdc42p GTPase is known to be required for vacuole fusion. We now show that proteins of the Cdc42p-regulated actin remodeling cascade (Cdc42p --> Cla4p --> Las17p/Vrp1p --> Arp2/3 complex --> actin) are enriched on isolated vacuoles. Vacuole fusion is dramatically altered by perturbation of the vacuole-bound actin, either by mutation of the ACT1 gene, addition of specific actin ligands such as latrunculin B or jasplakinolide, antibody to the actin regulatory proteins Las17p (yeast Wiskott-Aldrich syndrome protein) or Arp2/3, or deletion of actin regulatory genes. On docked vacuoles, actin is enriched at the "vertex ring" membrane microdomain where fusion occurs and is required for the terminal steps leading to membrane fusion. This role for actin may extend to other trafficking systems.

Rab3A and calmodulin regulate acrosomal exocytosis by mechanisms that do not require a direct interaction

The interaction between Rab3A and calmodulin is necessary for the inhibitory effect of Rab3A in neuroendocrine cells. Contrastingly, Rab3A triggers the exocytosis known as acrosome reaction in permeabilized spermatozoa. Here we show that a Rab3A mutant that cannot bind calmodulin was fully capable of triggering acrosomal exocytosis. Additionally, calmodulin by itself abrogated the exocytosis triggered by Rab3A. The effect was observed with both the wild type protein and the calmodulin binding deficient mutant. Our results indicate that the inhibitory and stimulatory effects of Rab3A in different exocytic processes are mediated by different effectors.

Membrane fusion

Fusion of biological membranes is governed by physical principles but it is unclear whether the transition states are primarily determined by lipid physics or by protein-lipid interactions. Recent advances in the field include the physical description of bilayer fusion, particularly new models beyond continuum models and the role of the SNARE proteins. Despite substantial progress, an integrated concept for protein-mediated membrane fusion is not yet available, and many open questions remain to be answered.

Calmodulin and lipid binding to synaptobrevin regulates calcium-dependent exocytosis

Neurotransmitter release involves the assembly of a heterotrimeric SNARE complex composed of the vesicle protein synaptobrevin (VAMP 2) and two plasma membrane partners, syntaxin 1 and SNAP-25. Calcium influx is thought to control this process via Ca(2+)-binding proteins that associate with components of the SNARE complex. Ca(2+)/calmodulin or phospholipids bind in a mutually exclusive fashion to a C-terminal domain of VAMP (VAMP(77-90)), and residues involved were identified by plasmon resonance spectroscopy. Microinjection of wild-type VAMP(77-90), but not mutant peptides, inhibited catecholamine release from chromaffin cells monitored by carbon fibre amperometry. Pre-incubation of PC12 pheochromocytoma cells with the irreversible calmodulin antagonist ophiobolin A inhibited Ca(2+)-dependent human growth hormone release in a permeabilized cell assay. Treatment of permeabilized cells with tetanus toxin light chain (TeNT) also suppressed secretion. In the presence of TeNT, exocytosis was restored by transfection of TeNT-resistant (Q(76)V, F(77)W) VAMP, but additional targeted mutations in VAMP(77-90) abolished its ability to rescue release. The calmodulin- and phospholipid-binding domain of VAMP 2 is thus required for Ca(2+)-dependent exocytosis, possibly to regulate SNARE complex assembly.