She2p, a novel RNA-binding protein tethers ASH1 mRNA to the Myo4p myosin motor via She3p

Membrane-associated mRNAs: A Post-transcriptional Pathway for Fine-turning Gene Expression

Gene expression is a fundamental and highly regulated process involving a series of tightly coordinated steps, including transcription, post-transcriptional processing, translation, and post-translational modifications. A growing number of studies have revealed an additional layer of complexity in gene expression through the phenomenon of mRNA subcellular localization. mRNAs can be organized into membraneless subcellular structures within both the cytoplasm and the nucleus, but they can also targeted to membranes. In this review, we will summarize in particular our knowledge on localization of mRNAs to organelles, focusing on important regulators and available techniques for studying organellar localization, and significance of this localization in the broader context of gene expression regulation.

Polarity-dependent expression and localization of secretory glucoamylase mRNA in filamentous fungal cells

In multinuclear and multicellular filamentous fungi little is known about how mRNAs encoding secreted enzymes are transcribed and localized spatiotemporally. To better understand this process we analyzed mRNA encoding GlaA, a glucoamylase secreted in large amounts by the industrial filamentous fungus Aspergillus oryzae, by the MS2 system, in which mRNA can be visualized in living cells. We found that glaA mRNA was significantly transcribed and localized near the hyphal tip and septum, which are the sites of protein secretion, in polarity-dependent expression and localization manners. We also revealed that glaA mRNA exhibits long-range dynamics in the vicinity of the endoplasmic reticulum (ER) in a manner that is dependent on the microtubule motor proteins kinesin-1 and kinesin-3, but independent of early endosomes. Moreover, we elucidated that although glaA mRNA localized to stress granules (SGs) and processing bodies (PBs) under high temperature, glaA mRNA was not seen under ER stress, suggesting that there are different regulatory mechanisms of glaA mRNA by SG and PB under high temperature and ER stress. Collectively, this study uncovers a dynamic regulatory mechanism of mRNA encoding a secretory enzyme in filamentous fungi.

Hitting the mark: Localization of mRNA and biomolecular condensates in health and disease

Subcellular mRNA localization is critical to a multitude of biological processes such as development of cellular polarity, embryogenesis, tissue differentiation, protein complex formation, cell migration, and rapid responses to environmental stimuli and synaptic depolarization. Our understanding of the mechanisms of mRNA localization must now be revised to include formation and trafficking of biomolecular condensates, as several biomolecular condensates that transport and localize mRNA have recently been discovered. Disruptions in mRNA localization can have catastrophic effects on developmental processes and biomolecular condensate biology and have been shown to contribute to diverse diseases. A fundamental understanding of mRNA localization is essential to understanding how aberrations in this biology contribute the etiology of numerous cancers though support of cancer cell migration and biomolecular condensate dysregulation, as well as many neurodegenerative diseases, through misregulation of mRNA localization and biomolecular condensate biology. This article is categorized under: RNA Export and Localization > RNA Localization RNA in Disease and Development > RNA in Disease RNA in Disease and Development > RNA in Development.

Phosphorylation controls the oligomeric state of She2 and mRNA localization in yeast

Messenger RNA (mRNA) localization is an important mechanism controlling local protein synthesis. In budding yeast, asymmetric localization of transcripts such as ASH1 mRNA to the bud tip depends on the She2 RNA-binding protein. She2 assembles as a tetramer to bind RNA, but the regulation of this process as part of the mRNA locasome is still unclear. Here, we performed a phosphoproteomic analysis of She2 in vivo and identified new phosphosites, several of which are located at the dimerization or tetramerization interfaces of She2. Remarkably, phosphomimetic mutations at these residues disrupt the capacity of She2 to promote Ash1 asymmetric accumulation. A detailed analysis of one of these residues, T109, shows that a T109D mutation inhibits She2 oligomerization and its interaction with She3 and the importin-α Srp1. She2 proteins harboring the T109D mutation also display reduced expression. More importantly, this phosphomimetic mutation strongly impairs the capacity of She2 to bind RNA and disrupts ASH1 mRNA localization. These results demonstrate that the control of She2 oligomerization by phosphorylation constitutes an important regulatory step in the mRNA localization pathway.

Opposing Roles of FACT for Euchromatin and Heterochromatin in Yeast

DNA is stored in the nucleus of a cell in a folded state; however, only the necessary genetic information is extracted from the required group of genes. The key to extracting genetic information is chromatin ambivalence. Depending on the chromosomal region, chromatin is characterized into low-density "euchromatin" and high-density "heterochromatin", with various factors being involved in its regulation. Here, we focus on chromatin regulation and gene expression by the yeast FACT complex, which functions in both euchromatin and heterochromatin. FACT is known as a histone H2A/H2B chaperone and was initially reported as an elongation factor associated with RNA polymerase II. In budding yeast, FACT activates promoter chromatin by interacting with the transcriptional activators SBF/MBF via the regulation of G1/S cell cycle genes. In fission yeast, FACT plays an important role in the formation of higher-order chromatin structures and transcriptional repression by binding to Swi6, an HP1 family protein, at heterochromatin. This FACT property, which refers to the alternate chromatin-regulation depending on the binding partner, is an interesting phenomenon. Further analysis of nucleosome regulation within heterochromatin is expected in future studies.

Auxin alone provokes retention of ASH1 mRNA in Saccharomyces cerevisiae mother cells

The auxin-inducible degradation (AID) system can elicit conditional and reversible protein degradation as a tool to assess the role of essential proteins. Indeed, AID enables functional studies without the possibility of adaptation, which can occur with permanent gene deletions. The AID system relies on the addition of auxin molecules, such as indole-3-acetic acid (IAA), as a means to launch the degradation of the protein of interest. In this context, it is extremely important to control for the effect of auxin addition alone. To study the role of essential proteins in the process of selective mRNA delivery to daughter cells in Saccharomyces cerevisiae , we first controlled for the effect of adding IAA to cells that cannot perform AID-mediated degradation. We found that auxin alone restricted ASH1 delivery to daughter cells, as ASH1 mRNA started being retained in the mother cell as soon as thirty minutes after IAA addition. Thus, our data warn about the danger of not systematically including auxin-treated cells incapable of degradation in every AID-related experiment. Furthermore, given previous data reporting the ability of auxin to inhibit the master growth regulator TORC1 in S. cerevisiae , our data suggest that TORC1 could control the selective delivery of mRNAs to daughter cells.

mRNA Localization to the Endoplasmic Reticulum in Plant Endosperm Cells

Subcellular mRNA localization is an evolutionarily conserved mechanism to spatially and temporally drive local translation and, in turn, protein targeting. Hence, this mechanism achieves precise control of gene expression and establishes functional and structural networks during cell growth and development as well as during stimuli response. Since its discovery in ascidian eggs, mRNA localization has been extensively studied in animal and yeast cells. Although our knowledge of subcellular mRNA localization in plant cells lags considerably behind other biological systems, mRNA localization to the endoplasmic reticulum (ER) has also been well established since its discovery in cereal endosperm cells in the early 1990s. Storage protein mRNA targeting to distinct subdomains of the ER determines efficient accumulation of the corresponding proteins in different endosomal storage sites and, in turn, underlies storage organelle biogenesis in cereal grains. The targeting process requires the presence of RNA localization elements, also called zipcodes, and specific RNA-binding proteins that recognize and bind these zipcodes and recruit other factors to mediate active transport. Here, we review the current knowledge of the mechanisms and functions of mRNA localization to the ER in plant cells and address directions for future research.

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.

Cotranslational Mechanisms of Protein Biogenesis and Complex Assembly in Eukaryotes

The formation of protein complexes is crucial to most biological functions. The cellular mechanisms governing protein complex biogenesis are not yet well understood, but some principles of cotranslational and posttranslational assembly are beginning to emerge. In bacteria, this process is favored by operons encoding subunits of protein complexes. Eukaryotic cells do not have polycistronic mRNAs, raising the question of how they orchestrate the encounter of unassembled subunits. Here we review the constraints and mechanisms governing eukaryotic co- and posttranslational protein folding and assembly, including the influence of elongation rate on nascent chain targeting, folding, and chaperone interactions. Recent evidence shows that mRNAs encoding subunits of oligomeric assemblies can undergo localized translation and form cytoplasmic condensates that might facilitate the assembly of protein complexes. Understanding the interplay between localized mRNA translation and cotranslational proteostasis will be critical to defining protein complex assembly in vivo.

Regulation of spatially restricted gene expression: linking RNA localization and phase separation

Subcellular restriction of gene expression is crucial to the functioning of a wide variety of cell types. The cellular machinery driving spatially restricted gene expression has been studied for many years, but recent advances have highlighted novel mechanisms by which cells can generate subcellular microenvironments with specialized gene expression profiles. Particularly intriguing are recent findings that phase separation plays a role in certain RNA localization pathways. The burgeoning field of phase separation has revolutionized how we view cellular compartmentalization, revealing that, in addition to membrane-bound organelles, phase-separated cytoplasmic microenvironments - termed biomolecular condensates - are compositionally and functionally distinct from the surrounding cytoplasm, without the need for a lipid membrane. The coupling of phase separation and RNA localization allows for precise subcellular targeting, robust translational repression and dynamic recruitment of accessory proteins. Despite the growing interest in the intersection between RNA localization and phase separation, it remains to be seen how exactly components of the localization machinery, particularly motor proteins, are able to associate with these biomolecular condensates. Further studies of the formation, function, and transport of biomolecular condensates promise to provide a new mechanistic understanding of how cells restrict gene expression at a subcellular level.

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.

Seeking a Role for Translational Control by Alternative Polyadenylation in Saccharomyces cerevisiae

Alternative polyadenylation (APA) represents an important mechanism for regulating isoform-specific translation efficiency, stability, and localisation. Though some progress has been made in understanding its consequences in metazoans, the role of APA in the model organism Saccharomyces cerevisiae remains a relative mystery because, despite abundant studies on the translational state of mRNA, none differentiate mRNA isoforms’ alternative 3′-end. This review discusses the implications of alternative polyadenylation in S. cerevisiae using other organisms to draw inferences. Given the foundational role that research in this yeast has played in the discovery of the mechanisms of cleavage and polyadenylation and in the drivers of APA, it is surprising that such an inference is required. However, because advances in ribosome profiling are insensitive to APA, how it impacts translation is still unclear. To bridge the gap between widespread observed APA and the discovery of any functional consequence, we also provide a review of the experimental techniques used to uncover the functional importance of 3′ UTR isoforms on translation.

Intracellular mRNA transport and localized translation

Fine-tuning cellular physiology in response to intracellular and environmental cues requires precise temporal and spatial control of gene expression. High-resolution imaging technologies to detect mRNAs and their translation state have revealed that all living organisms localize mRNAs in subcellular compartments and create translation hotspots, enabling cells to tune gene expression locally. Therefore, mRNA localization is a conserved and integral part of gene expression regulation from prokaryotic to eukaryotic cells. In this Review, we discuss the mechanisms of mRNA transport and local mRNA translation across the kingdoms of life and at organellar, subcellular and multicellular resolution. We also discuss the properties of messenger ribonucleoprotein and higher order RNA granules and how they may influence mRNA transport and local protein synthesis. Finally, we summarize the technological developments that allow us to study mRNA localization and local translation through the simultaneous detection of mRNAs and proteins in single cells, mRNA and nascent protein single-molecule imaging, and bulk RNA and protein detection methods.

Walking the line: mechanisms underlying directional mRNA transport and localisation in neurons and beyond

Messenger RNA (mRNA) localisation enables a high degree of spatiotemporal control on protein synthesis, which contributes to establishing the asymmetric protein distribution required to set up and maintain cellular polarity. As such, a tight control of mRNA localisation is essential for many biological processes during development and in adulthood, such as body axes determination in Drosophila melanogaster and synaptic plasticity in neurons. The mechanisms controlling how mRNAs are localised, including diffusion and entrapment, local degradation and directed active transport, are largely conserved across evolution and have been under investigation for decades in different biological models. In this review, we will discuss the standing of the field regarding directional mRNA transport in light of the recent discovery that RNA can hitchhike on cytoplasmic organelles, such as endolysosomes, and the impact of these transport modalities on our understanding of neuronal function during development, adulthood and in neurodegeneration.

Myosin V-mediated transport of Snc1 and Vps10 toward the trans-Golgi network

Retrieval of cargo proteins from the endosome towards the trans-Golgi network (TGN) is a crucial intracellular process for cellular homeostasis. Its dysfunction is associated with pathogenesis of Alzheimer and Parkinson's diseases. Myosin family proteins are cellular motors walking along actin filaments by utilizing the chemical energy from ATP hydrolysis, known to involve in pleiotropic cellular trafficking pathways. However, the question of whether myosins play a role in the trafficking of Snc1 and Vps10 has not been addressed yet. The present study assesses the potential roles of all five yeast myosins in the recycling of two membrane cargo, Snc1 and Vps10. It appears that all myosins except Myo2 are not required for the Snc1 traffic, while it was found that Myo1 and 2 play important roles for Vps10 retrieval from the endosome and the vacuole. Multiple myo2 mutants harboring a point mutation in the actin binding or the cargo binding tail domain were characterized to demonstrate abnormal Vps10-GFP and GFP-Snc1 distribution phenotypes, suggesting a severe defect in their sorting and trafficking at the endosome. Furthermore, Vps10-GFP patches in all tested myo2 mutants were found to be near stationary with quantitative live cell imaging. Finally, we found that actin cables in the myo2 mutant cells were considerably disrupted, which may aggravate the trafficking of Vps10 from the endosome. Together, our results provide novel insights into the function of Myo-family proteins in the recycling traffic of Vps10 and Snc1 destined for the TGN.

Localization elements and zip codes in the intracellular transport and localization of messenger RNAs in Saccharomyces cerevisiae

Intracellular trafficking and localization of mRNAs provide a mechanism of regulation of expression of genes with excellent spatial control. mRNA localization followed by localized translation appears to be a mechanism of targeted protein sorting to a specific cell-compartment, which is linked to the establishment of cell polarity, cell asymmetry, embryonic axis determination, and neuronal plasticity in metazoans. However, the complexity of the mechanism and the components of mRNA localization in higher organisms prompted the use of the unicellular organism Saccharomyces cerevisiae as a simplified model organism to study this vital process. Current knowledge indicates that a variety of mRNAs are asymmetrically and selectively localized to the tip of the bud of the daughter cells, to the vicinity of endoplasmic reticulum, mitochondria, and nucleus in this organism, which are connected to diverse cellular processes. Interestingly, specific cis-acting RNA localization elements (LEs) or RNA zip codes play a crucial role in the localization and trafficking of these localized mRNAs by providing critical binding sites for the specific RNA-binding proteins (RBPs). In this review, we present a comprehensive account of mRNA localization in S. cerevisiae, various types of localization elements influencing the mRNA localization, and the RBPs, which bind to these LEs to implement a number of vital physiological processes. Finally, we emphasize the significance of this process by highlighting their connection to several neuropathological disorders and cancers. This article is categorized under: RNA Export and Localization > RNA Localization.

mRNA Localization in Plant Cells

Localization of mRNAs at the subcellular level is an essential mechanism for specific protein targeting and local control of protein synthesis in both eukaryotes and bacteria. While mRNA localization is well documented in metazoans, somatic cells, and microorganisms, only a handful of well-defined mRNA localization examples have been reported in vascular plants and algae. This review summarizes the function and mechanism of mRNA localization and highlights recent studies of mRNA localization in vascular plants. While the emphasis focuses on storage protein mRNA localization in rice endosperm cells, information on targeting of RNAs to organelles (chloroplasts and mitochondria) and plasmodesmata is also discussed.

The endoplasmic reticulum-the caring mother of the cell

In eukaryotic cells, various cellular functions are compartmentalized and performed by sophisticated and specialized organelles. However, the membrane-bounded organelles need to communicate with each other and with the cytoplasm, and sense the outside through the plasma membrane to coordinate various functions and to maintain cellular homeostasis. To maintain homeostasis, the information on the cellular state must be collected and appropriate responses initiated. The endoplasmic reticulum fulfils these functions. In this review, I will discuss various aspects of how the ER senses and relays information and acts to protect the cell, in what sometimes could be interpreted as an altruistic behavior.

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.

RNA Live Imaging in the Model Microorganism Ustilago maydis

An essential feature of protein expression is the tight regulation of when and where a protein is translated from its cognate mRNA. This spatiotemporal expression is particularly important in guaranteeing the correct and efficient targeting of proteins to defined subcellular sites. In order to achieve local translation, mRNAs must be deposited at specific locations. A common mechanism is the active transport of mRNAs along the actin or microtubule cytoskeleton. To study such dynamic transport processes in vivo RNA live imaging is the method of choice. This method is based on the principle that defined binding sites for a heterologous RNA-binding protein (RBP) are inserted in the 3' UTR of target mRNAs. Coexpression of the RBP fused to a fluorescent protein enables mRNA detection in vivo using fluorescence microscopy techniques. In this chapter we describe the well-established method of studying microtubule-dependent mRNA transport in the eukaryotic model microorganism Ustilago maydis. The presented experimental design and the microscopic techniques are applicable to a broad range of other organisms.

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.

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.

A jack of all trades: the RNA-binding protein vigilin

The vigilin family of proteins is evolutionarily conserved from yeast to humans and characterized by the proteins' 14 or 15 hnRNP K homology (KH) domains, typically associated with RNA-binding. Vigilin is the largest RNA-binding protein (RBP) in the KH domain-containing family and one of the largest RBP known to date. Since its identification 30 years ago, vigilin has been shown to bind over 700 mRNAs and has been associated with cancer progression and cardiovascular disease. We provide a brief historic overview of vigilin research and outline the proteins' different functions, focusing on maintenance of genome ploidy, heterochromatin formation, RNA export, as well as regulation of translation, mRNA transport, and mRNA stability. The multitude of associated functions is reflected by the large number of identified interaction partners, ranging from tRNAs, mRNAs, ribosomes and ribosome-associated proteins, to histone methyltransferases and DNA-dependent protein kinases. Most of these partners bind to vigilin's carboxyterminus, and the two most C-terminal KH domains of the protein, KH13 and KH14, represent the main mRNA-binding interface. Since the nuclear functions of vigilins in particular are not conserved, we outline a model for the basal functions of vigilins, as well as those which were acquired during the transition from unicellular organisms to metazoa. WIREs RNA 2017, 8:e1448. doi: 10.1002/wrna.1448 For further resources related to this article, please visit the WIREs website.

Saccharomyces cerevisiae MSA1 mRNA has a sequence for localization at the bud tip

MSA1 mRNA encodes Msa1p, a protein associated with the SCB-binding factor (SBF) and MCB-binding factor (MBF) complex. Msa1p promotes the transcription of G1 phase-specific genes, and is subjected to cell cycle-dependent regulation for its abundance and subcellular localization. MSA1 mRNA and Msa1p levels oscillate in the cell cycle with peaks at the late M/early G1 phase and early G1 phase, respectively. Phosphorylation by CDK1 negatively regulates the nuclear localization of Msa1p. In the present study, we identified MSA1 mRNA as a bud tip-localized mRNA in screening using a Tag-GFP system. A fragmentation analysis revealed a sequence of ~145 bases for the bud tip localization. Endogenous MSA1 mRNA localized at the bud tip in a manner that depended on SHE2. Msa1p levels were also affected by SHE2 in cells constitutively expressing MSA1 mRNA. These results suggest the existence of a regulatory mechanism for Msa1p through the localized control of MSA1 mRNA.

Messenger RNA transport in the opportunistic fungal pathogen Candida albicans

Candida albicans, a common commensal fungus, can cause disease in immunocompromised hosts ranging from mild mucosal infections to severe bloodstream infections with high mortality rates. The ability of C. albicans cells to switch between a budding yeast form and an elongated hyphal form is linked to pathogenicity in animal models. Hyphal-specific proteins such as cell-surface adhesins and secreted hydrolases facilitate tissue invasion and host cell damage, but the specific mechanisms leading to asymmetric protein localization in hyphae remain poorly understood. In many eukaryotes, directional cytoplasmic transport of messenger RNAs that encode asymmetrically localized proteins allows efficient local translation at the site of protein function. Over the past two decades, detailed mechanisms for polarized mRNA transport have been elucidated in the budding yeast Saccharomyces cerevisiae and the filamentous fungus Ustilago maydis. This review highlights recent studies of RNA-binding proteins in C. albicans that have revealed intriguing similarities to and differences from known fungal mRNA transport systems. I also discuss outstanding questions that will need to be answered to reach an in-depth understanding of C. albicans mRNA transport mechanisms and the roles of asymmetric mRNA localization in polarized growth, hyphal function, and virulence of this opportunistic pathogen.

Post-translational modification directs nuclear and hyphal tip localization of Candida albicans mRNA-binding protein Slr1

The morphological transition of the opportunistic fungal pathogen Candida albicans from budding to hyphal growth has been implicated in its ability to cause disease in animal models. Absence of SR‐like RNA‐binding protein Slr1 slows hyphal formation and decreases virulence in a systemic candidiasis model, suggesting a role for post‐transcriptional regulation in these processes. SR (serine–arginine)‐rich proteins influence multiple steps in mRNA metabolism and their localization and function are frequently controlled by modification. We now demonstrate that Slr1 binds to polyadenylated RNA and that its intracellular localization is modulated by phosphorylation and methylation. Wildtype Slr1‐GFP is predominantly nuclear, but also co‐fractionates with translating ribosomes. The non‐phosphorylatable slr1‐6SA‐GFP protein, in which six serines in SR/RS clusters are substituted with alanines, primarily localizes to the cytoplasm in budding cells. Intriguingly, hyphal cells display a slr1‐6SA‐GFP focus at the tip near the Spitzenkörper, a vesicular structure involved in molecular trafficking to the tip. The presence of slr1‐6SA‐GFP hyphal tip foci is reduced in the absence of the mRNA‐transport protein She3, suggesting that unphosphorylated Slr1 associates with mRNA–protein complexes transported to the tip. The impact of SLR1 deletion on hyphal formation and function thus may be partially due to a role in hyphal mRNA transport.

Molecular architecture and dynamics of ASH1 mRNA recognition by its mRNA-transport complex

mRNA localization is an essential mechanism of gene regulation and is required for processes such as stem-cell division, embryogenesis and neuronal plasticity. It is not known which features in the cis-acting mRNA localization elements (LEs) are specifically recognized by motor-containing transport complexes. To the best of our knowledge, no high-resolution structure is available for any LE in complex with its cognate protein complex. Using X-ray crystallography and complementary techniques, we carried out a detailed assessment of an LE of the ASH1 mRNA from yeast, its complex with its shuttling RNA-binding protein She2p, and its highly specific, cytoplasmic complex with She3p. Although the RNA alone formed a flexible stem loop, She2p binding induced marked conformational changes. However, only joining by the unstructured She3p resulted in specific RNA recognition. The notable RNA rearrangements and joint action of a globular and an unfolded RNA-binding protein offer unprecedented insights into the step-wise maturation of an mRNA-transport complex.

The Role of Microtubule Motors in mRNA Localization and Patterning Within the Drosophila Oocyte

Messenger RNA (mRNA) localization is a powerful and prevalent mechanism of post-transcriptional gene regulation, enabling the cell to produce protein at the exact location at which it is needed. The phenomenon of mRNA localization has been observed in many types of cells in organisms ranging from yeast to man. Thus, the process appears to be widespread and highly conserved. Several model systems have been used to understand the mechanism by which mRNAs are localized. One such model, and the focus of this chapter, is the egg chamber of the female Drosophila melanogaster. The polarity of the developing Drosophila oocyte and resulting embryo relies on the specific localization of three critical mRNAs: gurken, bicoid, and oskar. If these mRNAs are not localized during oogenesis, the resulting progeny will not survive. The study of these mRNAs has served as a model for understanding the general mechanisms by which mRNAs are sorted. In this chapter, we will discuss how the localization of these mRNAs enables polarity establishment. We will also discuss the role of motor proteins in the localization pathway. Finally, we will consider potential mechanisms by which mRNAs can be anchored at their site of localization. It is likely that the lessons learned using the Drosophila oocyte model system will be applicable to mRNAs that are localized in other organisms as well.

Control of Candida albicans morphology and pathogenicity by post-transcriptional mechanisms

Candida albicans is a major human fungal pathogen responsible for both systemic and mucosal infections in a wide variety of immunocompromised individuals. Because the ability of C. albicans to undergo a reversible morphological transition from yeast to filaments is important for virulence, significant research efforts have focused on mechanisms that control this transition. While transcriptional and post-translational mechanisms have been well-studied, considerably less is known about the role of post-transcriptional mechanisms. However, in recent years several discoveries have begun to shed light on this important, but understudied, area. Here, I will review a variety of post-transcriptional mechanisms that have recently been shown to control C. albicans morphology, virulence and/or virulence-related processes, including those involving alternative transcript localization, mRNA stability and translation. I will also discuss the role that these mechanisms play in other pathogens as well as the potential they may hold to serve as targets for new antifungal strategies. Ultimately, gaining a better understanding of C. albicans post-transcriptional mechanisms will significantly improve our knowledge of how morphogenesis and virulence are controlled in fungal pathogens and open new avenues for the development of novel and more effective antifungals.

ASH1 mRNP-core factors form stable complexes in absence of cargo RNA at physiological conditions

Asymmetric ASH1 mRNA transport during mitosis of budding yeast constitutes one of the best-studied examples of mRNA localization. Recently, 2 studies used in vitro motility assays to prove that motile ASH1 mRNA-transport complexes can be reconstituted entirely from recombinant factors. Both studies, however, differed in their conclusions on whether cargo RNA itself is required for particle assembly and thus activation of directional transport. Here we provide direct evidence that stable complexes do assemble in absence of RNA at physiologic conditions and even at ionic strengths above cellular levels. These results directly confirm the previous notion that the ASH1 transport machinery is not activated by the cargo RNA itself, but rather through protein-protein interactions.

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.

Afadin regulates RhoA/Rho-associated protein kinase signaling to control formation of actin stress fibers in kidney podocytes

The function of kidney podocytes is closely associated with actin cytoskeleton. Rho family small GTPase RhoA promotes stress fiber assembly through Rho-associated protein kinase (ROCK)-dependent myosin II phosphorylation and plays an important role in maintenance of actin stress fibers of podocytes. However, little is known how stress fiber assembly is regulated in podocytes. Here, we found that afadin, an actin filament-binding protein, is required for RhoA/ROCK-dependent formation of actin stress fibers in rat podocyte C7 cells. We show that depletion of afadin in C7 cells induced loss of actin stress fibers. Conversely, forced expression of afadin increased the formation of actin stress fibers. Depletion of afadin inactivated RhoA and reduced the phosphorylation of myosin II. Moreover, the DIL domain of afadin appears to be responsible for actin stress fiber formation. Thus, afadin mediates RhoA/ROCK signaling and contributes to the formation of actin stress fibers in podocyte cells.

In the right place at the right time: visualizing and understanding mRNA localization

The spatial regulation of protein translation is an efficient way to create functional and structural asymmetries in cells. Recent research has furthered our understanding of how individual cells spatially organize protein synthesis, by applying innovative technology to characterize the relationship between mRNAs and their regulatory proteins, single-mRNA trafficking dynamics, physiological effects of abrogating mRNA localization in vivo and for endogenous mRNA labelling. The implementation of new imaging technologies has yielded valuable information on mRNA localization, for example, by observing single molecules in tissues. The emerging movements and localization patterns of mRNAs in morphologically distinct unicellular organisms and in neurons have illuminated shared and specialized mechanisms of mRNA localization, and this information is complemented by transgenic and biochemical techniques that reveal the biological consequences of mRNA mislocalization.

Hooking She3p onto She2p for myosin-mediated cytoplasmic mRNA transport

The segregation of approximately two dozen distinct mRNAs from yeast mother to daughter cell cytoplasm is a classical paradigm for eukaryotic mRNA transport. The information for transport resides in an mRNA element 40-100 nt in length, known as "zipcode." Targeted transport requires properly positioned actin filaments and cooperative loading of mRNA cargo to myosin. Cargo loading to myosin uses myosin 4 protein (Myo4p), swi5p-dependent HO expression 2 protein (She2p) and 3 protein (She3p), and zipcode. We previously determined a crystal structure of Myo4p and She3p, their 1:2 stoichiometry and interactome; we furthermore showed that the motor complex assembly requires two Myo4p⋅She3p heterotrimers, one She2p tetramer, and at least a single zipcode to yield a stable complex of [Myo4p⋅She3p⋅She2p⋅zipcode] in 2:4:4:1 stoichiometry in vitro. Here, we report a structure at 2.8-Å resolution of a cocrystal of a She2p tetramer bound to a segment of She3p. In this crystal structure, the She3p segment forms a striking hook that binds to a shallow hydrophobic pocket on the surface of each She2p subunit of the tetramer. Both She3p hook and cognate She2p binding pocket are composed of highly conserved residues. We also discovered a highly conserved region of She3p upstream of its hook region. Because this region consists of basic and aromatic residues, it likely represents part of She3p's binding activity for zipcode. Because She2p also exhibits zipcode-binding activity, we suggest that "hooking" She3p onto She2p aligns each of their zipcode-binding activities into a high-affinity site, thereby linking motor assembly to zipcode.

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.

RNA localization in bacteria

One of the most important discoveries in the field of microbiology in the last two decades is that bacterial cells have intricate subcellular organization. This understanding has emerged mainly from the depiction of spatial and temporal organization of proteins in specific domains within bacterial cells, e.g., midcell, cell poles, membrane and periplasm. Because translation of bacterial RNA molecules was considered to be strictly coupled to their synthesis, they were not thought to specifically localize to regions outside the nucleoid. However, the increasing interest in RNAs, including non-coding RNAs, encouraged researchers to explore the spatial and temporal localization of RNAs in bacteria. The recent technological improvements in the field of fluorescence microscopy allowed subcellular imaging of RNAs even in the tiny bacterial cells. It has been reported by several groups, including ours that transcripts may specifically localize in such cells. Here we review what is known about localization of RNA and of the pathways that determine RNA fate in bacteria, and discuss the possible cues and mechanisms underlying these distribution patterns.

A single molecule approach to mRNA transport by a class V myosin

mRNA localization ensures correct spatial and temporal control of protein synthesis in the cell. We show that an in vitro single molecule approach, using purified recombinant full-length proteins and synthesized mRNA, provides insight into the mechanism by which localizing mRNAs are carried to their destination. A messenger ribonucleoprotein (mRNP) complex was reconstituted from a budding yeast class V myosin motor complex (Myo4p-She3p), an mRNA-binding adaptor protein (She2p), and a localizing mRNA (ASH1). The motion of the mRNP was tracked with high spatial (∼10 nm) and temporal (70 ms) resolution. Using this "bottom-up" methodology, we show that mRNA triggers the assembly of a high affinity double-headed motor-mRNA complex that moves continuously for long distances on actin filaments at physiologic ionic strength. Without mRNA, the myosin is monomeric and unable to move continuously on actin. This finding reveals an elegant strategy to ensure that only cargo-bound motors are activated for transport. Increasing the number of localization elements ("zip codes") in the mRNA enhanced both the frequency of motile events and their run length, features which likely enhance cellular localization. Future in vitro reconstitution of mRNPs with kinesin and dynein motors should similarly yield mechanistic insight into mRNA transport by microtubule-based motors.

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.

Ribonomic approaches to study the RNA-binding proteome

Gene expression is controlled through a complex interplay among mRNAs, non-coding RNAs and RNA-binding proteins (RBPs), which all assemble along with other RNA-associated factors in dynamic and functional ribonucleoprotein complexes (RNPs). To date, our understanding of RBPs is largely limited to proteins with known or predicted RNA-binding domains. However, various methods have been recently developed to capture an RNA of interest and comprehensively identify its associated RBPs. In this review, we discuss the RNA-affinity purification methods followed by mass spectrometry analysis (AP-MS); RBP screening within protein libraries and computational methods that can be used to study the RNA-binding proteome (RBPome).

In vitro reconstitution of an mRNA-transport complex reveals mechanisms of assembly and motor activation

The budding yeast SHE mRNA-transport complex dimerizes to activate processive RNA transport, irrespective of the presence of RNA cargo, and multimerizes upon binding RNAs with multiple localization elements.

The assembly and composition of ribonucleic acid (RNA)–transporting particles for asymmetric messenger RNA (mRNA) localization is not well understood. During mitosis of budding yeast, the Swi5p-dependent HO expression (SHE) complex transports a set of mRNAs into the daughter cell. We recombinantly reconstituted the core SHE complex and assessed its properties. The cytoplasmic precomplex contains only one motor and is unable to support continuous transport. However, a defined interaction with a second, RNA-bound precomplex after its nuclear export dimerizes the motor and activates processive RNA transport. The run length observed in vitro is compatible with long-distance transport in vivo. Surprisingly, SHE complexes that either contain or lack RNA cargo show similar motility properties, demonstrating that the RNA-binding protein and not its cargo activates motility. We further show that SHE complexes have a defined size but multimerize into variable particles upon binding of RNAs with multiple localization elements. Based on these findings, we provide an estimate of number, size, and composition of such multimeric SHE particles in the cell.

Multiple RNA binding protein complexes interact with the rice prolamine RNA cis-localization zipcode sequences

RNAs for the storage proteins, glutelins and prolamines, contain zipcode sequences, which target them to specific subdomains of the cortical endoplasmic reticulum in developing rice (Oryza sativa) seeds. Fifteen RNA binding proteins (RBPs) specifically bind to the prolamine zipcode sequences and are likely to play an important role in the transport and localization of this storage protein RNA. To understand the underlying basis for the binding of multiple protein species to the prolamine zipcode sequences, the relationship of five of these RBPs, RBP-A, RBP-I, RBP-J, RBP-K, and RBP-Q, were studied. These five RBPs, which belong to the heterogeneous nuclear ribonucleoprotein class, bind specifically to the 5' coding regions as well as to the 3' untranslated region zipcode RNAs but not to a control RNA sequence. Coimmunoprecipitation-immunoblot analyses in the presence or absence of ribonuclease showed that these five RBPs are assembled into three multiprotein complexes to form at least two zipcode RNA-protein assemblies. One cytoplasmic-localized zipcode assembly contained two multiprotein complexes sharing a common core consisting of RBP-J and RBP-K and either RBP-A (A-J-K) or RBP-I (I-J-K). A second zipcode assembly of possibly nuclear origin consists of a multiprotein complex containing RBP-Q and modified forms of the other protein complexes. These results suggest that prolamine RNA transport is initiated in the nucleus to form a zipcode-protein assembly, which is remodeled in the cytoplasm to target the RNA to its proper location on the cortical endoplasmic reticulum.

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.

Scp160p is required for translational efficiency of codon-optimized mRNAs in yeast

The budding yeast multi-K homology domain RNA-binding protein Scp160p binds to >1000 messenger RNAs (mRNAs) and polyribosomes, and its mammalian homolog vigilin binds transfer RNAs (tRNAs) and translation elongation factor EF1alpha. Despite its implication in translation, studies on Scp160p's molecular function are lacking to date. We applied translational profiling approaches and demonstrate that the association of a specific subset of mRNAs with ribosomes or heavy polysomes depends on Scp160p. Interaction of Scp160p with these mRNAs requires the conserved K homology domains 13 and 14. Transfer RNA pairing index analysis of Scp160p target mRNAs indicates a high degree of consecutive use of iso-decoding codons. As shown for one target mRNA encoding the glycoprotein Pry3p, Scp160p depletion results in translational downregulation but increased association with polysomes, suggesting that it is required for efficient translation elongation. Depletion of Scp160p also decreased the relative abundance of ribosome-associated tRNAs whose codons show low potential for autocorrelation on mRNAs. Conversely, tRNAs with highly autocorrelated codons in mRNAs are less impaired. Our data indicate that Scp160p might increase the efficiency of tRNA recharge, or prevent diffusion of discharged tRNAs, both of which were also proposed to be the likely basis for the translational fitness effect of tRNA pairing.

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.

The functions and regulatory principles of mRNA intracellular trafficking

The subcellular localization of RNA molecules is a key step in the control of gene expression that impacts a broad array of biological processes in different organisms and cell types. Like other aspects of posttranscriptional gene regulation discussed in this collection of reviews, the intracellular trafficking of mRNAs is modulated by a complex regulatory code implicating specific cis-regulatory elements, RNA-binding proteins, and cofactors that function combinatorially to dictate precise localization mechanisms. In this review, we first discuss the functional benefits of transcript localization, the regulatory principles involved, and specific molecular mechanisms that have been described for a few well-characterized mRNAs. We also overview some of the emerging genomic and imaging technologies that have provided significant insights into this layer of gene regulation. Finally, we highlight examples of human diseases where defective transcript localization has been documented.

Role of Loc1p in assembly and reorganization of nuclear ASH1 messenger ribonucleoprotein particles in yeast

Directional transport of mRNA is a universal feature in eukaryotes, requiring the assembly of motor-dependent RNA-transport particles. The cytoplasmic transport of mRNAs is preceded by the nuclear assembly of pre-messenger ribonucleoprotein particles (mRNPs). In budding yeast, the asymmetric synthesis of HO 1 (ASH1) pre-mRNP originates already cotranscriptionally and passes through the nucleolus before its nuclear export. The nucleolar localization of ASH1 mRNA protein 1 (Loc1p) is required for efficient ASH1 mRNA localization. Immunoprecipitation experiments have revealed that Loc1p forms cocomplexes with other components of the ASH1 transport complex. However, it remains unclear how Loc1p is recruited into this mRNP and why Loc1p is important for ASH1 mRNA localization. Here we demonstrate that Loc1p undergoes a direct and specific interaction with the ASH1 mRNA-binding Swi5p-dependent HO expression protein 2 (She2p). This cocomplex shows higher affinity and specificity for RNA bearing localization elements than the individual proteins. It also stabilizes the otherwise transient binding of She2p to ASH1 mRNA, suggesting that cooperative mRNA binding of Loc1p with She2p is the required nuclear function of Loc1p for ASH1 mRNA localization. After nuclear export, myosin-bound She3p joins the ASH1 mRNP to form a highly specific cocomplex with She2p and ASH1 mRNA. Because Loc1p is found only in the nucleus, it must be removed from the complex directly before or after export. In vitro and in vivo experiments indicate that the synergistic interaction of She2p and She3p displaces Loc1p from the ASH1 complex, allowing free Loc1p to rapidly reenter the nucle(ol)us. Together these findings suggest an ordered process of nuclear assembly and reorganization for the maturation of localizing ASH1 mRNPs.

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.