REVIEWS POST-TRANSCRI P TIONAL CONTROL Translational control of localized mRNAs: restricting protein synthesis in space and time Florence Besse*‡ and Anne Ephrussi* Abstract | As highlighted by recent genome-wide analyses in diverse organisms and cell types, subcellular targeting of mRNAs has emerged as a major mechanism for cells to establish functionally distinct compartments and structures. For protein synthesis to be spatially restricted, translation of localizing mRNAs is silenced during their transport and is activated when they reach their final destination. Such a precise translation pattern is controlled by repressors, which are specifically recruited to transport ribonucleoprotein particles and block translation at different steps. Functional studies have revealed that the inactivation of these repressors, either by pre-localized proteins or in response to conserved signalling pathways, triggers local protein synthesis. Ribonucleoprotein complex Over the past 20 years, mRNA targeting coupled to local complex), are localized apically, thus contributing to 3,4 A multimolecular complex that translation has been recognized as a powerful means to the establishment of epithelial cell polarity . Localized is composed of mRNAs and spatially restrict the synthesis of specific proteins in mRNAs also regulate directed cell migration. In fibro- associated trans-acting factors cells. In particular, for proteins that can be deleterious blasts, the localization of B-actin mRNA coupled with its (proteins or non-coding RNAs). to the cell when expressed ectopically, the ability to translation at the leading edge promotes local cytoskeletal precisely restrict their synthesis has obvious impor- assembly, cell polarization and directional movement5. tance1. Furthermore, transcript localization allows for Similarly, during neuronal development, axonal growth the superimposition of multiple layers of control. The cones are guided by external cues that induce local syn- prevalence of this phenomenon has become apparent thesis of cytoskeleton regulators6. Semaphorin-3A, for with the advent of genome-wide analyses in diverse example, provokes growth cone collapse, which triggers organisms and cell types (Supplementary information S1 local translation of axonally targeted RHOA mRNA7. (table)). These studies have revealed that a vast number In differentiated neurons, up to hundreds of mRNAs of mRNAs display specific subcellular localizations (for are dendritically enriched, as estimated from primary example, apical–basal, membrane associated, centro- cultures (Supplementary information S1 (table)). Local some, spindle pole or astral-microtubule associated), and specific translation of a subset of these mRNAs can which indicates that mRNA localization is an important allow rapid and synapse-restricted responses to neuronal mechanism that is used by cells to establish functionally stimulation8. Segregation of cell fate and embryonic polar- distinct compartments and structures2. ity determinants is also frequently achieved by mRNA 9–11 *European Molecular What is the purpose of localizing mRNAs? Answers localization coupled to local translation . Among Biology Laboratory, can be found in diverse types of somatic cells, most of the best-studied examples is ASH1 mRNA localization Meyerhofstrasse 1, which display some form of polarization and functional to the tip of the daughter cell in the budding yeast 69117 Heidelberg, Germany. compartmentalization. For instance, whereas much is Saccharomyces cerevisiae (FIG. 1). Localization-dependent ‡Institute of Developmental Biology and Cancer, known about the composition of the complexes that translational activation of ASH1 mRNA, which encodes a CNRS UMR6543, establish and maintain epithelial cell polarity, an out- repressor of mating-type switching, ensures its restriction University of Nice Sophia standing issue is that of how their individual protein com- to the daughter cell and thus the generation of two cells of Antipolis, Parc Valrose, ponents, many of them cytoplasmic, achieve their apical distinct types, a prerequisite for mating11,12. 06108 Nice cedex 2, or basolateral membrane localization. Recent genetic Localizing mRNAs are packaged into ribonucleo- France. protein complexes e-mails: [email protected]; studies have revealed that mRNAs that encode two key (RNP complexes) that engage with [email protected] polarity regulators, Stardust and Crumbs (both are com- cytoskeletal motors for directed transport along cyto- doi:10.1038/nrm2548 ponents of the conserved apical Crumbs–Stardust–PATJ skeletal tracks (BOX 1) and ensure their translational NATURE REVIEWS | MOLECULAR CELL BIOLOGY VOLUME 9 | DECEMBER 2008 | 971 REVIEWS Mechanisms for ASH1 translational repression 60S 40S (A)n (A)n Puf6 eIF5B eIF4G eIF4G Khd1 She2 40S She2 eIF4E eIF4E Ck2 P 2 Assembly of mature Puf6 transport RNP 4 Phosphorylation (A)n and release of 2 60S Puf6 (A)n e translational Sh 1 Nuclear loading of repressors Yck1 first RNP components 40S ? Khd1 3 Transport Khd1 (A)n P Puf6 She2 Actin filament Motor (A)n Ck2 ASH1 mRNA She2 complex 5 Translation Nucleus 60S 40S Yck1 Cytoplasm Yck1 Yck1 Figure 1 | Spatial translational activation of ASH1 mRNA in budding yeast. Trans-actingNature Re factors,views |such Molecular as She2, Cell first Biolog y associate with ASH1 mRNA in the nucleus (step 1), and are subsequently exported together with the mRNA to the cytoplasm. A mature transport ribonucleoprotein particle (RNP) is then assembled (step 2) by further recruitment of motor proteins and translational repressors (Khd1 (also known as Hek2) and pumilio-homology domain family-6 (Puf6)). Note that Puf6 strongly accumulates in the nucleus but has not been shown to associate with the mRNA in this compartment. During transport along actin filaments (step 3), ASH1 mRNA translation initiation is blocked by two complementary mechanisms (inset) that prevent assembly of the eukaryotic translation initiation factor-4F (eIF4F) complex and recruitment of the 40S ribosomal subunit (Khd1-mediated mechanism; left), and prevent recruitment of the 60S ribosomal subunit (Puf6-mediated mechanism; right). After reaching the bud tip, ASH1 RNP contacts the membrane-associated kinases Yck1 (type I casein kinase) and casein kinase-II (Ck2). Phosphorylation of Khd1 and Puf6 by Yck1 and Ck2, respectively, (step 4) induces their release from the complex, and leads to translational activation of ASH1 mRNA (step 5). (A)n, polyadenine. silencing. The precise composition of these complexes Assembling a silenced RNP is dictated by the combination of cis-regulatory ele- Following export to the cell cytoplasm, the mRNAs to ments that are present on the mRNA and recognized be localized are specifically recognized by the cellular by specific trans-acting factors. Among these factors, transport machinery and must be sequestered from the conserved RNA-binding proteins have been shown to translational machinery until they reach their destination. control both targeting of the mRNA and translational Such precise sorting is achieved through the recognition repression, thereby providing a molecular link between of RNA cis-regulatory elements by trans-acting factors these two processes13–16. Their inactivation at the final and through the subsequent assembly of RNP complexes subcellular destination releases the mRNA from the of unique composition and structure. These complexes transport machinery and allows translational activation. start to assemble co-transcriptionally in the nucleus, but Depending on the cell type, mRNAs are either trans- undergo dynamic remodelling at different steps17. lationally derepressed upon arrival at their destination, or are maintained in a repressed state until specific signals Composition of transport RNP complexes. Systematic lead to their activation. proteomics analyses of RNA granule components have Here, we provide a general overview of the composi- revealed that these complexes contain a large number tion and maturation of transport RNPs. We subsequently of associated proteins, including RNA-binding proteins describe the main steps of translation that are targeted that regulate both mRNA transport and translation18–20. by translational repressors. The final two sections deal Although transport RNPs might share components with Processing body with the mechanisms that lead to spatially and tempo- processing bodies (P bodies) — general cytoplasmic sites (P body). A cytoplasmic site for mRNA degradation and the rally restricted activation of translation. In these sec- for translational silencing — they correspond to distinct storage of translationally tions, signals and their effects on downstream targets are and specific structures (BOX 2). Furthermore, whereas silenced mRNAs. mentioned and the question of specificity is discussed. some conserved RNA-binding proteins are present in 972 | DECEMBER 2008 | VOLUME 9 www.nature.com/reviews/molcellbio REVIEWS 34 Box 1 | mRNA transport mechanisms and differentially tagged in vivo in yeast or have been co-injected in cultured oligodendrocytes35, further sug- Asymmetric distribution of mRNAs in a cell can be achieved through different gest that several RNA species that are localized by the mechanisms, including localized protection from degradation (as best shown for the same transport machinery might also be co-transported Drosophila melanogaster heat-shock protein-83 (Hsp83) mRNA105) or passive diffusion in common RNPs. Formation of higher-order RNP coupled with local entrapment (as shown for D.