Ca2+/calmodulin causes Rab3A to dissociate from synaptic membranes

The Role of Calmodulin vs. Synaptotagmin in Exocytosis

Exocytosis is a Ca²⁺-regulated process that requires the participation of Ca²⁺ sensors. In the 1980s, two classes of Ca²⁺-binding proteins were proposed as putative Ca²⁺ sensors: EF-hand protein calmodulin, and the C2 domain protein synaptotagmin. In the next few decades, numerous studies determined that in the final stage of membrane fusion triggered by a micromolar boost in the level of Ca²⁺, the low affinity Ca²⁺-binding protein synaptotagmin, especially synaptotagmin 1 and 2, acts as the primary Ca²⁺ sensor, whereas calmodulin is unlikely to be functional due to its high Ca²⁺ affinity. However, in the meantime emerging evidence has revealed that calmodulin is involved in the earlier exocytotic steps prior to fusion, such as vesicle trafficking, docking and priming by acting as a high affinity Ca²⁺ sensor activated at submicromolar level of Ca²⁺. Calmodulin directly interacts with multiple regulatory proteins involved in the regulation of exocytosis, including VAMP, myosin V, Munc13, synapsin, GAP43 and Rab3, and switches on key kinases, such as type II Ca²⁺/calmodulin-dependent protein kinase, to phosphorylate a series of exocytosis regulators, including syntaxin, synapsin, RIM and Ca²⁺ channels. Moreover, calmodulin interacts with synaptotagmin through either direct binding or indirect phosphorylation. In summary, calmodulin and synaptotagmin are Ca²⁺ sensors that play complementary roles throughout the process of exocytosis. In this review, we discuss the complementary roles that calmodulin and synaptotagmin play as Ca²⁺ sensors during exocytosis.

Pleiotropic Roles of Calmodulin in the Regulation of KRas and Rac1 GTPases: Functional Diversity in Health and Disease

Calmodulin is a ubiquitous signalling protein that controls many biological processes due to its capacity to interact and/or regulate a large number of cellular proteins and pathways, mostly in a Ca²⁺-dependent manner. This complex interactome of calmodulin can have pleiotropic molecular consequences, which over the years has made it often difficult to clearly define the contribution of calmodulin in the signal output of specific pathways and overall biological response. Most relevant for this review, the ability of calmodulin to influence the spatiotemporal signalling of several small GTPases, in particular KRas and Rac1, can modulate fundamental biological outcomes such as proliferation and migration. First, direct interaction of calmodulin with these GTPases can alter their subcellular localization and activation state, induce post-translational modifications as well as their ability to interact with effectors. Second, through interaction with a set of calmodulin binding proteins (CaMBPs), calmodulin can control the capacity of several guanine nucleotide exchange factors (GEFs) to promote the switch of inactive KRas and Rac1 to an active conformation. Moreover, Rac1 is also an effector of KRas and both proteins are interconnected as highlighted by the requirement for Rac1 activation in KRas-driven tumourigenesis. In this review, we attempt to summarize the multiple layers how calmodulin can regulate KRas and Rac1 GTPases in a variety of cellular events, with biological consequences and potential for therapeutic opportunities in disease settings, such as cancer.

Modeling and subtleties of K-Ras and Calmodulin interaction

K-Ras, one of the most common small GTPases of the cell, still presents many riddles, despite the intense efforts to unveil its mysteries. Such is the case of its interaction with Calmodulin, a small acidic protein known for its role as a calcium ion sensor. Although the interaction between these two proteins and its biological implications have been widely studied, a model of their interaction has not been performed. In the present work we analyse this intriguing interaction by computational means. To do so, both conventional molecular dynamics and scaled molecular dynamics have been used. Our simulations suggest a model in which Calmodulin would interact with both the hypervariable region and the globular domain of K-Ras, using a lobe to interact with each of them. According to the presented model, the interface of helixes α4 and α5 of the globular domain of K-Ras would be relevant for the interaction with a lobe of Calmodulin. These results were also obtained when bringing the proteins together in a step wise manner with the umbrella sampling methodology. The computational results have been validated using SPR to determine the relevance of certain residues. Our results demonstrate that, when mutating residues of the α4-α5 interface described to be relevant for the interaction with Calmodulin, the interaction of the globular domain of K-Ras with Calmodulin diminishes. However, it is to be considered that our simulations indicate that the bulk of the interaction would fall on the hypervariable region of K-Ras, as many more interactions are identified in said region. All in all our simulations present a suitable model in which K-Ras could interact with Calmodulin at membrane level using both its globular domain and its hypervariable region to stablish an interaction that leads to an altered signalling.

K-Ras is one of the most mutated oncogenes in human cancer. Although several studies validate K-Ras protein as good candidate for direct therapeutic targeting, pharmacologic targeting has not been successful. During the last years increasing evidences demonstrate that oncogenic K-Ras activity can be modulated in vivo by dimerization, nanoclustering at the plasma membrane or interaction with non-effector proteins, consequently opening new therapeutic strategies. We have previously demonstrated that Calmodulin, an ubiquitous Ca2+-binding protein, is one of this K-Ras interacting proteins and that it negatively modulates K-Ras signaling. Although experimental data were available showing the relevant regions for this interaction, a model of K-Ras and Calmodulin interaction was missing. In the present work by using different computational modeling techniques we obtained a model for this interaction that agrees with the experimental data. We believe the present model will help to better understand K-Ras regulation, and to design new inhibitors. For instance, base on our model, we can predict that the interaction can take place at the plasma membrane, and that since the surface of K-Ras that interact with Calmodulin is the same that it uses for dimerization, that Calmodulin could be inhibiting K-Ras dimerization.

Calmodulin interacts with Rab3D and modulates osteoclastic bone resorption

Calmodulin is a highly versatile protein that regulates intracellular calcium homeostasis and is involved in a variety of cellular functions including cardiac excitability, synaptic plasticity and signaling transduction. During osteoclastic bone resorption, calmodulin has been reported to concentrate at the ruffled border membrane of osteoclasts where it is thought to modulate bone resorption activity in response to calcium. Here we report an interaction between calmodulin and Rab3D, a small exocytic GTPase and established regulator osteoclastic bone resorption. Using yeast two-hybrid screening together with a series of protein-protein interaction studies, we show that calmodulin interacts with Rab3D in a calcium dependent manner. Consistently, expression of a calcium insensitive form of calmodulin (i.e. CaM1234) perturbs calmodulin-Rab3D interaction as monitored by bioluminescence resonance energy transfer (BRET) assays. In osteoclasts, calmodulin and Rab3D are constitutively co-expressed during RANKL-induced osteoclast differentiation, co-occupy plasma membrane fractions by differential gradient sedimentation assay and colocalise in the ruffled border as revealed by confocal microscopy. Further, functional blockade of calmodulin-Rab3D interaction by calmidazolium chloride coincides with an attenuation of osteoclastic bone resorption. Our data imply that calmodulin- Rab3D interaction is required for efficient bone resorption by osteoclasts in vitro.

Mapping of interactions between human macrophages and Staphylococcus aureus reveals an involvement of MAP kinase signaling in the host defense

Staphylococcus aureus is a dangerous opportunistic human pathogen that causes serious invasive diseases when it reaches the bloodstream. Recent studies have shown that S. aureus is highly resistant to killing by professional phagocytes and that such cells even provide a favorable environment for intracellular survival of S. aureus. Importantly, the reciprocal interactions between phagocytes and S. aureus have remained largely elusive. Here we have employed kinase profiling to define the nature and time resolution of the human THP-1 macrophage response toward S. aureus and proteomics to identify the response of S. aureus toward macrophages. The results of these studies reveal major macrophage signaling pathways triggered by S. aureus and proteomic signatures of the responses of S. aureus to macrophages. We also identify human proteins bound to S. aureus that have potential roles in bacterial killing and internalization. Most noticeably, our observations challenge the classical concept that macrophage responses are mainly mediated through Toll-like receptor 2 and NF-κB signaling and highlight the important role of the stress-activated MAP kinase signaling in orchestrating the host defense.

Both the C-terminal polylysine region and the farnesylation of K-RasB are important for its specific interaction with calmodulin

Background

Ras protein, as one of intracellular signal switches, plays various roles in several cell activities such as differentiation and proliferation. There is considerable evidence showing that calmodulin (CaM) binds to K-RasB and dissociates K-RasB from membrane and that the inactivation of CaM is able to induce K-RasB activation. However, the mechanism for the interaction of CaM with K-RasB is not well understood.

Methodology/Principal Findings

Here, by applying fluorescence spectroscopy and isothermal titration calorimetry, we have obtained thermodynamic parameters for the interaction between these two proteins and identified the important elements of K-RasB for its interaction with Ca²⁺/CaM. One K-RasB molecule interacts with one CaM molecule in a GTP dependent manner with moderate, micromolar affinity at physiological pH and physiologic ionic strength. Mutation in the polybasic domain of K-Ras decreases the binding affinity. By using a chimera in which the C-terminal polylysine region of K-RasB has been replaced with that of H-Ras and vice versa, we find that at physiological pH, H-Ras-(KKKKKK) and Ca²⁺/CaM formed a 1∶1 complex with an equilibrium association constant around 10⁵ M⁻¹, whereas no binding reaction of K-RasB-(DESGPC) with Ca²⁺/CaM is detected. Furthermore, the interaction of K-RasB with Ca²⁺/CaM is found to be enhanced by the farnesylation of K-RasB.

Conclusions/Significance

We demonstrate that the polylysine region of K-RasB not only contributes importantly to the interaction of K-RasB with Ca²⁺/CaM, but also defines its isoform specific interaction with Ca²⁺/CaM. The farnesylation of K-RasB is also important for its specific interaction with Ca²⁺/CaM. Information obtained here can enhance our understanding of how CaM interacts with K-RasB in physiological environments.

Ca/Calmodulin and presynaptic short-term plasticity

Synaptic efficacy is remodeled by neuronal firing activity at the presynaptic terminal. Presynaptic activity-dependent changes in transmitter release induce postsynaptic plasticity, including morphological change in spine, gene transcription, and protein synthesis and trafficking. The presynaptic transmitter release is triggered and regulated by Ca²⁺, which enters through voltage-gated Ca²⁺ (Ca_V) channels and diffuses into the presynaptic terminal accompanying action potential firings. Residual Ca²⁺ is sensed by Ca²⁺-binding proteins, among other potential actions, it mediates time- and space-dependent synaptic facilitation and depression via effects on Ca_V2 channel gating and vesicle replenishment in the readily releasable pool (RRP). Calmodulin, a Ca²⁺-sensor protein with an EF-hand motif that binds Ca²⁺, interacts with Ca_V2 channels and autoreceptors in modulation of SNARE-mediated exocytosis.

Synapsin-dependent vesicle recruitment modulated by forskolin, phorbol ester and ca in mouse excitatory hippocampal synapses

Repeated release of transmitter from presynaptic elements depends on stimulus-induced Ca²⁺ influx together with recruitment and priming of synaptic vesicles from different vesicle pools. We have compared three different manipulations of synaptic strength, all of which are known to increase short-term synaptic efficacy through presynaptic mechanisms, in the glutamatergic CA3-to-CA1 stratum radiatum synapse in the mouse hippocampal slice preparation. Synaptic responses elicited from the readily releasable vesicle pool during low-frequency synaptic activation (0.1 Hz) were significantly enhanced by both the adenylate cyclase activator forskolin, the priming activator β-phorbol-12,13-dibutyrate (PDBu) and 4 mM [Ca²⁺]_o′ whereas during 20 Hz stimulation, the same manipulations reduced the time needed to reach the peak and increased the magnitude of the resulting frequency facilitation. In contrast, paired-pulse facilitations were unchanged in the presence of forskolin, decreased by 4 mM [Ca²⁺]_o and essentially abolished by PDBu. The subsequent delayed response enhancement (DRE) responses, elicited during continuous 20 Hz stimulations and mediated by recruited vesicles, were enhanced by forskolin, essentially unchanged by PDBu and slightly decreased by 4 mM [Ca²⁺]_o· Similar experiments done on slices devoid of the vesicle-associated synapsin I and II proteins indicated that synapsin I/II-induced enhancements of vesicle recruitment were restricted to Ca²⁺-induced frequency facilitations and forskolin-induced enhancements of the early DRE phase, whereas the proteins had minor effects during PDBu-treatment and represented constraints on late Ca²⁺-induced responses. The data indicate that in these glutamatergic synapses, the comparable enhancements of single synaptic responses induced by these biochemical mechanisms can be transformed during prolonged synaptic stimulation into highly distinct short-term plasticity patterns, which are partly dependent on synapsins I/II.

Role for Rab3a in oligodendrocyte morphological differentiation

Rab3a, a small GTPase important for exocytosis, is uniquely up-regulated as oligodendrocytes enter terminal differentiation and initiate myelin biosynthesis. In this study, we analyze the role of this protein in oligodendrocyte morphological differentiation by using Rab3a overexpression and siRNAi-mediated Rab3a silencing. We found that Rab3a silencing delayed mature oligodendrocyte morphological differentiation but did not interfere with lineage progression of OL progenitors; this is consistent with the high levels of Rab3a expressed by mature oligodendrocytes compared with progenitor cells. Overexpression of GTP-bound, but not that of wild-type, Rab3a delayed OL morphological differentiation; this suggests that expression of a GTP-bound Rab3a mutant interferes with the normal function of endogenous Rab3a. We have also identified in oligodendrocytes two other exocytic small GTPases, Rab27B and RalA. Together, these findings indicate that Rab3a specifically stimulates morphological differentiation of mature oligodendrocytes and thus may be part of the necessary machinery for myelin membrane biogenesis.

Small G proteins as key regulators of pancreatic digestive enzyme secretion

Small GTP-binding (G) proteins act as molecular switches to regulate a number of cellular processes, including vesicular transport. Emerging evidence indicates that small G proteins regulate a number of steps in the secretion of pancreatic acinar cells. Diverse small G proteins have been localized at discrete compartments along the secretory pathway and particularly on the secretory granule. Rab3D, Rab27B, and Rap1 are present on the granule membrane and play a role in the steps leading up to exocytosis. Whether the function of these G proteins is simply to ensure appropriate targeting or if they are involved as regulatory molecules is discussed. Most evidence suggests that Rab3D and Rab27B play a role in tethering the secretory granule to its target membrane. Other Rabs have been identified on the secretory granule that are associated with different steps in the secretory pathway. The Rho family small G proteins RhoA and Rac1 also regulate secretion through remodeling of the actin cytoskeleton. Possible mechanisms for regulation of these G proteins and their effector molecules are considered.

Roles of calmodulin and calmodulin-binding proteins in synaptic vesicle recycling during regulated exocytosis at submicromolar Ca2+ concentrations

Calcium ion is required at various concentrations for vesicular recycling in the presynaptic terminal. Although calmodulin (CaM) is the most abundant Ca2+-binding protein and has a submicromolar affinity for Ca2+, it is not the Ca2+ sensor for vesicular fusion because this process requires Ca2+ concentrations above 1 microM. Several lines of evidence, however, suggest that CaM mediates the regulation of vesicular recycling by submicromolar Ca2+ via novel protein-protein interactions. In this review, we discuss recent findings on how CaM regulates synaptic vesicle recycling by controlling the SNARE mechanism, which is the molecular machinery that mediates exocytosis.

Visualization of Rab3A dissociation during exocytosis: a study by total internal reflection microscopy

Rab3A is a small G protein in the Rab3 subfamily, and is thought to act at late stage of exocytosis. However, the detailed mechanism of its action is not completely understood. To study the role of Rab3A in exocytosis, we used a total internal reflection fluorescence microscope to examine the fluorescence changes of EGFP-Rab3A-labeled and NPY-EGFP-labeled vesicles in PC12 cells upon stimulation. The fluorescence of EGFP-Rab3A-labeled and NPY-EGFP-labeled vesicles decreased while showing different patterns. The NPY-EGFP-labeled vesicles that exocytosed showed a transient fluorescence increase before NPY-EGFP fluorescence disappearance, which represents fusion and NPY release. This transient increase was diminished in cells that co-expressed the GDP-bound Rab3A mutant. The fluorescence of EGFP-Rab3A-labeled vesicles dispersed before disappearance, which represents the dissociation of Rab3A from the vesicles. The dispersion was not found in GTP-bound Rab3A mutant-labeled vesicles. Interestingly, EGFP-Rab3A F59S, a mutant unable to bind rabphilin, dissociates slower from the vesicles than wild type Rab3A and caused a slower release of NPY-EGFP. The results provide direct evidence to support the hypothesis that GTP hydrolysis and rabphilin are involved in Rab3A dissociation from the vesicles and the occurrence of exocytosis.

Protein-protein interaction analysis by C-terminally specific fluorescence labeling and fluorescence cross-correlation spectroscopy

Here, we describe novel puromycin derivatives conjugated with iminobiotin and a fluorescent dye that can be linked covalently to the C-terminus of full-length proteins during cell-free translation. The iminobiotin-labeled proteins can be highly purified by affinity purification with streptavidin beads. We confirmed that the purified fluorescence-labeled proteins are useful for quantitative protein–protein interaction analysis based on fluorescence cross-correlation spectroscopy (FCCS). The apparent dissociation constants of model protein pairs such as proto-oncogenes c-Fos/c-Jun and archetypes of the family of Ca²⁺-modulated calmodulin/related binding proteins were in accordance with the reported values. Further, detailed analysis of the interactions of the components of polycomb group complex, Bmi1, M33, Ring1A and RYBP, was successfully conducted by means of interaction assay for all combinatorial pairs. The results indicate that FCCS analysis with puromycin-based labeling and purification of proteins is effective and convenient for in vitro protein–protein interaction assay, and the method should contribute to a better understanding of protein functions by using the resource of available nucleotide sequences.

Calmodulin binding to the small GTPase Ral requires isoprenylated Ral

Ral, a member of the Ras-p21 superfamily of small GTPases, has been shown to require the calcium-signaling protein calmodulin (CaM) for activation. In the present work, we investigated the properties of the Ral-CaM interaction. Using CaM affinity binding assay with lysates from mammalian cells overexpressing various Ral mutants, we found that RalB(V23, DeltaCAAX) lacking the C-terminal isoprenylation region bound significantly less efficiently to CaM. Binding of other mutants containing critical amino acid changes in the nucleotide or substrate binding regions (residues 23, 28, and 49) was not affected. In addition, all mutants bound significantly better in the presence of calcium versus the calcium chelator EGTA. Using in vitro transcription-translation in the presence of geranylgeranyl pyrophosphate, we demonstrate enhanced Ral binding to CaM. Inhibition of isoprenylation in cells in culture with lovastatin resulted in decreased binding of CaM to Ral. The present results show that post-translational isoprenylation of Ral is important in Ral-CaM interaction.

Reversible intracellular translocation of KRas but not HRas in hippocampal neurons regulated by Ca2+/calmodulin

The Ras/MAPK pathway regulates synaptic plasticity and cell survival in neurons of the central nervous system. Here, we show that KRas, but not HRas, acutely translocates from the plasma membrane (PM) to the Golgi complex and early/recycling endosomes in response to neuronal activity. Translocation is reversible and mediated by the polybasic-prenyl membrane targeting motif of KRas. We provide evidence that KRas translocation occurs through sequestration of the polybasic-prenyl motif by Ca²⁺/calmodulin (Ca²⁺/CaM) and subsequent release of KRas from the PM, in a process reminiscent of GDP dissociation inhibitor–mediated membrane recycling of Rab and Rho GTPases. KRas translocation was accompanied by partial intracellular redistribution of its activity. We conclude that the polybasic-prenyl motif acts as a Ca²⁺/CaM-regulated molecular switch that controls PM concentration of KRas and redistributes its activity to internal sites. Our data thus define a novel signaling mechanism that differentially regulates KRas and HRas localization and activity in neurons.

Large-scale profiling of Rab GTPase trafficking networks: the membrome

Rab GTPases and SNARE fusion proteins direct cargo trafficking through the exocytic and endocytic pathways of eukaryotic cells. We have used steady state mRNA expression profiling and computational hierarchical clustering methods to generate a global overview of the distribution of Rabs, SNAREs, and coat machinery components, as well as their respective adaptors, effectors, and regulators in 79 human and 61 mouse nonredundant tissues. We now show that this systems biology approach can be used to define building blocks for membrane trafficking based on Rab-centric protein activity hubs. These Rab-regulated hubs provide a framework for an integrated coding system, the membrome network, which regulates the dynamics of the specialized membrane architecture of differentiated cells. The distribution of Rab-regulated hubs illustrates a number of facets that guides the overall organization of subcellular compartments of cells and tissues through the activity of dynamic protein interaction networks. An interactive website for exploring datasets comprising components of the Rab-regulated hubs that define the membrome of different cell and organ systems in both human and mouse is available at http://www.membrome.org/.

Calcium-mediated protein secretion potentiates motility in Toxoplasma gondii

Apicomplexans such as Toxoplasma gondii actively invade host cells using a unique parasite-dependent mechanism termed gliding motility. Calcium-mediated protein secretion by the parasite has been implicated in this process, but the precise role of calcium signaling in motility remains unclear. Here we used calmidazolium as a tool to stimulate intracellular calcium fluxes and found that this drug led to enhanced motility by T. gondii. Treatment with calmidazolium increased the duration of gliding and resulted in trails that were twice as long as those formed by control parasites. Calmidazolium also increased microneme secretion by T. gondii, and studies with a deletion mutant of the accessory protein m2AP specifically implicated that adhesin MIC2 was important for gliding. The effects of calmidazolium on gliding and secretion were due to increased release of calcium from intracellular stores and calcium influx from the extracellular milieu. In addition, we demonstrate that calmidazolium-stimulated increases in intracellular calcium were highly dynamic, and that rapid fluxes in calcium levels were associated with parasite motility. Our studies suggest that oscillations in intracellular calcium levels may regulate microneme secretion and control gliding motility in T. gondii.

Rab3D redistribution and function in rat parotid acini

Rab3D is a low molecular weight GTP-binding protein believed to be involved with regulated secretion in many cell types. In parotid, Rab3D is localized to secretory granule membranes or present in the cytosol as a complex with Rab escort protein. In the present study, we examined the redistribution of membrane-associated Rab3D during secretion in permeabilized parotid acini. When permeabilized acini were stimulated with calcium and cAMP, amylase release increased greater than twofold over basal. Quantitative immunoblotting of subcellular fractions revealed that Rab3D did not dissociate from parotid membranes during secretion. Immunohistochemical staining demonstrated that Rab3D co-localizes with amylase containing granules that are found in the apical pole of the cell. Upon stimulation with calcium and cAMP, Rab3D and amylase immunostaining of granules appeared to be more dispersed. However, Rab3D immunostaining was not observed on the plasma membrane and appeared to reside in the apical cytoplasm. To examine the role of Rab3D in amylase release, cytosolic extracts containing myc-tagged Rab3D and Rab3DQ81L, a GTP-binding mutant, were prepared and incubated with streptolysin O-permeabilized acini. Rab3D, but not Rab3DQ81L, bound to parotid membranes suggesting that Rab3D-binding to parotid membranes is guanine nucleotide-dependent. Moreover, wild-type and mutant Rab3D inhibited agonist-induced amylase release from permeabilized parotid acini. These observations indicate that in parotid acini, Rab3D does not dissociate from parotid membranes or redistribute to the plasma membrane during secretion, and may play an inhibitory role in regulated secretion. The fact that both wild-type Rab3D and the GTP-binding mutant inhibit amylase release suggests that binding of Rab3D to the membrane is not essential for secretory inhibition.

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.

Ca2+/calmodulin binds and dissociates K-RasB from membrane

We have investigated the interaction of calmodulin (CaM) with Ras-p21 and the significance of this association. All Ras-p21 isoforms tested (H-, K-, and N-Ras) were detected in the particulate fraction of human platelets and MCF-7 cells, a human breast cancer cell line. In MCF-7 cells, H- and N-Ras were also detected in the cytosolic fraction. K-RasB from platelet and MCF-7 cell lysates was found to bind CaM in a Ca2+ -dependent but GTPgammaS-independent manner. The yeast two-hybrid analysis demonstrated that K-RasB binds to CaM in vivo. Incubation of isolated membranes from platelet and MCF-7 cells with CaM caused dissociation of only K-RasB from membranes in a Ca2+ -dependent manner. CaM antagonist, W7, inhibited dissociation of K-RasB. Addition of platelet or MCF-7 cytosol alone to isolated platelet membranes did not cause dissociation of K-RasB and only addition of exogenous CaM caused dissociation. The results suggest a potential role for Ca2+/CaM in the regulation of K-RasB function.

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.

Role of Rab Proteins in Epithelial Membrane Traffic

Small GTPase rab proteins play an important role in various aspects of membrane traffic, including cargo selection, vesicle budding, vesicle motility, tethering, docking, and fusion. Recent data suggest also that rabs, and their divalent effector proteins, organize organelle subdomains and as such may define functional organelle identity. Most rabs are ubiquitously expressed. However, some rabs are preferentially expressed in epithelial cells where they appear intimately associated with the epithelial-specific transcytotic pathway and/or tight junctions. This review discusses the role of rabs in epithelial membrane transport.

Mlc1p promotes septum closure during cytokinesis via the IQ motifs of the vesicle motor Myo2p

Little is known about the molecular machinery that directs secretory vesicles to the site of cell separation during cytokinesis. We show that in Saccharomyces cerevisiae, the class V myosin Myo2p and the Rab/Ypt Sec4p, that are required for vesicle polarization processes at all stages of the cell cycle, form a complex with each other and with a myosin light chain, Mlc1p, that is required for actomyosin ring assembly and cytokinesis. Mlc1p travels on secretory vesicles and forms a complex(es) with Myo2p and/or Sec4p. Its functional interaction with Myo2p is essential during cytokinesis to target secretory vesicles to fill the mother bud neck. The role of Mlc1p in actomyosin ring assembly instead is dispensable for this process. Therefore, in yeast, as recently shown in mammals, class V myosins associate with vesicles via the formation of a complex with Rab/Ypt proteins. Further more, myosin light chains, via their ability to be transported by secretory vesicles and to interact with class V myosin IQ motifs, can regulate vesicle polarization processes at a specific location and stage of the cell cycle.

SNAREs and associated regulators in the control of exocytosis in the RBL-2H3 mast cell line

Mast cells participate in inflammation and allergies by releasing biologically active mediators stored in numerous cytoplasmic granules. Degranulation is tightly controlled and requires activation of cell surface receptors, such as the high affinity IgE receptor (FcepsilonRI). Here, we discuss some of the key components of the molecular machinery that regulates the final steps of fusion between the granular and plasma membrane based on results obtained with the rat mast cell line RBL-2H3. We emphasize the role of soluble N-ethylmaleimide attachment protein receptors (SNAREs) proteins such as syntaxin 4 that can promote membrane fusion through formation of a stable complex with SNAP-23. We also highlight the role of a Ser/Thr kinase found to be associated with Rab3D, a negative regulator of degranulation. Associated kinase activity, which diminishes after stimulation as a consequence of intracellular calcium increases, specifically phosphorylates syntaxin 4 thereby affecting its capacity to bind to its t-SNARE partner SNAP-23. Our results suggest a new way of how Rab3 GTPases may intersect with the function of SNAREs thought to be universal mediators of membrane fusion.

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.

GTP binds to Rab3A in a complex with Ca2+/calmodulin

Ras-like small GTP-binding proteins of the Rab family regulate trafficking of the secretory or endocytic pathways. Rab3 proteins within the Rab family are expressed at high levels in neurons and endocrine cells, where they regulate release of dense-core granules and synaptic vesicles (SVs). Rab3A is present as either the soluble or the SV membrane-bound form in neurons that are dependent on the GDP- or GTP-bound states respectively. GDP dissociation inhibitor (GDI) is known to induce the dissociation of Rab3A from synaptic membranes when GTP is depleted. In an earlier study, Ca(2+)/calmodulin (CaM) was also shown to dissociate Rab3A from synaptic membranes by forming an equimolar complex with Rab3A in vitro. We have examined a possible role for Ca(2+)/CaM in modulating both the binding of guanine nucleotides to Rab3A and the GTPase activity of Rab3A. The basal level of Rab3A GTPase activity was not affected by an association with Ca(2+)/CaM. Ca(2+)/CaM-Rab3A complex that was formed in synaptic membranes was able to bind guanine nucleotides, whereas the Rab3A-GDI complex could not. In addition, Ca(2+)/CaM led to the replacement of the GDP molecule in the Rab3A-GDI complex with GTP in Rab3A. Taken together, these results suggest that CaM may have a role in stimulating GTP binding to Rab3A that is complexed with GDI, which leads to the formation of an active GTP-bound form of the Rab3A-Ca(2+)/CaM complex.

Direct interaction between Rab3b and the polymeric immunoglobulin receptor controls ligand-stimulated transcytosis in epithelial cells

We have examined the role of rab3b in epithelial cells. In MDCK cells, rab3b localizes to vesicular structures containing the polymeric immunoglobulin receptor (pIgR) and located subjacent to the apical surface. We found that GTP-bound rab3b directly interacts with the cytoplasmic domain of pIgR. Binding of dIgA to pIgR causes a dissociation of the interaction with rab3b, a process that requires dIgA-mediated signaling, Arg657 in the cytoplasmic domain of pIgR, and possibly GTP hydrolysis by rab3b. Binding of dIgA to pIgR at the basolateral surface stimulates subsequent transcytosis to the apical surface. Overexpression of GTP-locked rab3b inhibits dIgA-stimulated transcytosis. Together, our data demonstrate that a rab protein can bind directly to a specific cargo protein and thereby control its trafficking.

Rab3B in human platelet is membrane bound and interacts with Ca(2+)/calmodulin

The subcellular distribution of Rab3B in fresh and aged platelets was determined and majority of the protein was localized with the particulate fraction with only a minor amount detected in the cytosol. Rab3B was pulled out from platelet particulate fraction with GST-RabGDI-alpha fusion protein. Using GST-Rab3B in in vitro pull-down experiments, the binding of calmodulin from platelet cytosol to Rab3B was demonstrated. In the reverse experiment, binding of Rab3B from platelet particulate and cytosolic fractions to Sepharose-CaM beads was also observed. The interaction between Rab3B and calmodulin was Ca(2+)-dependent but independent of the guanine nucleotide status of Rab3B. These findings provide evidence that Rab3B is primarily localized with the particulate fraction and that Ca(2+)/calmodulin could regulate function of this GTPase in the platelet.

Calmodulin mediates rapid recruitment of fast-releasing synaptic vesicles at a calyx-type synapse

In many synapses, depletion and recruitment of releasable synaptic vesicles contribute to use-dependent synaptic depression and recovery. Recently it has been shown that high-frequency presynaptic stimulation enhances recovery from depression, which may be mediated by Ca2+. We addressed this issue by measuring quantal release rates at the calyx of Held synapse and found that transmission is mediated by a heterogeneous population of vesicles, with one subset releasing rapidly and recovering slowly and another one releasing reluctantly and recovering rapidly. Ca2+ promotes refilling of the rapidly releasing synaptic vesicle pool and calmodulin inhibitors block this effect. We propose that calmodulin-dependent refilling supports recovery from synaptic depression during high-frequency trains in concert with rapid recovery of the slowly releasing vesicles.

IgE receptor type I-dependent regulation of a Rab3D-associated kinase: a possible link in the calcium-dependent assembly of SNARE complexes

Following activation through high affinity IgE receptors (FcepsilonRI), mast cells release, within a few minutes, their granule content of inflammatory and allergic mediators. FcepsilonRI-induced degranulation is a SNARE (soluble N-ethylmaleimide attachment protein receptors)-dependent fusion process. It is regulated by Rab3D, a subfamily member of Rab GTPases. Evidence exists showing that Rab3 action is calcium-regulated although the molecular mechanisms remain unclear. To obtain an understanding of Rab3D function we have searched for Rab3D-associated effectors that respond to allergic triggering through FcepsilonRI. Using the RBL-2H3 mast cell line we detected a Ser/Thr kinase activity, termed here Rak3D (from Rab3D-associated kinase), because it was specifically co-immunoprecipitated with anti-Rab3D antibody. Rak3D activity, as measured by its auto- or transphosphorylation, was maximal in resting cells and decreased upon stimulation. The down-regulation of the observed activity was blocked with EGTA, but not with other degranulation inhibitors, suggesting that its activity functions downstream of calcium influx. We found that Rak3D phosphorylates the NH(2)-terminal regulatory domain of the t-SNARE syntaxin 4, but not syntaxin 2 or 3. The phosphorylation of syntaxin 4 decreased its binding to its partner SNAP23. Thus, we propose a novel phosphorylation-dependent mechanism by which Rab3D controls SNARE assembly in a calcium-dependent manner.

Ypt/rab gtpases: regulators of protein trafficking

Ypt/Rab guanosine triphosphatases (GTPases) have emerged in the last decade as key regulators of protein transport in all eukaryotic cells. They seem to be involved in all aspects of vesicle trafficking: vesicle formation, motility, and docking, and membrane remodeling and fusion. The functions of Ypt/Rabs are themselves controlled by upstream regulators that stimulate both their nucleotide cycling and their cycling between membranes. Ypt/Rabs transmit signals to downstream effectors in a guanosine triphosphate (GTP)-dependent manner. The identity of upstream regulators and downstream effectors is known for a number of Ypt/Rabs, and models for their mechanisms of action are emerging. In at least two cases, Ypt/Rab upstream regulators and downstream effectors are found together in a single complex. In agreement with the idea that Ypt/Rabs function in all aspects of vesicular transport, their diverse effectors have recently been shown to function in all identified aspects of vesicle transport. Activators and effectors for individual Ypt/Rabs share no similarity, but are conserved between yeast and mammalian cells. Finally, cross talk demonstrated among the various Ypt/Rabs, and between Ypt/Rabs and other signaling factors, suggests possible coordination among secretory steps, as well as between protein transport and other cellular processes.

A low-affinity Ca2+-dependent association of calmodulin with the Rab3A effector domain inversely correlates with insulin exocytosis

The stimulus-response coupling pathway for glucose-regulated insulin secretion has implicated a rise in cytosolic [Ca2+]i as a key factor to induce insulin exocytosis. However, it is unclear how elevated [Ca2+]i communicates with the pancreatic beta-cell's exocytotic apparatus. As Rab3A is a model protein involved in regulated exocytosis, we have focused on its role in regulating insulin exocytosis. By using a photoactivatable cross-linking synthetic peptide that mimics the effector domain of Rab3A and microsequence analysis, we found calmodulin to be a major Rab3A target effector protein in pancreatic beta-cells. Coimmunoprecipitation analysis from pancreatic islets confirmed a Rab3A-calmodulin interaction in vivo, and that it inversely correlated with insulin exocytosis. Calmodulin affected neither GTPase nor guanine nucleotide exchange activity of Rab3A. The calmodulin-Rab3A interaction was pH- and Ca2+-dependent, and it was preferential for GTP-bound Rab3A. However, Rab3A affinity for calmodulin was relatively low (Kd = 18-22 micromol/l at 10(-5) mol/l [Ca2+]) and competed by other calmodulin-binding proteins that had higher affinity (e.g., Ca2+/calmodulin-dependent protein kinase-2 [CaMK-2] [Kd = 300-400 nmol/l at 10(-5) mol/l [Ca2+]]). Moreover, the Ca2+ dependence of the calmodulin-Rab3A interaction (K0.5 = 15-18 micromol/l [Ca2+], maximal at 100 micromol/l [Ca2+]) was significantly lower compared with that of the calmodulin-CaMK-2 association (K0.5 = 40 micromol/l [Ca2+], maximal at 1 mmol/l [Ca2+]). The data suggested that a transient Rab3A-calmodulin interaction might represent a means of directing calmodulin to the cytoplasmic face of a beta-granule, where it can be subsequently transferred for activation of other beta-granule-associated calmodulin-binding proteins as local [Ca2+]i rises to promote insulin exocytosis.

Rab3a controls exocytosis in cholecystokinin-secreting cells

The expression of rab3A and rab3D isoforms in the enteroendocrine, cholecystokinin-secreting, cell lines STC-1 and GLUTag is here demonstrated. In contrast, rab3B is undetectable in these two cell lines, and rab3C is only slightly expressed in GLUTag cells. Using a transient co-transfection system with human growth hormone as reporter protein, we show that overexpression of the GTPase-deficient mutant rab3AQ81L, but not rab3DQ81L, significantly decreases human growth hormone secretory responses to various agonists in STC-1 cells. These results indicate that endocrine cell lines of intestinal origin express rab3A and rab3D proteins, but the GTP-bound form of rab3A only acts as a negative modulator in the control of cholecystokinin secretion from STC-1 cells.

A direct inhibitory role for the Rab3-specific effector, Noc2, in Ca2+-regulated exocytosis in neuroendocrine cells

Rab proteins comprise a family of GTPases, conserved from yeast to mammals, which are integral components of membrane trafficking pathways. Rab3A is a neural/neuroendocrine-specific member of the Rab family involved in Ca(2+) -regulated exocytosis, where it functions in an inhibitory capacity controlling recruitment of secretory vesicles into a releasable pool at the plasma membrane. The effector by which Rab3A exerts its inhibitory effect is unclear as the Rab3A effectors Rabphilin and RIM have been excluded from for this role. One putative Rab3A effector in dense-core granule exocytosis is the cytosolic zinc finger protein, Noc2. We have established that overexpression of Noc2 in PC12 cells has a direct inhibitory effect upon Ca(2+)-triggered exocytosis in permeabilized cells. We demonstrate specific nucleotide-dependent binding of Noc2 to Rab3A and show that the inhibition of exocytosis is dependent upon this interaction since Rab3A binding-deficient mutants of Noc2 do not inhibit exocytosis. We propose that Noc2 may be a negative effector for Rab3A in regulated exocytosis of dense-core granules from endocrine cells.

Mycobacterium tuberculosis phagosomes exhibit altered calmodulin-dependent signal transduction: contribution to inhibition of phagosome-lysosome fusion and intracellular survival in human macrophages

Mycobacterium tuberculosis successfully parasitizes macrophages by disrupting the maturation of its phagosome, creating an intracellular compartment with endosomal rather than lysosomal characteristics. We have recently demonstrated that live M. tuberculosis infect human macrophages in the absence of an increase in cytosolic Ca(2+) ([Ca(2+)](c)), which correlates with inhibition of phagosome-lysosome fusion and intracellular viability. In contrast, killed M. tuberculosis induces an elevation in [Ca(2+)](c) that is coupled to phagosome-lysosome fusion. We tested the hypothesis that defective activation of the Ca(2+)-dependent effector proteins calmodulin (CaM) and CaM-dependent protein kinase II (CaMKII) contributes to the intracellular pathogenesis of tuberculosis. Phagosomes containing live M. tuberculosis exhibited decreased levels of CaM and the activated form of CaMKII compared with phagosomes encompassing killed tubercle bacilli. Furthermore, ionophore-induced elevations in [Ca(2+)](c) resulted in recruitment of CaM and activation of CaMKII on phagosomes containing live M. tuberculosis. Specific inhibitors of CaM or CaMKII blocked Ca(2+) ionophore-induced phagosomal maturation and enhanced the bacilli's intracellular viability. These results demonstrate a novel role for CaM and CaMKII in the regulation of phagosome-lysosome fusion and suggest that defective activation of these Ca(2+)-activated signaling components contributes to the successful parasitism of human macrophages by M. tuberculosis.

Biochemical characterization of Rab3-GTPase-activating protein reveals a mechanism similar to that of Ras-GAP

Small G proteins of the Rab family are regulators of intracellular vesicle traffic. Their intrinsic rate of GTP hydrolysis is very low but is enhanced by specific GTPase-activating proteins (GAPs) that switch G proteins to their inactive form. We have characterized the activity of recombinant Rab3-GAP on Rab3A in solution. The K(m) and K(d) values (75 microm) indicate a low affinity of Rab3-GAP for its substrate. The affinity is higher for the transition state analog Rab3A:GDP:AlF(x) (15 microm). The k(cat) (1 s(-)(1)) is within the range of values reported for other GAPs. A mutation in the switch I region of Rab3A disrupted the interaction with Rab3-GAP. Furthermore, Rabphilin, a putative target of Rab3, inhibited the activity of Rab3-GAP on Rab3. Therefore, the Rab3-GAP-binding site involves the switch I region of Rab3 and overlaps with the Rabphilin-binding domain. Substitution of a single arginine residue (Arg-728) of Rab3-GAP disrupted its catalytic activity but not its interaction with Rab3A. We propose that Rab3-GAP, like Ras- and Rho-GAPs, stabilizes the transition state of Rab3 and provides a critical arginine residue to accelerate the GTPase reaction.

Beta-granule transport and exocytosis

Regulated beta -granule exocytosis is critical for the ability of the beta -cell to finely control body glucose homeostasis. This is now understood to be a multistage process whereby beta -granules are transported from biosynthetic/storage sites in the cell cytoplasm and targeted to specific regions of the plasma membrane. Exocytosis is achieved when these granules are triggered to fuse with the membrane by an elevated cytosolic Ca(2+). Dramatic advances have been made recently in our understanding of the protein-protein interactions and regulatory signals that govern intracellular transport and fusion. Although best understood for exocytosis from neurons and neuroendocrine cells, similar processes are thought to be conserved in the beta -cell.

The Rab3-interacting molecule RIM is expressed in pancreatic beta-cells and is implicated in insulin exocytosis

The putative Rab3 effector RIM (Rab3-interacting molecule) was detected by Northern blotting, RT-PCR and Western blotting in native pancreatic beta-cells as well as in the derived cell lines INS-1E and HIT-T15. RIM was localized on the plasma membrane of INS-1E cells and beta-cells. An involvement of RIM in insulin exocytosis was indicated by transfection experiments of INS-1E cells with the Rab3 binding domain of RIM. This domain enhanced glucose-stimulated secretion in intact cells and Ca(2+)-stimulated exocytosis in permeabilized cells. Co-expression of Rab3A reversed the effect of RIM on exocytosis. These results suggest an implication of RIM in the control of insulin secretion.

Multiple aspects of Rab protein action in the secretory pathway: focus on Rab3 and Rab6

Rab proteins form the largest branch of the Ras superfamily of GTPases. They are localized to the cytoplasmic face of organelles and vesicles involved in the biosynthetic/secretory and endocytic pathways in eukaryotic cells. It is now well established that Rab proteins play an essential role in the processes that underlie the targeting and fusion of transport vesicles with their appropriate acceptor membranes. They perform this task through interactions with a wide variety of effector molecules. In this review, we illustrate recent advances in the field of Rab GTPases, taking as examples two proteins involved in the biosynthetic pathway, Rab3 and Rab6.

Binding of Rab3A to synaptic vesicles

Prenylated Rab GTPases cycle between membrane-bound and soluble forms. Membrane-bound GDP-Rabs interact with GDP dissociation inhibitor (GDI), resulting in the dissociation of a Rab.GDI complex, which in turn serves as a precursor for the membrane re-association of Rabs. We have now characterized the binding of Rab3A to synaptic vesicles in vitro using either purified complexes or rat brain cytosol as source for GDI.Rab3A. Binding of Rab3A results in the immediate release of GDI from the membrane. Furthermore, binding does not require the presence of additional guanine nucleotides (GDP or GTP) or of cytosolic factors. Although nucleotide exchange follows binding, binding is initially reversible, suggesting that binding of GDP-Rab3A and nucleotide exchange are separate and independent events. Comparison with the binding of Rab1B revealed that both Rab proteins bind preferentially to their respective resident membranes although some promiscuity was observable. Binding is saturable and involves a protease-sensitive binding site that is tightly associated with the vesicle membrane.