Association of the yeast RNA-binding protein She2p with the tubular endoplasmic reticulum depends on membrane curvature

PtdIns4P is required for the autophagosomal recruitment of STX17 (syntaxin 17) to promote lysosomal fusion

The autophagosomal SNARE STX17 (syntaxin 17) promotes lysosomal fusion and degradation, but its autophagosomal recruitment is incompletely understood. Notably, PtdIns4P is generated on autophagosomes and promotes fusion through an unknown mechanism. Here we show that soluble recombinant STX17 is spontaneously recruited to negatively charged liposomes and adding PtdIns4P to liposomes containing neutral lipids is sufficient for its recruitment. Consistently, STX17 colocalizes with PtdIns4P-positive autophagosomes in cells, and specific inhibition of PtdIns4P synthesis on autophagosomes prevents its loading. Molecular dynamics simulations indicate that C-terminal positively charged amino acids establish contact with membrane bilayers containing negatively charged PtdIns4P. Accordingly, Ala substitution of Lys and Arg residues in the C terminus of STX17 abolishes membrane binding and impairs its autophagosomal recruitment. Finally, only wild type but not Ala substituted STX17 expression rescues the autophagosome-lysosome fusion defect of STX17 loss-of-function cells. We thus identify a key step of autophagosome maturation that promotes lysosomal fusion.Abbreviations: Cardiolipin: 1',3'-bis[1-palmitoyl-2-oleoyl-sn-glycero-3-phospho]-glycerol; DMSO: dimethyl sulfoxide; GST: glutathione S-transferase; GUV: giant unilamellar vesicles; LAMP1: lysosomal associated membrane protein 1; MAP1LC3/LC3: microtubule associated protein 1 light chain 3; PA: 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphate; PC/POPC: 1-palmitoyl-2-oleoyl-glycero-3-phosphocholine; PG: 1-palmitoyl-2-linoleoyl-sn-glycero-3-phospho-(1'-rac-glycerol); PI: L-α-phosphatidylinositol; PI4K2A: phosphatidylinositol 4-kinase type 2 alpha; PIK3C3/VPS34: phosphatidylinositol 3-kinase catalytic subunit type 3; POPE/PE: 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine; PS: 1-stearoyl-2-linoleoyl-sn-glycero-3-phospho-L-serine; PtdIns(3,5)P2: 1,2-dioleoyl-sn-glycero-3-phospho-(1"-myo-inositol-3',5'-bisphosphate); PtdIns3P: 1,2- dioleoyl-sn-glycero-3-phospho-(1'-myo-inositol-3'-phosphate); PtdIns4P: 1,2-dioleoyl-sn-glycero-3-phospho-(1"-myo-inositol-4'-phosphate); SDS-PAGE: sodium dodecyl sulfate-polyacrylamide gel electrophoresis; STX17: syntaxin 17.

Membrane Proteins and Membrane Curvature: Mutual Interactions and a Perspective on Disease Treatments

The pathogenesis of various diseases often involves an intricate interplay between membrane proteins and membrane curvature. Understanding the underlying mechanisms of this interaction could offer novel perspectives on disease treatment. In this review, we provide an introduction to membrane curvature and its association with membrane proteins. Furthermore, we delve into the impact and potential implications of this interaction in the context of disease treatment. Lastly, we discuss the prospects and challenges associated with harnessing these interactions for effective disease management, aiming to provide fresh insights into therapeutic strategies.

Post-transcriptional control of fungal cell wall synthesis

Pathogenic fungi hide from their hosts by camouflage, obscuring immunogenic cell wall components such as beta-glucan with innocuous coverings such as mannoproteins and alpha-glucan that are less readily recognised by the host. Attempts to understand how such processes are regulated have met with varying success. Typically studies focus on understanding the transcriptional response of fungi to either their reservoir environment or the host. However, such approaches do not fully address this research question, due to the layers of post-transcriptional and post-translational regulation that occur within a cell. Although in animals the impact of post-transcriptional and post-translational regulation has been well characterised, our knowledge of these processes in the fungal kingdom is more limited. Mutations in RNA-binding proteins, like Ssd1 and Candida albicans Slr1, affect cell wall composition and fungal virulence indicating that post-transcriptional regulation plays a key role in these processes. Here, we review the current state of knowledge of fungal post-transcriptional regulation, and link this to potential mechanisms of immune evasion by drawing on studies from model yeast and plant pathogenic fungi. We highlight several RNA-binding proteins that regulate cell wall synthesis and could be involved in local translation of cell wall components. Expanding our knowledge on post-transcriptional regulation in human fungal pathogens is essential to fully comprehend fungal virulence strategies and for the design of novel antifungal therapies.

Hitchhiking Across Kingdoms: Cotransport of Cargos in Fungal, Animal, and Plant Cells

Eukaryotic cells across the tree of life organize their subcellular components via intracellular transport mechanisms. In canonical transport, myosin, kinesin, and dynein motor proteins interact with cargos via adaptor proteins and move along filamentous actin or microtubule tracks. In contrast to this canonical mode, hitchhiking is a newly discovered mode of intracellular transport in which a cargo attaches itself to an already-motile cargo rather than directly associating with a motor protein itself. Many cargos including messenger RNAs, protein complexes, and organelles hitchhike on membrane-bound cargos. Hitchhiking-like behaviors have been shown to impact cellular processes including local protein translation, long-distance signaling, and organelle network reorganization. Here, we review instances of cargo hitchhiking in fungal, animal, and plant cells and discuss the potential cellular and evolutionary importance of hitchhiking in these different contexts.

DDX3 modulates the tumor microenvironment via its role in endoplasmic reticulum-associated translation

Using antibody arrays, we found that the RNA helicase DDX3 modulates the expression of secreted signaling factors in oral squamous cell carcinoma (OSCC) cells. Ribo-seq analysis confirmed amphiregulin (AREG) as a translational target of DDX3. AREG exerts important biological functions in cancer, including promoting cell migration and paracrine effects of OSCC cells and reprogramming the tumor microenvironment (TME) of OSCC in mice. DDX3-mediated translational control of AREG involves its 3′-untranslated region. Proteomics identified the signal recognition particle (SRP) as an unprecedented interacting partner of DDX3. DDX3 and SRP54 were located near the endoplasmic reticulum, regulated the expression of a common set of secreted factors, and were essential for targeting AREG mRNA to membrane-bound polyribosomes. Finally, OSCC-associated mutant DDX3 increased the expression of AREG, emphasizing the role of DDX3 in tumor progression via SRP-dependent, endoplasmic reticulum-associated translation. Therefore, pharmacological targeting of DDX3 may inhibit the tumor-promoting functions of the TME.

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DDX3-AREG axis promotes cancer progression through microenvironment remodeling

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DDX3 activates AREG translation via binding to its 3′ UTR

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DDX3 interacts with the signal recognition particle (SRP)

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DDX3-SRP-mediated mRNA recruitment assists ER-associated translation

Daughter cell-targeted mRNAs can achieve segregation without the universal Endoplasmic Reticulum docker She2p

The establishment of cell polarity in eukaryotes involves the asymmetric distribution of messenger RNAs (mRNAs). In Saccharomyces cerevisiae, establishment of the cell polarity that gives rise to mother and daughter cells concurs with the selective targeting of more than 30 mRNAs toward the bud tip. Different mRNAs are segregated at different cell cycle stages, namely early during S phase, in a process dependent on anchoring to the endoplasmic reticulum (ER), or later in G₂ or mitosis, in an ER-independent manner. In spite of this difference, this transport requires in all cases the Myo4p motor and its interaction with actin, the adaptor protein She3p and a third, RNA-binding protein docking this complex at the mRNA itself. This protein is universally considered to be She2p. Yet, the majority of mRNAs whose segregation was shown to be She2p-dependent are not S-phase segregated ones. In other processes aimed at establishing polarity, such as during pheromone-stimulated G₁ arrest, the coupling of mRNAs to the ER during their transport is She2p-independent. We have therefore asked if the segregation to the bud of a model S-phase-specific mRNA, EAR1, is dependent on She2p or not. We report that a modest yet consistent percentage of EAR1 segregating particles achieves polarization without She2p. Our data invite to a re-evaluation of the absolute necessity for She2p for daughter cell-targeted mRNAs distribution.

mRNA Targeting, Transport and Local Translation in Eukaryotic Cells: From the Classical View to a Diversity of New Concepts

Spatial organization of protein biosynthesis in the eukaryotic cell has been studied for more than fifty years, thus many facts have already been included in textbooks. According to the classical view, mRNA transcripts encoding secreted and transmembrane proteins are translated by ribosomes associated with endoplasmic reticulum membranes, while soluble cytoplasmic proteins are synthesized on free polysomes. However, in the last few years, new data has emerged, revealing selective translation of mRNA on mitochondria and plastids, in proximity to peroxisomes and endosomes, in various granules and at the cytoskeleton (actin network, vimentin intermediate filaments, microtubules and centrosomes). There are also long-standing debates about the possibility of protein synthesis in the nucleus. Localized translation can be determined by targeting signals in the synthesized protein, nucleotide sequences in the mRNA itself, or both. With RNA-binding proteins, many transcripts can be assembled into specific RNA condensates and form RNP particles, which may be transported by molecular motors to the sites of active translation, form granules and provoke liquid-liquid phase separation in the cytoplasm, both under normal conditions and during cell stress. The translation of some mRNAs occurs in specialized "translation factories," assemblysomes, transperons and other structures necessary for the correct folding of proteins, interaction with functional partners and formation of oligomeric complexes. Intracellular localization of mRNA has a significant impact on the efficiency of its translation and presumably determines its response to cellular stress. Compartmentalization of mRNAs and the translation machinery also plays an important role in viral infections. Many viruses provoke the formation of specific intracellular structures, virus factories, for the production of their proteins. Here we review the current concepts of the molecular mechanisms of transport, selective localization and local translation of cellular and viral mRNAs, their effects on protein targeting and topogenesis, and on the regulation of protein biosynthesis in different compartments of the eukaryotic cell. Special attention is paid to new systems biology approaches, providing new cues to the study of localized translation.

Multiplexed mRNA assembly into ribonucleoprotein particles plays an operon-like role in the control of yeast cell physiology

Prokaryotes utilize polycistronic messages (operons) to co-translate proteins involved in the same biological processes. Whether eukaryotes achieve similar regulation by selectively assembling and translating monocistronic messages derived from different chromosomes is unknown. We employed transcript-specific RNA pulldowns and RNA-seq/RT-PCR to identify yeast mRNAs that co-precipitate as ribonucleoprotein (RNP) complexes. Consistent with the hypothesis of eukaryotic RNA operons, mRNAs encoding components of the mating pathway, heat shock proteins, and mitochondrial outer membrane proteins multiplex in trans, forming discrete messenger ribonucleoprotein (mRNP) complexes (called transperons). Chromatin capture and allele tagging experiments reveal that genes encoding multiplexed mRNAs physically interact; thus, RNA assembly may result from co-regulated gene expression. Transperon assembly and function depends upon histone H4, and its depletion leads to defects in RNA multiplexing, decreased pheromone responsiveness and mating, and increased heat shock sensitivity. We propose that intergenic associations and non-canonical histone H4 functions contribute to transperon formation in eukaryotic cells and regulate cell physiology.

The functional organization of axonal mRNA transport and translation

Axons extend for tremendously long distances from the neuronal soma and make use of localized mRNA translation to rapidly respond to different extracellular stimuli and physiological states. The locally synthesized proteins support many different functions in both developing and mature axons, raising questions about the mechanisms by which local translation is organized to ensure the appropriate responses to specific stimuli. Publications over the past few years have uncovered new mechanisms for regulating the axonal transport and localized translation of mRNAs, with several of these pathways converging on the regulation of cohorts of functionally related mRNAs - known as RNA regulons - that drive axon growth, axon guidance, injury responses, axon survival and even axonal mitochondrial function. Recent advances point to these different regulatory pathways as organizing platforms that allow the axon's proteome to be modulated to meet its physiological needs.

Local Translation in Axons: When Membraneless RNP Granules Meet Membrane-Bound Organelles

Eukaryotic cell compartmentalization relies on long-known membrane-delimited organelles, as well as on more recently discovered membraneless macromolecular condensates. How these two types of organelles interact to regulate cellular functions is still largely unclear. In this review, we highlight how membraneless ribonucleoprotein (RNP) organelles, enriched in RNAs and associated regulatory proteins, cooperate with membrane-bound organelles for tight spatio-temporal control of gene expression in the axons of neuronal cells. Specifically, we present recent evidence that motile membrane-bound organelles are used as vehicles by RNP cargoes, promoting the long-range transport of mRNA molecules to distal axons. As demonstrated by recent work, membrane-bound organelles also promote local protein synthesis, by serving as platforms for the local translation of mRNAs recruited to their outer surface. Furthermore, dynamic and specific association between RNP cargoes and membrane-bound organelles is mediated by bi-partite adapter molecules that interact with both types of organelles selectively, in a regulated-manner. Maintaining such a dynamic interplay is critical, as alterations in this process are linked to neurodegenerative diseases. Together, emerging studies thus point to the coordination of membrane-bound and membraneless organelles as an organizing principle underlying local cellular responses.

Conserved role for Ataxin-2 in mediating endoplasmic reticulum dynamics

Ataxin-2, a conserved RNA-binding protein, is implicated in the late-onset neurodegenerative disease Spinocerebellar ataxia type-2 (SCA2). SCA2 is characterized by shrunken dendritic arbors and torpedo-like axons within the Purkinje neurons of the cerebellum. Torpedo-like axons have been described to contain displaced endoplasmic reticulum (ER) in the periphery of the cell; however, the role of Ataxin-2 in mediating ER function in SCA2 is unclear. We utilized the Caenorhabditis elegans and Drosophila homologs of Ataxin-2 (ATX-2 and DAtx2, respectively) to determine the role of Ataxin-2 in ER function and dynamics in embryos and neurons. Loss of ATX-2 and DAtx2 resulted in collapse of the ER in dividing embryonic cells and germline, and ultrastructure analysis revealed unique spherical stacks of ER in mature oocytes and fragmented and truncated ER tubules in the embryo. ATX-2 and DAtx2 reside in puncta adjacent to the ER in both C. elegans and Drosophila embryos. Lastly, depletion of DAtx2 in cultured Drosophila neurons recapitulated the shrunken dendritic arbor phenotype of SCA2. ER morphology and dynamics were severely disrupted in these neurons. Taken together, we provide evidence that Ataxin-2 plays an evolutionary conserved role in ER dynamics and morphology in C. elegans and Drosophila embryos during development and in fly neurons, suggesting a possible SCA2 disease mechanism.

Membrane-Associated RNA-Binding Proteins Orchestrate Organelle-Coupled Translation

Proteins are positioned and act at defined subcellular locations. This is particularly important in eukaryotic cells that deliver proteins to membrane-bound organelles such as the endoplasmic reticulum (ER), mitochondria, or endosomes. It is axiomatic that organelle targeting depends mainly on polypeptide signals. However, recent results demonstrate that targeting elements within the encoding transcripts are essential for efficient protein localisation. Key readers of these elements are membrane-associated RNA-binding proteins (memRBPs) that orchestrate organelle-coupled translation. The translation products then either cross the membrane for organelle entry or hitchhike on organelle surfaces for complex assembly and co-transport. Understanding the interaction of protein- and RNA-based targeting signals is essential to decipher the molecular basis for mutant phenotypes in disease.

H1.0 Linker Histone as an Epigenetic Regulator of Cell Proliferation and Differentiation

H1 linker histones are a class of DNA-binding proteins involved in the formation of supra-nucleosomal chromatin higher order structures. Eleven non-allelic subtypes of H1 are known in mammals, seven of which are expressed in somatic cells, while four are germ cell-specific. Besides having a general structural role, H1 histones also have additional epigenetic functions related to DNA replication and repair, genome stability, and gene-specific expression regulation. Synthesis of the H1 subtypes is differentially regulated both in development and adult cells, thus suggesting that each protein has a more or less specific function. The somatic variant H1.0 is a linker histone that was recognized since long ago to be involved in cell differentiation. Moreover, it has been recently found to affect generation of epigenetic and functional intra-tumor heterogeneity. Interestingly, H1.0 or post-translational forms of it have been also found in extracellular vesicles (EVs) released from cancer cells in culture, thus suggesting that these cells may escape differentiation at least in part by discarding H1.0 through the EV route. In this review we will discuss the role of H1.0 in development, differentiation, and stem cell maintenance, also in relation with tumorigenesis, and EV production.

Signal sequence-independent targeting of MID2 mRNA to the endoplasmic reticulum by the yeast RNA-binding protein Khd1p

Localization of mRNAs depends on specific RNA-binding proteins (RBPs) and critically contributes not only to cell polarization but also to basal cell function. The yeast RBP Khd1p binds to several hundred mRNAs, the majority of which encodes secreted or membrane proteins. We demonstrate that a subfraction of Khd1p associates with artificial liposomes and endoplasmic reticulum (ER), and that Khd1p endomembrane association is partially dependent on its binding to RNA. ER targeting of at least two mRNAs, MID2 and SLG1/WSC1, requires KHD1 but is independent of their translation. Together, our results suggest interdependence of Khd1p and mRNA for their targeting to the ER and presents additional evidence for signal sequence-independent, RBP-mediated mRNA targeting.

mRNA transport in fungal top models

Eukaryotic cells rely on the precise determination of when and where proteins are synthesized. Spatiotemporal expression is supported by localization of mRNAs to specific subcellular sites and their subsequent local translation. This holds true for somatic cells as well as for oocytes and embryos. Most commonly, mRNA localization is achieved by active transport of the molecules along the actin or microtubule cytoskeleton. Key factors are molecular motors, adaptors, and RNA-binding proteins that recognize defined sequences or structures in cargo mRNAs. A deep understanding of this process has been gained from research on fungal model systems such as Saccharomyces cerevisiae and Ustilago maydis. Recent highlights of these studies are the following: (1) synergistic binding of two RNA-binding proteins is needed for high affinity recognition; (2) RNA sequences undergo profound structural rearrangements upon recognition; (3) mRNA transport is tightly linked to membrane trafficking; (4) mRNAs and ribosomes are transported on the cytoplasmic surface of endosomes; and (5) heteromeric protein complexes are, most likely, assembled co-translationally during endosomal transport. Thus, the study of simple fungal model organisms provides valuable insights into fundamental mechanisms of mRNA transport boosting the understanding of similar events in higher eukaryotes. WIREs RNA 2018, 9:e1453. doi: 10.1002/wrna.1453 This article is categorized under: RNA Interactions with Proteins and Other Molecules > Protein-RNA Recognition RNA Interactions with Proteins and Other Molecules > RNA-Protein Complexes RNA Export and Localization > RNA Localization.

Hitchhiking: A Non-Canonical Mode of Microtubule-Based Transport

The long-range movement of organelles, vesicles, and macromolecular complexes by microtubule-based transport is crucial for cell growth and survival. The canonical view of intracellular transport is that each cargo directly recruits molecular motors via cargo-specific adaptor molecules. Recently, a new paradigm called 'hitchhiking' has emerged: some cargos can achieve motility by interacting with other cargos that have already recruited molecular motors. In this way, cargos are co-transported together and their movements are directly coupled. Cargo hitchhiking was discovered in fungi. However, the observation that organelle dynamics are coupled in mammalian cells suggests that this paradigm may be evolutionarily conserved. We review here the data for hitchhiking and discuss the biological significance of this non-canonical mode of microtubule-based transport.

Membrane-Coupled mRNA Trafficking in Fungi

Intracellular logistics are essential for delivery of newly synthesized material during polar growth of fungal hyphae. Proteins and lipids are actively transported throughout the cell by motor-dependent movement of small vesicles or larger units such as endosomes and the endoplasmic reticulum. A remarkably tight link is emerging between active membrane trafficking and mRNA transport, a process that determines the precise subcellular localization of translation products within the cell. Here, we report on recent insights into the mechanism and biological role of these intricate cotransport processes in fungal models such as Saccharomyces cerevisiae, Candida albicans, and Ustilago maydis. In the latter, we focus on the new finding of endosomal mRNA transport and its implications for protein targeting, complex assembly, and septin biology.

A FYVE zinc finger domain protein specifically links mRNA transport to endosome trafficking

An emerging theme in cellular logistics is the close connection between mRNA and membrane trafficking. A prominent example is the microtubule-dependent transport of mRNAs and associated ribosomes on endosomes. This coordinated process is crucial for correct septin filamentation and efficient growth of polarised cells, such as fungal hyphae. Despite detailed knowledge on the key RNA-binding protein and the molecular motors involved, it is unclear how mRNAs are connected to membranes during transport. Here, we identify a novel factor containing a FYVE zinc finger domain for interaction with endosomal lipids and a new PAM2-like domain required for interaction with the MLLE domain of the key RNA-binding protein. Consistently, loss of this FYVE domain protein leads to specific defects in mRNA, ribosome, and septin transport without affecting general functions of endosomes or their movement. Hence, this is the first endosomal component specific for mRNP trafficking uncovering a new mechanism to couple mRNPs to endosomes.

DOI:http://dx.doi.org/10.7554/eLife.06041.001

DNA contains the instructions to build proteins. These instructions are first copied to make a molecule of messenger RNA (or mRNA for short). A large machine called the ribosome then reads the mRNA molecule and translates it to build a protein.

Many proteins must get to particular locations in a cell to carry out their roles. For some proteins, this is achieved by transporting the mRNAs to the right location before they get translated, via a process called ‘mRNA trafficking’. However, mRNAs do not move by themselves; instead they bind to a host of mRNA-binding proteins, and the ribosomes that are required for translation to take place. Cells also move proteins between different locations using small bubble-like structures called vesicles. These vesicles are surrounded by a membrane, and so this process is known as ‘membrane trafficking’. Previous work has shown that these two processes are often linked, as vesicles can also carry mRNA molecules. But it is not fully understood how mRNA molecules are connected to vesicles.

Now, Pohlmann et al. have used a fungus called Ustilago maydis as a model system to investigate how mRNAs and vesicles can move together in cells that grow to form filament-like structures called hyphae. This fungus uses these filaments to penetrate into plant tissues and causes a disease called corn smut. The experiments revealed a vesicle protein called Upa1 that contains a new type of binding site that allows Upa1 to bring an important RNA-binding protein to the surface of vesicles. Since the RNA-binding protein binds mRNA and the translating ribosomes, this can explain how mRNAs can associate with membranes to move together along hyphae.

When Pohlmann et al. engineered fungi that lacked the gene for Upa1, these mutants had problems transporting their mRNAs and associated ribosomes. These findings reveal a direct connection between mRNA trafficking and membrane trafficking. Future studies could now investigate whether similar processes take place in other cells that grow as long filaments, such as plant pollen tubes or nerve cells. These studies might provide new insights into plant reproduction or brain activity.

DOI:http://dx.doi.org/10.7554/eLife.06041.002

Here, there, everywhere. mRNA localization in budding yeast

mRNA localization and localized translation is a common mechanism that contributes to cell polarity and cellular asymmetry. In metazoan, mRNA transport participates in embryonic axis determination and neuronal plasticity. Since the mRNA localization process and its molecular machinery are rather complex in higher eukaryotes, the unicellular yeast Saccharomyces cerevisiae has become an attractive model to study mRNA localization. Although the focus has so far been on the mechanism of ASH1 mRNA transport, it has become evident that mRNA localization also assists in protein sorting to organelles, as well as in polarity establishment and maintenance. A diversity of different pathways has been identified that targets mRNA to their destination site, ranging from motor protein-dependent trafficking of translationally silenced mRNAs to co-translational targeting, in which mRNAs hitch-hike to organelles on ribosomes during nascent polypeptide chain elongation. The presence of these diverse pathways in yeast allows a systemic analysis of the contribution of mRNA localization to the physiology of a cell.

Of social molecules: The interactive assembly of ASH1 mRNA-transport complexes in yeast

Asymmetric, motor-protein dependent transport of mRNAs and subsequent localized translation is an important mechanism of gene regulation. Due to the high complexity of such motile particles, our mechanistic understanding of mRNA localization is limited. Over the last two decades, ASH1 mRNA localization in budding yeast has served as comparably simple and accessible model system. Recent advances have helped to draw an increasingly clear picture on the molecular mechanisms governing ASH1 mRNA localization from its co-transcriptional birth to its delivery at the site of destination. These new insights help to better understand the requirement of initial nuclear mRNPs, the molecular basis of specific mRNA-cargo recognition via cis-acting RNA elements, the different stages of RNP biogenesis and reorganization, as well as activation of the motile activity upon cargo binding. We discuss these aspects in context of published findings from other model organisms.

In Candida albicans hyphae, Sec2p is physically associated with SEC2 mRNA on secretory vesicles

Candida albicans hyphae grow in a highly polarized fashion from their tips. This polarized growth requires the continuous delivery of secretory vesicles to the tip region. Vesicle delivery depends on Sec2p, the Guanine Exchange Factor (GEF) for the Rab GTPase Sec4p. GTP bound Sec4p is required for the transit of secretory vesicles from the trans-Golgi to sites of polarized growth. We previously showed that phosphorylation of Sec2p at residue S584 was necessary for Sec2p to support hyphal, but not yeast growth. Here we show that on secretory vesicles SEC2 mRNA is physically associated with Sec2p. Moreover, we show that the phosphorylation of S584 allows SEC2 mRNA to dissociate from Sec2p and we speculate that this is necessary for Sec2p function and/or translation. During hyphal extension, the growing tip may be separated from the nucleus by up to 15 μm. Transport of SEC2 mRNA on secretory vesicles to the tip localizes SEC2 translation to tip allowing a sufficient accumulation of this key protein at the site of polarized growth.

Structure of a myosin•adaptor complex and pairing by cargo

Myosin 4 protein (Myo4p), one of five distinct myosins of yeast, is dedicated to cytoplasmic transport of two types of cargos, zipcoded messenger ribonucleoprotein particles (mRNPs) and tubular endoplasmic reticulum (tER). Neither cargo binds directly to Myo4p. Instead, swi5p-dependent HO expression 3 protein (She3p) serves as an "adaptor" that contains three binding modules, one for Myo4p and one each for zipcoded mRNP and tER. The assembly of a transport-competent motor complex is poorly understood. Here, we report that Myo4p•She3p forms a stable 1:2 heterotrimer in solution. In the Myo4p•She3p crystal structure, Myo4p's C-terminal domain (CTD) assumes a lobster claw-shaped form, the minor prong of which adheres to a pseudocoiled-coil region of She3p. The extensive Myo4p•She3p interactome buries 3,812 Å(2) surface area and is primarily hydrophobic. Because the Myo4p•She3p heterotrimer contains only one myosin molecule, it is not transport-competent. By stepwise reconstitution, we found a single molecule of synthetic oligonucleotide (representing the mRNA zipcode element) bound to a single tetramer of zipcode binding protein She2p to be sufficient for Myo4p•She3p dimerization. Therefore, cargo initiates cross-linking of two Myo4p•She3p heterotrimers to an ensemble that contains two myosin molecules obligatory for movement. An additional crystal structure comprising an overlapping upstream portion of She3p showed continuation of the pseudocoiled-coil structure and revealed another highly conserved surface region. We suggest this region as a candidate binding site for a yet unidentified tER ligand. We propose a model whereby zipcoded mRNP and/or tER ligands couple two Myo4p•She3p heterotrimers and thereby generate a transport-competent motor complex either for separate transport or cotransport of these two cargos.

Hop-on hop-off: polysomes take a tour of the cell on endosomes

After export from the nucleus, ribosomes need to be distributed throughout the entire cell so that protein synthesis can occur even at distant sites. In the elongated hyphal cell of the fungus Ustilago maydis, Higuchi et al. (2014. J. Cell Biol. http://dx.doi.org/10.1083/jcb.201307164) now demonstrate that polysomes associate with early endosomes that undergo kinesin 3- and dynein-dependent long-range motility. The bidirectional movement of early endosomes randomly distributes polysomes, which may ensure the even distribution of the translation machinery across the entire cell.

Yeast phospholipid biosynthesis is linked to mRNA localization

Localization of mRNAs and local translation are universal features in eukaryotes and contribute to cellular asymmetry and differentiation. In Saccharomyces cerevisiae, localization of mRNAs that encode membrane proteins requires the She protein machinery including the RNA-binding protein She2p as well as movement of the cortical endoplasmic reticulum (cER) to the yeast bud. In a screen for ER-specific proteins necessary for directional transport of WSC2 and EAR1 mRNAs, we have identified enzymes of the phospholipid metabolism. Loss of the phospholipid methyltransferase Cho2p, which showed the strongest impact on mRNA localization, disturbs mRNA localization as well as ER morphology and segregation due to an increase in cellular phosphatidylethanolamine (PE). Mislocalized mRNPs containing She2p co-localize with aggregated cER structures suggesting entrapment of mRNA and She2p by the elevated PE level, which is confirmed by elevated binding of She2p to PE-containing liposomes. These findings underscore the importance of ER membrane integrity in mRNA transport.

Protein targeting via mRNA in bacteria

Proteins of all living organisms must reach their subcellular destination to sustain the cell structure and function. The proteins are transported to one of the cellular compartments, inserted into the membrane, or secreted across the membrane to the extracellular milieu. Cells have developed various mechanisms to transport proteins across membranes, among them localized translation. Evidence for targeting of Messenger RNA for the sake of translation of their respective protein products at specific subcellular sites in many eukaryotic model organisms have been accumulating in recent years. Cis-acting RNA localizing elements, termed RNA zip-codes, which are embedded within the mRNA sequence, are recognized by RNA-binding proteins, which in turn interact with motor proteins, thus coordinating the intracellular transport of the mRNA transcripts. Despite the rareness of conventional organelles, first and foremost a nucleus, pieces of evidence for mRNA localization to specific subcellular domains, where their protein products function, have also been obtained for prokaryotes. Although the underlying mechanisms for transcript localization in bacteria are yet to be unraveled, it is now obvious that intracellular localization of mRNA is a common mechanism to spatially localize proteins in both eukaryotes and prokaryotes. This article is part of a Special Issue entitled: Protein trafficking and secretion in bacteria. Guest Editors: Anastassios Economou and Ross Dalbey.