Myosin V transports secretory vesicles via a Rab GTPase cascade and interaction with the exocyst complex

The Cdc42 effectors Gic1 and Gic2 regulate polarized post-Golgi secretion

Background

Cell polarity refers to spatial difference in morphology, structure, and function within different parts of a single cell, which plays important roles in a wide range of cellular processes. In eukaryotic cells, the small GTPase Cdc42 and phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P₂) are critical components for cell polarity and required for polarized exocytosis and cell growth. Previous data showed that the GTPase-interacting components, Gic1 and Gic2, control cell polarity through its binding with Cdc42 and PtdIns(4,5)P₂ in the plasma membrane in budding yeast. However, whether the Gic proteins regulate polarized exocytosis is unknown.

Results

In this study, we found that Gic2 co-immunoprecipitates with the exocyst complex, suggesting Gic proteins may be involved in exocytosis. Although we could not show the direct interaction between Gic2 and exocyst, we found gic1Δgic2Δ are synthetically sick with sec3ΔN. We demonstrated that Gic1 and Gic2 are required for polarized exocytosis in a yeast strain harboring the N-terminal domain deletion of Sec3, which is also known as an effector of Cdc42 GTPase. Gic proteins are required for polarized localization of exocyst, growth, and efficient secretion in sec3∆N mutant. In addition, we found that the N-terminal domain of both Gic2 and Sec3 share the similar binding sites of Cdc42. Surprisingly, not all the Sec3/Gic binding deficient cdc42 mutants displayed defects of growth and secretion, indicating that disruption of Cdc42 binding with Gic proteins and Sec3 does not necessarily show secretion defects in cdc42 mutants.

Conclusions

We conclude that Gic1/2 and Sec3 act in parallel to regulate polarized post-Golgi secretion, but this regulation is not solely controlled by their upstream factor Cdc42. Considering that N-terminal domain of Gic2 and Sec3 can bind to both Cdc42 and PtdIns(4,5)P₂, the regulation of Gic protein and Sec3 on polarized secretion may also be controlled by PtdIns(4,5)P₂. Further experiments need to be performed to test this hypothesis. Our findings provide important clues for understanding the molecular mechanism of cell polarity establishment in eukaryotic cells.

Electronic supplementary material

The online version of this article (10.1186/s13578-019-0295-x) contains supplementary material, which is available to authorized users.

Myosins XI Are Involved in Exocytosis of Cellulose Synthase Complexes

In plants, cellulose is synthesized at the cell surface by plasma membrane (PM)-localized cellulose synthase (CESA) complexes (CSCs). The molecular and cellular mechanisms that underpin delivery of CSCs to the PM, however, are poorly understood. Cortical microtubules have been shown to interact with CESA-containing compartments and mark the site for CSC delivery, but are not required for the delivery itself. Here, we demonstrate that myosin XI and the actin cytoskeleton mediate CSC delivery to the PM by coordinating the exocytosis of CESA-containing compartments. Measurement of cellulose content indicated that cellulose biosynthesis was significantly reduced in a myosin xik xi1 xi2 triple-knockout mutant. By combining genetic and pharmacological disruption of myosin activity with quantitative live-cell imaging, we observed decreased abundance of PM-localized CSCs and reduced delivery rate of CSCs in myosin-deficient cells. These phenotypes correlated with a significant increase in failed vesicle secretion events at the PM as well as an abnormal accumulation of CESA-containing compartments at the cell cortex. Through high-resolution spatiotemporal assays of cortical vesicle behavior, we identified defects in CSC vesicle tethering and fusion at the PM. Furthermore, disruption of myosin activity reduced the delivery of several other secretory markers to the PM and reduced constitutive and receptor-mediated endocytosis. These findings reveal a previously undescribed role for myosin in vesicle secretion and cellulose production at the cytoskeleton-PM-cell wall nexus.

Off the wall: The rhyme and reason of Neurospora crassa hyphal morphogenesis

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Chitin and β-1,3-glucan synthases are transported separately in chitosomes and macrovesicles.

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Chitin synthases occupy the core of the SPK; β-1,3-glucan synthases the outer layer.

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CHS-4 arrival to the SPK and septa is CSE-7 dependent.

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Rabs YPT-1 and YPT-31 localization at the SPK mimics that of chitosomes and macrovesicles.

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The exocyst acts as a tether between the SPK outer layer vesicles and the apical PM.

The fungal cell wall building processes are the ultimate determinants of hyphal shape. In Neurospora crassa the main cell wall components, β-1,3-glucan and chitin, are synthesized by enzymes conveyed by specialized vesicles to the hyphal tip. These vesicles follow different secretory routes, which are delicately coordinated by cargo-specific Rab GTPases until their accumulation at the Spitzenkörper. From there, the exocyst mediates the docking of secretory vesicles to the plasma membrane, where they ultimately get fused. Although significant progress has been done on the cellular mechanisms that carry cell wall synthesizing enzymes from the endoplasmic reticulum to hyphal tips, a lot of information is still missing. Here, the current knowledge on N. crassa cell wall composition and biosynthesis is presented with an emphasis on the underlying molecular and cellular secretory processes.

Molecular mechanisms of contractile-ring constriction and membrane trafficking in cytokinesis

In this review, we discuss the molecular mechanisms of cytokinesis from plants to humans, with a focus on contribution of membrane trafficking to cytokinesis. Selection of the division site in fungi, metazoans, and plants is reviewed, as well as the assembly and constriction of a contractile ring in fungi and metazoans. We also provide an introduction to exocytosis and endocytosis, and discuss how they contribute to successful cytokinesis in eukaryotic cells. The conservation in the coordination of membrane deposition and cytoskeleton during cytokinesis in fungi, metazoans, and plants is highlighted.

Exposing the Elusive Exocyst Structure

A major challenge for a molecular understanding of membrane trafficking has been the elucidation of high-resolution structures of large, multisubunit tethering complexes that spatially and temporally control intracellular membrane fusion. Exocyst is a large hetero-octameric protein complex proposed to tether secretory vesicles at the plasma membrane to provide quality control of soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE)-mediated membrane fusion. Breakthroughs in methodologies, including sample preparation, biochemical characterization, fluorescence microscopy, and single-particle cryoelectron microscopy, are providing critical insights into the structure and function of the exocyst. These studies now pose more questions than answers for understanding fundamental functional mechanisms, and they open wide the door for future studies to elucidate interactions with protein and membrane partners, potential conformational changes, and molecular insights into tethering reactions.

A broadly conserved NERD genetically interacts with the exocyst to affect root growth and cell expansion

The exocyst, a conserved, octameric protein complex, helps mediate secretion at the plasma membrane, facilitating specific developmental processes that include control of root meristem size, cell elongation, and tip growth. A genetic screen for second-site enhancers in Arabidopsis identified NEW ENHANCER of ROOT DWARFISM1 (NERD1) as an exocyst interactor. Mutations in NERD1 combined with weak exocyst mutations in SEC8 and EXO70A1 result in a synergistic reduction in root growth. Alone, nerd1 alleles modestly reduce primary root growth, both by shortening the root meristem and by reducing cell elongation, but also result in a slight increase in root hair length, bulging, and rupture. NERD1 was identified molecularly as At3g51050, which encodes a transmembrane protein of unknown function that is broadly conserved throughout the Archaeplastida. A functional NERD1-GFP fusion localizes to the Golgi, in a pattern distinct from the plasma membrane-localized exocyst, arguing against a direct NERD1-exocyst interaction. Structural modeling suggests the majority of the protein is positioned in the lumen, in a β-propeller-like structure that has some similarity to proteins that bind polysaccharides. We suggest that NERD1 interacts with the exocyst indirectly, possibly affecting polysaccharides destined for the cell wall, and influencing cell wall characteristics in a developmentally distinct manner.

Disruption of actin motor function due to MoMyo5 mutation impairs host penetration and pathogenicity in Magnaporthe oryzae

Actin motor myosin proteins are the driving forces behind the active transport of vesicles, and more than 20 classes of myosin have been found to contribute to a wide range of cellular processes, including endocytosis and exocytosis, autophagy, cytokinesis and the actin cytoskeleton. In Saccharomyces cerevisiae, class V myosin Myo2 (ScMyo2p) is important for the transport of distinct sets of cargo to regions of the cell along the cytoskeleton for polarized growth. To study whether myosins play a role in the formation or function of the appressorium (infectious structure) of the rice blast fungus Magnaporthe oryzae, we identified MoMyo5 as an orthologue of ScMyo2p and characterized its function. Targeted gene disruption revealed that MoMyo5 is required for intracellular transport and is essential for hyphal growth and asexual reproduction. Although the ΔMomyo5 mutant could form appressorium-like structures, the structures were unable to penetrate host cells and were therefore non-pathogenic. We further found that MoMyo5 moves dynamically from the cytoplasm to the hyphal tip, where it interacts with MoSec4, a Rab GTPase involved in secretory transport, hyphal growth and fungal pathogenicity. Our studies indicate that class V myosin and its translocation are tightly coupled with hyphal growth, asexual reproduction, appressorium function and pathogenicity in the rice blast fungus.

Regulation of Cell Polarity by Exocyst-Mediated Trafficking

One requirement for establishing polarity within a cell is the asymmetric trafficking of intracellular vesicles to the plasma membrane. This tightly regulated process creates spatial and temporal differences in both plasma membrane composition and the membrane-associated proteome. Asymmetric membrane trafficking is also a critical mechanism to regulate cell differentiation, signaling, and physiology. Many eukaryotic cell types use the eight-protein exocyst complex to orchestrate polarized vesicle trafficking to certain membrane locales. Members of the exocyst were originally discovered in yeast while screening for proteins required for the delivery of secretory vesicles to the budding daughter cell. The same eight exocyst genes are conserved in mammals, in which the specifics of exocyst-mediated trafficking are highly cell-type-dependent. Some exocyst members bind to certain Rab GTPases on intracellular vesicles, whereas others localize to the plasma membrane at the site of exocytosis. Assembly of the exocyst holocomplex is responsible for tethering these vesicles to the plasma membrane before their soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE)-mediated exocytosis. In this review, we will focus on the role and regulation of the exocyst complex in targeted vesicular trafficking as related to the establishment and maintenance of cellular polarity. We will contrast exocyst function in apicobasal epithelial polarity versus front-back mesenchymal polarity, and the dynamic regulation of exocyst-mediated trafficking during cell phenotype transitions.

Regulation of class V myosin

Class V myosin (myosin-5) is a molecular motor that functions as an organelle transporter. The activation of myosin-5's motor function has long been known to be associated with a transition from the folded conformation in the off-state to the extended conformation in the on-state, but only recently have we begun to understand the underlying mechanism. The globular tail domain (GTD) of myosin-5 has been identified as the inhibitory domain and has recently been shown to function as a dimer in regulating the motor function. The folded off-state of myosin-5 is stabilized by multiple intramolecular interactions, including head-GTD interactions, GTD-GTD interactions, and interactions between the GTD and the C-terminus of the first coiled-coil segment. Any cellular factor that affects these intramolecular interactions and thus the stability of the folded conformation of myosin-5 would be expected to regulate myosin-5 motor function. Both the adaptor proteins of myosin-5 and Ca2+ are potential regulators of myosin-5 motor function, because they can destabilize its folded conformation. A combination of these regulators provides a versatile scheme in regulating myosin-5 motor function in the cell.

Polarized Exocytosis

Polarized exocytosis is generally considered as the multistep vesicular trafficking process in which membrane-bounded carriers are transported from the Golgi or endosomal compartments to specific sites of the plasma membrane. Polarized exocytosis in cells is achieved through the coordinated actions of membrane trafficking machinery and cytoskeleton orchestrated by signaling molecules such as the Rho family of small GTPases. Elucidating the molecular mechanisms of polarized exocytosis is essential to our understanding of a wide range of pathophysiological processes from neuronal development to tumor invasion.

The exocyst subunit Sec3 is regulated by a protein quality control pathway

Exocytosis involves fusion of secretory vesicles with the plasma membrane, thereby delivering membrane proteins to the cell surface and releasing material into the extracellular space. The tethering of the secretory vesicles before membrane fusion is mediated by the exocyst, an essential phylogenetically conserved octameric protein complex. Exocyst biogenesis is regulated by several processes, but the mechanisms by which the exocyst is degraded are unknown. Here, to unravel the components of the exocyst degradation pathway, we screened for extragenic suppressors of a temperature-sensitive fission yeast strain mutated in the exocyst subunit Sec3 (sec3-913). One of the suppressing DNAs encoded a truncated dominant-negative variant of the 26S proteasome subunit, Rpt2, indicating that exocyst degradation is controlled by the ubiquitin-proteasome system. The temperature-dependent growth defect of the sec3-913 strain was gene dosage-dependent and suppressed by blocking the proteasome, Hsp70-type molecular chaperones, the Pib1 E3 ubiquitin-protein ligase, and the deubiquitylating enzyme Ubp3. Moreover, defects in cell septation, exocytosis, and endocytosis in sec3 mutant strains were similarly alleviated by mutation of components in this pathway. We also found that, particularly under stress conditions, wild-type Sec3 degradation is regulated by Pib1 and the 26S proteasome. In conclusion, our results suggest that a cytosolic protein quality control pathway monitors folding and proteasome-dependent turnover of an exocyst subunit and, thereby, controls exocytosis in fission yeast.

Hitchhiking vesicular transport routes to the vacuole: Amyloid recruitment to the Insoluble Protein Deposit (IPOD)

Sequestration of aggregates into specialized deposition sites occurs in many species across all kingdoms of life ranging from bacteria to mammals and is commonly believed to have a cytoprotective function. Yeast cells possess at least 3 different spatially separated deposition sites, one of which is termed “Insoluble Protein Deposit (IPOD)” and harbors amyloid aggregates. We have recently discovered that recruitment of amyloid aggregates to the IPOD uses an actin cable based recruitment machinery that also involves vesicular transport.¹ Here we discuss how different proteins known to be involved in vesicular transport processes to the vacuole might act to guide amyloid aggregates to the IPOD. These factors include the Myosin V motor protein Myo2 involved in transporting vacuolar vesicles along actin cables, the transmembrane protein Atg9 involved in the recruitment of large precursor hydrolase complexes to the vacuole, the phosphatidylinositol/ phosphatidylcholine (PI/PC) transfer protein Sec 14 and the SNARE chaperone Sec 18. Furthermore, we present new data suggesting that the yeast dynamin homolog Vps1 is also crucial for faithful delivery of the amyloid model protein PrD-GFP to the IPOD. This is in agreement with a previously identified role for Vps1 in recruitment of heat-denatured aggregates to a perivacuolar deposition site.²

Crystal structure of Sec10, a subunit of the exocyst complex

The exocyst complex is a heterooctameric protein complex composed of Sec3, Sec5, Sec6, Sec8, Sec10, Sec15, Exo70 and Exo84. This complex plays an essential role in trafficking secretory vesicles to the plasma membrane through its interaction with phosphatidylinositol 4,5-bisphosphate and small GTPases. To date, the near-full-length structural information of each subunit has been limited to Exo70, although the C-terminal half structures of Sec6, Sec15 and Exo84 and the structures of the small GTPase-binding domains of Sec3, Sec5 and Exo84 have been reported. Here, we report the crystal structure of the near-full-length zebrafish Sec10 (zSec10) at 2.73 Å resolution. The structure of zSec10 consists of tandem antiparallel helix bundles that form a straight rod, like helical core regions of other exocyst subunits. This structure provides the first atomic details of Sec10, which may be useful for future functional and structural studies of this subunit and the exocyst complex.

The Role of the Cytoskeleton and Myosin-Vc in the Targeting of KCa3.1 to the Basolateral Membrane of Polarized Epithelial Cells

Understanding the targeting of KCa3.1 to the basolateral membrane (BLM) of polarized epithelial cells is still emerging. Here, we examined the role of the cytoskeleton (microtubules and microfilaments) and Myosin-Vc (Myo-Vc) in the targeting of KCa3.1 in Fischer rat thyroid epithelial cells. We used a pharmacological approach with immunoblot (for the BLM expression of KCa3.1), Ussing chamber (functional BLM expression of KCa3.1) and siRNA experiments. The actin cytoskeleton inhibitors cytochalasin D (10 μM, 5 h) and latrunculin A (10 μM, 5 h) reduced the targeting of KCa3.1 to the BLM by 88 ± 4 and 70 ± 5%, respectively. Colchicine (10 μM, 5 h) a microtubule inhibitor reduced targeting of KCa3.1 to the BLM by 63 ± 7% and decreased 1-EBIO-stimulated KCa3.1 K⁺ current by 46 ± 18%, compared with control cells. ML9 (10 μM, 5 h), an inhibitor of myosin light chain kinase, decreased targeting of the channel by 83 ± 2% and reduced K⁺ current by 54 ± 8% compared to control cells. Inhibiting Myo-V with 2,3-butanedione monoxime (10 mM, 5 h) reduced targeting of the channel to the BLM by 58 ± 5% and decreased the stimulated current of KCa3.1 by 48 ± 12% compared with control cells. Finally, using siRNA for Myo-Vc, we demonstrated that knockdown of Myo-Vc reduced the BLM expression of KCa3.1 by 44 ± 7% and KCa3.1 K⁺ current by 1.04 ± 0.14 μA compared with control cells. These data suggest that the microtubule and microfilament cytoskeleton and Myo-Vc are critical for the targeting of KCa3.1.

Testis-Specific GTPase (TSG): An oligomeric protein

BACKGROUND

Ras-related proteins in brain (Rab)-family proteins are key members of the membrane trafficking pathway in cells. In addition, these proteins have been identified to have diverse functions such as cross-talking with different kinases and playing a role in cellular signaling. However, only a few Rab proteins have been found to have a role in male germ cell development. The most notable functions of this process are performed by numerous testis-specific and/or germ cell-specific genes. Here, we describe a new Rab protein that is specifically expressed in male germ cells, having GTPase activity.

RESULTS

Testis-specific GTPase (TSG) is a male-specific protein that is highly expressed in the testis. It has an ORF of 1593 base pairs encoding a protein of 530 amino acids. This protein appears in testicular cells approximately 24 days postpartum and is maintained thereafter. Immunohistochemistry of testicular sections indicates localized expression in germ cells, particularly elongating spermatids. TSG has a bipartite nuclear localization signal that targets the protein to the nucleus. The C-terminal region of TSG contains the characteristic domain of small Rab GTPases, which imparts GTPase activity. At the N-terminal region, it has a coiled-coil motif that confers self-interaction properties to the protein and allows it to appear as an oligomer in the testis.

CONCLUSION

TSG, being expressed in the male gonad in a developmental stage-specific manner, may have a role in male germ cell development. Further investigation of TSG function in vivo may provide new clues for uncovering the secrets of spermatogenesis.

Phosphoinositides in Control of Membrane Dynamics

Most functions of eukaryotic cells are controlled by cellular membranes, which are not static entities but undergo frequent budding, fission, fusion, and sculpting reactions collectively referred to as membrane dynamics. Consequently, regulation of membrane dynamics is crucial for cellular functions. A key mechanism in such regulation is the reversible recruitment of cytosolic proteins or protein complexes to specific membranes at specific time points. To a large extent this recruitment is orchestrated by phosphorylated derivatives of the membrane lipid phosphatidylinositol, known as phosphoinositides. The seven phosphoinositides found in nature localize to distinct membrane domains and recruit distinct effectors, thereby contributing strongly to the maintenance of membrane identity. Many of the phosphoinositide effectors are proteins that control membrane dynamics, and in this review we discuss the functions of phosphoinositides in membrane dynamics during exocytosis, endocytosis, autophagy, cell division, cell migration, and epithelial cell polarity, with emphasis on protein effectors that are recruited by specific phosphoinositides during these processes.

Cingulin and actin mediate midbody-dependent apical lumen formation during polarization of epithelial cells

Coordinated polarization of epithelial cells is a key step during morphogenesis that leads to the formation of an apical lumen. Rab11 and its interacting protein FIP5 are necessary for the targeting of apical endosomes to the midbody and apical membrane initiation site (AMIS) during lumenogenesis. However, the machinery that mediates AMIS establishment and FIP5-endosome targeting remains unknown. Here we identify a FIP5-interacting protein, Cingulin, which localizes to the AMIS and functions as a tether mediating FIP5-endosome targeting. We analysed the machinery mediating AMIS recruitment to the midbody and determined that both branched actin and microtubules are required for establishing the site of the nascent lumen. We demonstrate that the Rac1-WAVE/Scar complex mediates Cingulin recruitment to the AMIS by inducing branched actin formation, and that Cingulin directly binds to microtubule C-terminal tails through electrostatic interactions. We propose a new mechanism for apical endosome targeting and AMIS formation around the midbody during epithelial lumenogenesis.

Polarisation of epithelial cells causes lumen formation, which is mediated by apical membrane initiation site (AMIS) and FIP5, but how this is regulated is unclear. Here, the authors identify cingulin as a FIP-5 interacting protein, recruiting the Rac1-WAVE/Scar complex to the AMIS and branched actin formation.

Regulation of membrane traffic by Rab GEF and GAP cascades

ASBTRACT Rab GTPases serve as master regulators of membrane traffic, each typically controlling several different aspects of a specific stage of membrane traffic by recruiting diverse effector proteins such as cytoskeletal motors, vesicle tethering proteins and regulators of SNARE complex assembly. Rabs, in turn, are regulated by specific guanine nucleotide exchange factors (GEFs), which catalyze the displacement of GDP and binding of GTP, as well as GTPase activating proteins (GAPs) that stimulate the slow intrinsic rate of GTP hydrolysis. Here I review our studies on the final stages of the yeast secretory pathway that have led us to propose that adjacent Rabs on a pathway are networked to one another through their regulators; specifically we have shown that the Rab, Ypt32, in its GTP-bound form recruits both Sec2, the GEF that activates the downstream Rab, Sec4, as well as Gyp1, the GAP that inactivates the upstream Rab, Ypt1. The postulated effect of these counter-current cascades is a programmed series of abrupt Rab transitions that lead to critical changes in the functional identity of the membrane as it flows along the exocytic pathway. Phosphoinositides also play key roles in the temporal and spatial regulation of membrane traffic. The Golgi pool of phosphatidylinositol 4-phosphate (PI(4)P) works in concert with Ypt32 to initially recruit Sec2, yet a subsequent drop in PI(4)P levels directs a regulatory switch in Sec2 function in which it binds to the Sec4 effector Sec15 generating a positive feedback loop. PI(4)P distribution together with Sec2 phosphorylation by a casein kinase determines when and where each regulatory circuit is used.

Kinesin-related Smy1 enhances the Rab-dependent association of myosin-V with secretory cargo

Smy1 is a kinesin-related protein that enhances the association of the Myo2 myosin-V motor with its receptor, the Rab Sec4, on secretory vesicles. This function requires Smy1’s head, coiled-coil, and tail domains and is specific for secretory vesicle transport but not for mitochondrial segregation by Myo2, which also uses a Rab protein, Ypt11.

The mechanisms by which molecular motors associate with specific cargo is a central problem in cell organization. The kinesin-like protein Smy1 of budding yeast was originally identified by the ability of elevated levels to suppress a conditional myosin-V mutation (myo2-66), but its function with Myo2 remained mysterious. Subsequently, Myo2 was found to provide an essential role in delivery of secretory vesicles for polarized growth and in the transport of mitochondria for segregation. By isolating and characterizing myo2 smy1 conditional mutants, we uncover the molecular function of Smy1 as a factor that enhances the association of Myo2 with its receptor, the Rab Sec4, on secretory vesicles. The tail of Smy1—which binds Myo2—its central dimerization domain, and its kinesin-like head domain are all necessary for this function. Consistent with this model, overexpression of full-length Smy1 enhances the number of Sec4 receptors and Myo2 motors per transporting secretory vesicle. Rab proteins Sec4 and Ypt11, receptors for essential transport of secretory vesicles and mitochondria, respectively, bind the same region on Myo2, yet Smy1 functions selectively in the transport of secretory vesicles. Thus a kinesin-related protein can function intimately with a myosin-V and its receptor in the transport of a specific cargo.

EHBP1L1 coordinates Rab8 and Bin1 to regulate apical-directed transport in polarized epithelial cells

The highly conserved Rab guanosine triphosphatase (GTPase) Rab8 plays a role in exocytosis toward the polarized plasma membrane in eukaryotic cells. In murine Rab8-deficient small intestine cells, apical proteins are missorted into lysosomes. In this study, we identified a novel Rab8-interacting protein complex containing an EH domain-binding protein 1-like 1 (EHBP1L1), Bin1/amphiphysin II, and dynamin. Biochemical analyses showed that EHBP1L1 directly bound to GTP-loaded Rab8 and Bin1. The spatial dependency of these complexes at the endocytic recycling compartment (ERC) was demonstrated through overexpression and knockdown experiments. EHBP1L1- or Bin1-depleted or dynamin-inhibited small intestine organoids significantly accumulated apical membrane proteins but not basolateral membrane proteins in lysosomes. Furthermore, in EHBP1L1-deficient mice, small intestine cells displayed truncated and sparse microvilli, suggesting that EHBP1L1 maintains the apical plasma membrane by regulating apical transport. In summary, our data demonstrate that EHBP1L1 links Rab8 and the Bin1-dynamin complex, which generates membrane curvature and excises the vesicle at the ERC for apical transport.

Probing Functional Changes in Exocyst Configuration with Monoclonal Antibodies

Spatial regulation of exocytosis relies on the exocyst, a hetero-octameric protein complex that tethers vesicles to fusion sites at the plasma membrane. Nevertheless, our understanding of mechanisms regulating exocyst assembly/disassembly, localization, and function are incomplete. Here, we have exploited a panel of anti-Sec6 monoclonal antibodies (mAbs) to probe possible configurational changes accompanying transitions in exocyst function in epithelial MDCK cells. Sec6 is quantitatively associated with Sec8 in high molecular weight complexes, as shown by gel filtration and co-immunoprecipitation studies. We mapped epitopes recognized by more than 20 distinct mAbs to one of six Sec6 segments. Surprisingly, mAbs that bound epitopes in each segment labeled distinct subcellular structures. In general, antibodies to epitopes in N-terminal domains labeled Sec6 in either cytosolic or nuclear pools, whereas those that bound epitopes in C-terminal domains labeled membrane-associated Sec6. In this latter group, we identified antibodies that labeled distinct Sec6 populations at the apical junctional complex, desmosomes, endoplasmic reticulum and vimentin-type intermediate filaments. That each antibody was specific was verified by both Sec6 RNAi and competition with fusion proteins containing each domain. Comparison of non-polarized and polarized cells revealed that many Sec6 epitopes either redistribute or become concealed during epithelial polarization. Transitions in exocyst configurations may be regulated in part by the actions of Ral GTPases, because the exposure of Sec6 C-terminal domain epitopes at the plasma membrane is significantly reduced upon RalA RNAi. To determine whether spatio-temporal changes in epitope accessibility was correlated with differential stability of interactions between Sec6 and other exocyst subunits, we quantified relative amounts of each subunit that co-immunoprecipitated with Sec6 when antibodies to N-terminal or C-terminal epitopes were used. Antibodies to Sec6NT co-precipitated substantially more Sec5, -10, -15, Exo70 and -84 than did those to Sec6CT. In contrast, antibodies to Sec6CT co-precipitated more Sec3 and Sec8 than did those to Sec6NT. These results are consistent with a model in which exocyst activation during periods of rapid membrane expansion is accompanied by molecular rearrangements within the holocomplex or association with accessory proteins, which expose the Sec6 C-terminal domain when the complex is membrane-bound and conceal it when the complex is cytoplasmic.

Sec15 links bud site selection to polarised cell growth and exocytosis in Candida albicans

The exocyst plays a crucial role in the targeting of secretory vesicles to the plasma membrane during exocytosis. It has been shown to be involved in diverse cellular processes including yeast budding. However, the mechanism of the exocyst regulating yeast budding has not been fully elucidated. Here we report a novel interaction between the exocyst component Sec15 and the Ras-family GTPase Rsr1, a master regulator of bud-site-selection system, in the fungus Candida albicans. We present several lines of evidence indicating physical and genetic interaction of Sec15 with Rsr1. In vitro binding assays and co-immunoprecipitation studies showed that Sec15 associated physically with Rsr1. Deletion of RSR1 completely abolished the polarised localisation of Sec15 as well as all the other exocyst components in both yeast and hyphal cells, suggesting a functional interaction between Sec15 and Rsr1. We also show that C. albicans Sec15 interacts directly with the polarity determinant Bem1 and the type V myosin, Myo2. Disruption of the interaction by shutting off SEC15 results in mislocaliztion of Bem1-GFP. These findings highlight the important role of Sec15 in polarised cell growth by providing a direct functional link between bud-site-selection and exocytosis.

Roles of the TRAPP-II Complex and the Exocyst in Membrane Deposition during Fission Yeast Cytokinesis

The cleavage-furrow tip adjacent to the actomyosin contractile ring is believed to be the predominant site for plasma-membrane insertion through exocyst-tethered vesicles during cytokinesis. Here we found that most secretory vesicles are delivered by myosin-V on linear actin cables in fission yeast cytokinesis. Surprisingly, by tracking individual exocytic and endocytic events, we found that vesicles with new membrane are deposited to the cleavage furrow relatively evenly during contractile-ring constriction, but the rim of the cleavage furrow is the main site for endocytosis. Fusion of vesicles with the plasma membrane requires vesicle tethers. Our data suggest that the transport particle protein II (TRAPP-II) complex and Rab11 GTPase Ypt3 help to tether secretory vesicles or tubulovesicular structures along the cleavage furrow while the exocyst tethers vesicles at the rim of the division plane. We conclude that the exocyst and TRAPP-II complex have distinct localizations at the division site, but both are important for membrane expansion and exocytosis during cytokinesis.

Two putative vesicle tethers—the exocyst and TRAPP-II complexes—localize differently at the division plane to ensure efficient plasma-membrane deposition along the whole cleavage furrow during cytokinesis in the fission yeast Schizosaccharomyces pombe.

Cytokinesis partitions a mother cell into two daughter cells at the end of each cell-division cycle. A significant amount of new plasma membrane is needed at the cleavage furrow during cytokinesis in many cell types. Membrane expansion is achieved through the balance of exocytosis and endocytosis. It is poorly understood where and when the membrane is deposited and retrieved during cytokinesis. By tracking individual vesicles with high spatiotemporal resolution and using electron microscopy, we found that new membrane is deposited relatively evenly along the cleavage furrow in fission yeast, while the rim of the division plane is the predominant site for endocytosis. The secretory vesicles/compartments carrying new membrane are mainly delivered along formin-nucleated actin cables by myosin-V motors. Surprisingly, we find that both exocytosis and endocytosis at the division site are ramped up before contractile-ring constriction and last until daughter-cell separation. We discovered that two putative vesicle tethers, the exocyst and TRAPP-II complexes, localize to different sites at the cleavage furrow to promote tethering of different, yet overlapping, classes of secretory vesicles/compartments for exocytosis and new membrane deposition.

The Exocyst Complex in Health and Disease

Exocytosis involves the fusion of intracellular secretory vesicles with the plasma membrane, thereby delivering integral membrane proteins to the cell surface and releasing material into the extracellular space. Importantly, exocytosis also provides a source of lipid moieties for membrane extension. The tethering of the secretory vesicle before docking and fusion with the plasma membrane is mediated by the exocyst complex, an evolutionary conserved octameric complex of proteins. Recent findings indicate that the exocyst complex also takes part in other intra-cellular processes besides secretion. These various functions seem to converge toward defining a direction of membrane growth in a range of systems from fungi to plants and from neurons to cilia. In this review we summarize the current knowledge of exocyst function in cell polarity, signaling and cell-cell communication and discuss implications for plant and animal health and disease.

Cargo-selective apical exocytosis in epithelial cells is conducted by Myo5B, Slp4a, Vamp7, and Syntaxin 3

The motor protein Myo5B and t-SNARE Stx3 drive cargo-selective apical exocytosis in polarized epithelial cells in a pathway dependent on v-SNARE–like Slp4a, v-SNARE Vamp7, Sec1/Munc18-like protein Munc18-2, and the Rab11/8 cascade.

Mutations in the motor protein Myosin Vb (Myo5B) or the soluble NSF attachment protein receptor Syntaxin 3 (Stx3) disturb epithelial polarity and cause microvillus inclusion disease (MVID), a lethal hereditary enteropathy affecting neonates. To understand the molecular mechanism of Myo5B and Stx3 interplay, we used genome editing to introduce a defined Myo5B patient mutation in a human epithelial cell line. Our results demonstrate a selective role of Myo5B and Stx3 for apical cargo exocytosis in polarized epithelial cells. Apical exocytosis of NHE3, CFTR (cystic fibrosis transmembrane conductance regulator), and GLUT5 required an interaction cascade of Rab11, Myo5B, Slp4a, Munc18-2, and Vamp7 with Stx3, which cooperate in the final steps of this selective apical traffic pathway. The brush border enzymes DPPIV and sucrase-isomaltase still correctly localize at the apical plasma membrane independent of this pathway. Hence, our work demonstrates how Myo5B, Stx3, Slp4a, Vamp7, Munc18-2, and Rab8/11 cooperate during selective apical cargo trafficking and exocytosis in epithelial cells and thereby provides further insight into MVID pathophysiology.

Subunit connectivity, assembly determinants and architecture of the yeast exocyst complex

The exocyst is a hetero-octameric complex that has been proposed to serve as the tethering complex for exocytosis, although it remains poorly understood at the molecular level. Here, we purified endogenous exocyst complexes from Saccharomyces cerevisiae and showed that they are stable and consist of all eight subunits with equal stoichiometry. Using a combination of biochemical and auxin induced-degradation experiments in yeast, we mapped the subunit connectivity, identified two stable four-subunit modules within the octamer and demonstrated that several known exocyst-binding partners are not necessary for exocyst assembly and stability. Furthermore, we visualized the structure of the yeast complex by using negative-stain electron microscopy; our results indicate that the exocyst exists predominantly as a stable, octameric complex with an elongated architecture that suggests that the subunits are contiguous helical bundles packed together into a bundle of long rods.

The casein kinases Yck1p and Yck2p act in the secretory pathway, in part, by regulating the Rab exchange factor Sec2p

Sec2p is phosphorylated by the redundant casein kinases Yck1p and Yck2p. This promotes the interaction of Sec2p with the downstream effector, Sec15p, and contributes to Sec2p localization and function. Phosphorylation requires prior association of Sec2p with vesicles and reduction of the inhibitory Golgi lipid PI(4)P from the vesicle membrane.

Sec2p is a guanine nucleotide exchange factor that activates Sec4p, the final Rab GTPase of the yeast secretory pathway. Sec2p is recruited to secretory vesicles by the upstream Rab Ypt32p acting in concert with phosphatidylinositol-4-phosphate (PI(4)P). Sec2p also binds to the Sec4p effector Sec15p, yet Ypt32p and Sec15p compete against each other for binding to Sec2p. We report here that the redundant casein kinases Yck1p and Yck2p phosphorylate sites within the Ypt32p/Sec15p binding region and in doing so promote binding to Sec15p and inhibit binding to Ypt32p. We show that Yck2p binds to the autoinhibitory domain of Sec2p, adjacent to the PI(4)P binding site, and that addition of PI(4)P inhibits Sec2p phosphorylation by Yck2p. Loss of Yck1p and Yck2p function leads to accumulation of an intracellular pool of the secreted glucanase Bgl2p, as well as to accumulation of Golgi-related structures in the cytoplasm. We propose that Sec2p is phosphorylated after it has been recruited to secretory vesicles and the level of PI(4)P has been reduced. This promotes Sec2p function by stimulating its interaction with Sec15p. Finally, Sec2p is dephosphorylated very late in the exocytic reaction to facilitate recycling.

Sharing with your children: Mechanisms of peroxisome inheritance

Organelle inheritance is the process by which eukaryotic cells actively replicate and equitably partition their organelles between mother cell and daughter cell at cytokinesis to maintain the benefits of subcellular compartmentalization. The budding yeast Saccharomyces cerevisiae has proven invaluable in helping to define the factors involved in the inheritance of different organelles and in understanding how these factors act and interact to maintain balance in the organelle populations of actively dividing cells. Inheritance factors can be classified as motors that transport organelles, tethers that retain organelles, and connectors (receptors) that mediate the attachment of organelles to motors and anchors. This article will review how peroxisomes are inherited by cells, with a focus on budding yeast, and will discuss common themes and mechanisms of action that underlie the inheritance of all membrane-enclosed organelles.

Glut4 Is Sorted from a Rab10 GTPase-independent Constitutive Recycling Pathway into a Highly Insulin-responsive Rab10 GTPase-dependent Sequestration Pathway after Adipocyte Differentiation

The RabGAP AS160/TBC1D4 controls exocytosis of the insulin-sensitive glucose transporter Glut4 in adipocytes. Glut4 is internalized and recycled through a highly regulated secretory pathway in these cells. Glut4 also cycles through a slow constitutive endosomal pathway distinct from the fast transferrin (Tf) receptor recycling pathway. This slow constitutive pathway is the only Glut4 cycling pathway in undifferentiated fibroblasts. The α2-macroglobulin receptor LRP1 cycles with Glut4 and the Tf receptor through all three exocytic pathways. To further characterize these pathways, the effects of knockdown of AS160 substrates on the trafficking kinetics of Glut4, LRP1, and the Tf receptor were measured in adipocytes and fibroblasts. Rab10 knockdown decreased cell surface Glut4 in insulin-stimulated adipocytes by 65%, but not in basal adipocytes or in fibroblasts. This decrease was due primarily to a 62% decrease in the rate constant of Glut4 exocytosis (kex), although Rab10 knockdown also caused a 1.4-fold increase in the rate constant of Glut4 endocytosis (ken). Rab10 knockdown in adipocytes also decreased cell surface LRP1 by 30% by decreasing kex 30-40%. There was no effect on LRP1 trafficking in fibroblasts or on Tf receptor trafficking in either cell type. These data confirm that Rab10 is an AS160 substrate that limits exocytosis through the highly insulin-responsive specialized secretory pathway in adipocytes. They further show that the slow constitutive endosomal (fibroblast) recycling pathway is Rab10-independent. Thus, Rab10 is a marker for the specialized pathway in adipocytes. Interestingly, mathematical modeling shows that Glut4 traffics predominantly through the specialized Rab10-dependent pathway both before and after insulin stimulation.

Motors, anchors, and connectors: orchestrators of organelle inheritance

Organelle inheritance is a process whereby organelles are actively distributed between dividing cells at cytokinesis. Much valuable insight into the molecular mechanisms of organelle inheritance has come from the analysis of asymmetrically dividing cells, which transport a portion of their organelles to the bud while retaining another portion in the mother cell. Common principles apply to the inheritance of all organelles, although individual organelles use specific factors for their partitioning. Inheritance factors can be classified as motors, which are required for organelle transport; anchors, which immobilize organelles at distinct cell structures; or connectors, which mediate the attachment of organelles to motors and anchors. Here, we provide an overview of recent advances in the field of organelle inheritance and highlight how motor, anchor, and connector molecules choreograph the segregation of a multicopy organelle, the peroxisome. We also discuss the role of organelle population control in the generation of cellular diversity.

Myosin-Powered Membrane Compartment Drives Cytoplasmic Streaming, Cell Expansion and Plant Development

Using genetic approaches, particle image velocimetry and an inert tracer of cytoplasmic streaming, we have made a mechanistic connection between the motor proteins (myosins XI), cargo transported by these motors (distinct endomembrane compartment defined by membrane-anchored MyoB receptors) and the process of cytoplasmic streaming in plant cells. It is shown that the MyoB compartment in Nicotiana benthamiana is highly dynamic moving with the mean velocity of ~3 μm/sec. In contrast, Golgi, mitochondria, peroxisomes, carrier vesicles and a cytosol flow tracer share distinct velocity profile with mean velocities of 0.6-1.5 μm/sec. Dominant negative inhibition of the myosins XI or MyoB receptors using overexpression of the N. benthamiana myosin cargo-binding domain or MyoB myosin-binding domain, respectively, resulted in velocity reduction for not only the MyoB compartment, but also each of the tested organelles, vesicles and cytoplasmic streaming. Furthermore, the extents of this reduction were similar for each of these compartments suggesting that MyoB compartment plays primary role in cytosol dynamics. Using gene knockout analysis in Arabidopsis thaliana, it is demonstrated that inactivation of MyoB1-4 results in reduced velocity of mitochondria implying slower cytoplasmic streaming. It is also shown that myosins XI and MyoB receptors genetically interact to contribute to cell expansion, plant growth, morphogenesis and proper onset of flowering. These results support a model according to which myosin-dependent, MyoB receptor-mediated transport of a specialized membrane compartment that is conserved in all land plants drives cytoplasmic streaming that carries organelles and vesicles and facilitates cell growth and plant development.

The vacuole/lysosome is required for cell-cycle progression

Organelles are distributed to daughter cells, via inheritance pathways. However, it is unclear whether there are mechanisms beyond inheritance, which ensure that organelles are present in all cells. Here we present the unexpected finding that the yeast vacuole plays a positive essential role in initiation of the cell-cycle. When inheritance fails, a new vacuole is generated. We show that this occurs prior to the next cell-cycle, and gain insight into this alternative pathway. Moreover, we find that a combination of a defect in inheritance with an acute block in the vacuole biogenesis results in the loss of a functional vacuole and a specific arrest of cells in early G1 phase. Furthermore, this role for the vacuole in cell-cycle progression requires an intact TORC1-SCH9 pathway that can only signal from a mature vacuole. These mechanisms may serve as a checkpoint for the presence of the vacuole/lysosome.

DOI:http://dx.doi.org/10.7554/eLife.08160.001

Animals, fungi and other eukaryotes have cells that are divided into sub-compartments that are called organelles. Each type of organelle serves a specific purpose that is essential for the life of the cell. Yeast cells have a large organelle called a vacuole; the inside of the vacuole is acidic and contains enzymes that can break down other molecules.

Previous studies have shown that when a budding yeast cell buds to produce a new daughter cell, a process ensures that some of the mother's vacuole is transferred to its daughter. However, yeast mutants that fail to inherit some of their mother's vacuole can still survive. This is because an ‘alternative’ mechanism allows the newly forming daughter to generate its own vacuole from scratch.

Jin and Weisman now unexpectedly show that a new daughter cell cannot become a mother cell until its new vacuole is formed. The experiments made use of yeast mutants that were defective in the ‘inheritance’ mechanism, and double mutants that were defective in both the inheritance and alternative mechanisms. The experiments also revealed that a signal from the vacuole is required before the yeast cell's nucleus can start the cycle of events that lead to the cell dividing. Jin and Weisman suggest that this newly identified communication between the vacuole and the nucleus may help to ensure that critical organelles are present in all cells.

Though it remains unclear why the yeast vacuole is critical for a cell to divide, these findings suggest that the mammalian lysosome (which is similar to the yeast vacuole) may perform a similar critical role in mammals. If this is the case, then understanding how these organelles communicate with the nucleus may provide new insights into how to prevent the uncontrolled growth of tumors and cancer.

DOI:http://dx.doi.org/10.7554/eLife.08160.002

The Exocyst at a Glance

The exocyst is an octameric protein complex that is implicated in the tethering of secretory vesicles to the plasma membrane prior to SNARE-mediated fusion. Spatial and temporal control of exocytosis through the exocyst has a crucial role in a number of physiological processes, such as morphogenesis, cell cycle progression, primary ciliogenesis, cell migration and tumor invasion. In this Cell Science at a Glance poster article, we summarize recent works on the molecular organization, function and regulation of the exocyst complex, as they provide rationales to the involvement of this complex in such a diverse array of cellular processes.

Tracking individual secretory vesicles during exocytosis reveals an ordered and regulated process

Post-Golgi secretory vesicle trafficking is a coordinated process, with transport and regulatory mechanisms to ensure appropriate exocytosis. While the contributions of many individual regulatory proteins to this process are well studied, the timing and dependencies of events have not been defined. Here we track individual secretory vesicles and associated proteins in vivo during tethering and fusion in budding yeast. Secretory vesicles tether to the plasma membrane very reproducibly for ∼18 s, which is extended in cells defective for membrane fusion and significantly lengthened and more variable when GTP hydrolysis of the exocytic Rab is delayed. Further, the myosin-V Myo2p regulates the tethering time in a mechanism unrelated to its interaction with exocyst component Sec15p. Two-color imaging of tethered vesicles with Myo2p, the GEF Sec2p, and several exocyst components allowed us to document a timeline for yeast exocytosis in which Myo2p leaves 4 s before fusion, whereas Sec2p and all the components of the exocyst disperse coincident with fusion.

Structure-Function Analyses of the Interactions between Rab11 and Rab14 Small GTPases with Their Shared Effector Rab Coupling Protein (RCP)

Rab GTPases recruit effector proteins, via their GTP-dependent switch regions, to distinct subcellular compartments. Rab11 and Rab25 are closely related small GTPases that bind to common effectors termed the Rab11 family of interacting proteins (FIPs). The FIPs are organized into two subclasses (class I and class II) based on sequence and domain organization, and both subclasses contain a highly conserved Rab-binding domain at their C termini. Yeast two-hybrid and biochemical studies have revealed that the more distantly related Rab14 also interacts with class I FIPs. Here, we perform detailed structural, thermodynamic, and cellular analyses of the interactions between Rab14 and one of the class I FIPs, the Rab-coupling protein (RCP), to clarify the molecular aspects of the interaction. We find that Rab14 indeed binds to RCP, albeit with reduced affinity relative to conventional Rab11-FIP and Rab25-FIP complexes. However, in vivo, Rab11 recruits RCP onto biological membranes. Furthermore, biophysical analyses reveal a noncanonical 1:2 stoichiometry between Rab14-RCP in dilute solutions, in contrast to Rab11/25 complexes. The structure of Rab14-RCP reveals that Rab14 interacts with the canonical Rab-binding domain and also provides insight into the unusual properties of the complex. Finally, we show that both the Rab coupling protein and Rab14 function in neuritogenesis.

Head-to-tail regulation is critical for the in vivo function of myosin V

Myo2p is regulated by a head-to-tail interaction and defects in this regulatory mechanism lead to a host of cellular problems in vivo, including impaired cargo capture and delayed motor recycling

Cell organization requires regulated cargo transport along cytoskeletal elements. Myosin V motors are among the most conserved organelle motors and have been well characterized in both yeast and mammalian systems. Biochemical data for mammalian myosin V suggest that a head-to-tail autoinhibitory interaction is a primary means of regulation, but the in vivo significance of this interaction has not been studied. Here we generated and characterized mutations in the yeast myosin V Myo2p to reveal that it is regulated by a head-to-tail interaction and that loss of regulation renders the myosin V constitutively active. We show that an unregulated motor is very deleterious for growth, resulting in severe defects in Myo2-mediated transport processes, including secretory vesicle transport, mitochondrial inheritance, and nuclear orientation. All of the defects associated with motor misregulation could be rescued by artificially restoring regulation. Thus, spatial and temporal regulation of myosin V in vivo by a head-to-tail interaction is critical for the normal delivery functions of the motor.

Sharing the cell's bounty - organelle inheritance in yeast

Eukaryotic cells replicate and partition their organelles between the mother cell and the daughter cell at cytokinesis. Polarized cells, notably the budding yeast Saccharomyces cerevisiae, are well suited for the study of organelle inheritance, as they facilitate an experimental dissection of organelle transport and retention processes. Much progress has been made in defining the molecular players involved in organelle partitioning in yeast. Each organelle uses a distinct set of factors - motor, anchor and adaptor proteins - that ensures its inheritance by future generations of cells. We propose that all organelles, regardless of origin or copy number, are partitioned by the same fundamental mechanism involving division and segregation. Thus, the mother cell keeps, and the daughter cell receives, their fair and equitable share of organelles. This mechanism of partitioning moreover facilitates the segregation of organelle fragments that are not functionally equivalent. In this Commentary, we describe how this principle of organelle population control affects peroxisomes and other organelles, and outline its implications for yeast life span and rejuvenation.

Rab11a is required for apical protein localisation in the intestine

The small GTPase Rab11 plays an important role in the recycling of proteins to the plasma membrane as well as in polarised transport in epithelial cells and neurons. We generated conditional knockout mice deficient in Rab11a. Rab11a-deficient mice are embryonic lethal, and brain-specific Rab11a knockout mice show no overt abnormalities in brain architecture. In contrast, intestine-specific Rab11a knockout mice begin dying approximately 1 week after birth. Apical proteins in the intestines of knockout mice accumulate in the cytoplasm and mislocalise to the basolateral plasma membrane, whereas the localisation of basolateral proteins is unaffected. Shorter microvilli and microvillus inclusion bodies are also observed in the knockout mice. Elevation of a serum starvation marker was also observed, likely caused by the mislocalisation of apical proteins and reduced nutrient uptake. In addition, Rab8a is mislocalised in Rab11a knockout mice. Conversely, Rab11a is mislocalised in Rab8a knockout mice and in a microvillus atrophy patient, which has a mutation in the myosin Vb gene. Our data show an essential role for Rab11a in the localisation of apical proteins in the intestine and demonstrate functional relationships between Rab11a, Rab8a and myosin Vb in vivo.

Vesicle trafficking from a lipid perspective: Lipid regulation of exocytosis in Saccharomyces cerevisiae

The protein cargo transported by specific types of vesicles largely defines the different secretory trafficking pathways operating within cells. However, mole per mole the most abundant cargo contained within transport vesicles is not protein, but lipid. Taking a "lipid-centric" point-of-view, we examine the importance of lipid signaling, membrane lipid organization and lipid metabolism for vesicle transport during exocytosis in budding yeast. In fact, the essential requirement for some exocytosis regulatory proteins can be bypassed by making simple manipulations of the lipids involved. During polarized exocytosis the sequential steps required to generate post-Golgi vesicles and target them to the plasma membrane (PM) involves the interplay of several types of lipids that are coordinately linked through PI4P metabolism and signaling. In turn, PI4P levels are regulated by PI4P kinases, the Sac1p PI4P phosphatase and the yeast Osh proteins, which are homologs of mammalian oxysterol-binding protein (OSBP). Together these regulators integrate the transitional steps required for vesicle maturation directly through changes in lipid composition and organization.

Novel mechanism coupling cyclic AMP-protein kinase A signaling and golgi trafficking via Gyp1 phosphorylation in polarized growth

The cyclic AMP (cAMP)-protein kinase A (PKA) signaling activates virulence expression during hyphal development in the fungal human pathogen Candida albicans. The hyphal growth is characterized by Golgi polarization toward the hyphal tips, which is thought to enhance directional vesicle transport. However, how the hypha-induction signal regulates Golgi polarization is unknown. Gyp1, a Golgi-associated protein and the first GTPase-activating protein (GAP) in the Rab GAP cascade, critically regulates membrane trafficking from the endoplasmic reticulum to the plasma membrane. Here, we report a novel pathway by which the cAMP-PKA signaling triggers Golgi polarization during hyphal growth. We demonstrate that Gyp1 plays a crucial role in actin-dependent Golgi polarization. Hyphal induction activates PKA, which in turn phosphorylates Gyp1. Phosphomimetic mutation of four PKA sites identified by mass spectrometry (Gyp1(4E)) caused strong Gyp1 polarization to hyphal tips, whereas nonphosphorylatable mutations (Gyp1(4A)) abolished it. Gyp1(4E) exhibited enhanced association with the actin motor Myo2, while Gyp1(4A) showed the opposite effect, providing a possible mechanism for Golgi polarization. A GAP-dead Gyp1 (Gyp1(R292K)) showed strong polarization similar to that seen with Gyp1(4E), indicating a role for the GAP activity. Mutating the PKA sites on Gyp1 also impaired the recruitment of a late Golgi marker, Sec7. Furthermore, proper PKA phosphorylation and GAP activity of Gyp1 are required for virulence in mice. We propose that the cAMP-PKA signaling directly targets Gyp1 to promote Golgi polarization in the yeast-to-hypha transition, an event crucial for C. albicans infection.

Rapid glucose depletion immobilizes active myosin V on stabilized actin cables

Polarization of eukaryotic cells requires organelles and protein complexes to be transported to their proper destinations along the cytoskeleton. When nutrients are abundant, budding yeast grows rapidly transporting secretory vesicles for localized growth and actively segregating organelles. This is mediated by myosin Vs transporting cargos along F-actin bundles known as actin cables. Actin cables are dynamic structures regulated by assembly, stabilization, and disassembly. Polarized growth and actin filament dynamics consume energy. For most organisms, glucose is the preferred energy source and generally represses alternative carbon source usage. Thus, upon abrupt glucose depletion, yeast shuts down pathways consuming large amounts of energy, including the vacuolar-ATPase, translation, and phosphoinositide metabolism. Here we show that glucose withdrawal rapidly (<1 min) depletes ATP levels and that the yeast myosin V, Myo2, responds by relocalizing to actin cables, making it the fastest response documented. Myo2 immobilized on cables releases its secretory cargo, defining a new rigor-like state of a myosin V in vivo. Only actively transporting Myo2 can be converted to the rigor-like state. Glucose depletion has differential effects on the actin cytoskeleton, resulting in disassembly of actin patches with concomitant inhibition of endocytosis and strong stabilization of actin cables, thereby revealing a selective and previously unappreciated ATP requirement for actin cable disassembly. A similar response is seen in HeLa cells to ATP depletion. These findings reveal a new fast-acting energy conservation strategy halting growth by immobilizing myosin V in a newly described state on selectively stabilized actin cables.

SEC-10 and RAB-10 coordinate basolateral recycling of clathrin-independent cargo through endosomal tubules in Caenorhabditis elegans

Despite the increasing number of regulatory proteins identified in clathrin-independent endocytic (CIE) pathways, our understanding of the exact functions of these proteins and the sequential manner in which they function remains limited. In this study, using the Caenorhabditis elegans intestine as a model, we observed a unique structure of interconnected endosomal tubules, which is required for the basolateral recycling of several CIE cargoes including hTAC, GLUT1, and DAF-4. SEC-10 is a subunit of the octameric protein complex exocyst. Depleting SEC-10 and several other exocyst components disrupted the endosomal tubules into various ring-like structures. An epistasis analysis further suggested that SEC-10 operates at the intermediate step between early endosomes and recycling endosomes. The endosomal tubules were also sensitive to inactivation of the Rab GTPase RAB-10 and disruption of microtubules. Taken together, our data suggest that SEC-10 coordinates with RAB-10 and microtubules to form the endosomal tubular network for efficient recycling of particular CIE cargoes.

Asymmetric distribution of phosphatidylserine is generated in the absence of phospholipid flippases in Saccharomyces cerevisiae

In eukaryotic cells, phosphatidylserine (PS) is predominantly located in the cytosolic leaflet of the plasma membrane; this asymmetry is generated by an unknown mechanism. In this study, we used the PS-specific probe mRFP-Lact-C2 to investigate the possible involvement of type 4 P-type ATPases, also called phospholipid flippases, in the generation of this asymmetry in Saccharomyces cerevisiae. PS was not found in the trans-Golgi Network in wild-type cells, but it became exposed when vesicle formation was compromised in the sec7 mutant, and it was also exposed on secretory vesicles (SVs), as reported previously. However, flippase mutations did not reduce the exposure of PS in either case, even at low levels that would only be detectable by quantitative analysis of mRFP-Lact-C2 fluorescence in isolated SVs. Furthermore, no reduction in the PS level was observed in a mutant with multiple flippase mutations. Because PS was not exposed in a mutant that accumulates ER or cis/medial-Golgi membranes, Golgi maturation seems to be a prerequisite for PS translocation. Our results suggest that an unknown mechanism, possibly a protein with flippase-like activity, acts in conjunction with known flippases to regulate PS translocation.

Osh4p is needed to reduce the level of phosphatidylinositol-4-phosphate on secretory vesicles as they mature

In yeast, the oxysterol-binding proteins Osh1–Osh7 are collectively needed to maintain the normal distribution of PI4P. Osh4p is recruited to secretory vesicles in a PI4P-dependent manner and plays an important role in vesicle maturation.

Phosphatidylinositol-4-phosphate (PI4P) is produced on both the Golgi and the plasma membrane. Despite extensive vesicular traffic between these compartments, genetic analysis suggests that the two pools of PI4P do not efficiently mix with one another. Several lines of evidence indicate that the PI4P produced on the Golgi is normally incorporated into secretory vesicles, but the fate of that pool has been unclear. We show here that in yeast the oxysterol-binding proteins Osh1–Osh7 are collectively needed to maintain the normal distribution of PI4P and that Osh4p is critical in this function. Osh4p associates with secretory vesicles at least in part through its interaction with PI4P and is needed, together with lipid phosphatases, to reduce the level of PI4P as vesicles approach sites of exocytosis. This reduction in PI4P is necessary for a switch in the regulation of the Sec4p exchange protein, Sec2p, from an interaction with the upstream Rab, Ypt31/32, to an interaction with a downstream Sec4p effector, Sec15p. Spatial regulation of PI4P levels thereby plays an important role in vesicle maturation.

Actin is required for IFT regulation in Chlamydomonas reinhardtii

Assembly of cilia and flagella requires intraflagellar transport (IFT), a highly regulated kinesin-based transport system that moves cargo from the basal body to the tip of flagella [1]. The recruitment of IFT components to basal bodies is a function of flagellar length, with increased recruitment in rapidly growing short flagella [2]. The molecular pathways regulating IFT are largely a mystery. Because actin network disruption leads to changes in ciliary length and number, actin has been proposed to have a role in ciliary assembly. However, the mechanisms involved are unknown. In Chlamydomonas reinhardtii, conventional actin is found in both the cell body and the inner dynein arm complexes within flagella [3, 4]. Previous work showed that treating Chlamydomonas cells with the actin-depolymerizing compound cytochalasin D resulted in reversible flagellar shortening [5], but how actin is related to flagellar length or assembly remains unknown. Here we utilize small-molecule inhibitors and genetic mutants to analyze the role of actin dynamics in flagellar assembly in Chlamydomonas reinhardtii. We demonstrate that actin plays a role in IFT recruitment to basal bodies during flagellar elongation and that when actin is perturbed, the normal dependence of IFT recruitment on flagellar length is lost. We also find that actin is required for sufficient entry of IFT material into flagella during assembly. These same effects are recapitulated with a myosin inhibitor, suggesting that actin may act via myosin in a pathway by which flagellar assembly is regulated by flagellar length.

Maturation of late Golgi cisternae into RabE(RAB11) exocytic post-Golgi carriers visualized in vivo

The mechanism(s) by which proteins traverse and exit the Golgi are incompletely understood. Using Aspergillus nidulans hyphae, we show that late Golgi cisternae undergo changes in composition to gradually lose Golgi identity while acquiring post-Golgi RabE(RAB11) identity. This behavior of late Golgi cisternae is consistent with the cisternal maturation model. Post-Golgi RabE(RAB11) carriers travel to, and accumulate at, the apex, indicating that fusion is rate limiting for exocytosis. These carriers, which are loaded with kinesin, dynein, and MyoE(MYO5), move on a microtubule-based bidirectional conveyor belt relaying them to actin, which ultimately focuses exocytosis at the apex. Dynein drags RabE(RAB11) carriers away if engagement of MyoE(MYO5) to actin cables fails. Microtubules seemingly cooperating with F-actin capture can sustain secretion if MyoE(MYO5) is absent. Thus, filamentous fungal secretion involving post-Golgi carriers is remarkably similar, mechanistically, to the transport of melanosomes in melanocyte dendrites, even though melanosome biogenesis involves lysosomes rather than Golgi.

RhoGTPase-binding proteins, the exocyst complex and polarized vesicle trafficking

Cell polarity, the asymmetric distribution of proteins and lipids, is essential for a variety of cellular functions. One mechanism orchestrating cell polarity is polarized vesicle trafficking; whereby cargo loaded secretory vesicles are specifically transported to predetermined areas of the cell. The evolutionarily conserved exocyst complex and its small GTPase regulators play crucial roles in spatiotemporal control of polarized vesicle trafficking. In studies on neuronal membrane remodeling and synaptic plasticity, conserved mechanisms of exocyst regulation and cargo recycling during polarized vesicle trafficking are beginning to emerge as well. Recently, our lab demonstrated that RhoGTPase-binding proteins in both yeast (Bem3) and mammals (Ocrl1) are also required for the efficient traffic of secretory vesicles to sites of polarized growth and signaling. Together with our studies, we highlight the evolutionary conservation of the basic elements essential for polarized vesicle traffic across different cellular functions and model systems. In conclusion, we emphasize that studies on RhoGTPase-binding proteins in these processes should be included in the next level of investigation, for a more complete understanding of their hitherto unknown roles in polarized membrane traffic and exocyst regulation.