The Destinies and Destinations of RNAs
1998; Cell Press; Volume: 95; Issue: 4 Linguagem: Inglês
10.1016/s0092-8674(00)81613-x
ISSN1097-4172
Autores Tópico(s)Fungal and yeast genetics research
ResumoThe third biennial FASEB Summer Research Conference, "Intracellular RNA Sorting, Transport, and Localization," was held June 6–11 in Snowmass, Colorado. Topics included the biological functions of localized RNAs, the nature of nuclear–cytoplasmic RNA transport, the role of signaling pathways in RNA localization, the nature of cis-acting localization elements within RNAs, the proteins that bind these elements, and the cellular mechanisms that achieve cytoplasmic transport and anchoring of RNAs to specific domains within cells. Subcellular RNA localization is a means that cells use to achieve high local concentrations of protein products (reviewed in 73St. Johnston D The intracellular localization of messenger RNAs.Cell. 1995; 81: 161-170Abstract Full Text PDF PubMed Scopus (510) Google Scholar, 5Bashirullah A Cooperstock R.L Lipshitz H.D RNA localization in development.Annu. Rev. Biochem. 1998; 67: 335-394Crossref PubMed Scopus (290) Google Scholar). RNAs are localized in many different types of cells (Figure 1). Localization of mRNA is considered a more efficient way to localize protein products than targeting the proteins themselves, since a single mRNA can give rise to many protein molecules, provided that it is associated with components of the translation machinery. Indeed, in many cases mRNAs are localized as RNP complexes along with translation components, ensuring that localized translation can occur efficiently. At this meeting, abundant examples were reported that demonstrate the biological roles of localized RNAs. A striking example of mRNA localization regulating binary cell fate choices is ASH1 mRNA localization in budding cells of the yeast Saccharomyces cerevisae, where localization to the daughter cell is required for repression of mating type switching in this cell exclusively (50Long R.M Singer R.H Meng X Gonzalez I Nasmyth K Jansen R.P Mating type switching in yeast controlled by asymmetric localization of ASH1 mRNA.Science. 1997; 277: 383-387Crossref PubMed Scopus (416) Google Scholar, 80Takizawa P.A Sil A Swedlow J.R Herskowitz I Vale R.D Actin-dependent localization of an RNA encoding a cell-fate determinant in yeast.Nature. 1997; 389: 90-93Crossref PubMed Scopus (313) Google Scholar; discussed below in The Role of the Cytoskeleton, Motor Proteins, and Cellular Organelles in RNA Transport Pathways). RNA localization also underlies a binary cell fate choice that occurs in the Drosophila embryonic nervous system, described by Daniel St. Johnston (Wellcome/CRC Institute) and Chris Doe (University of Illinois) (reviewed in 28Fuerstenberg S Broadus J Doe C.Q Asymmetry and cell fate in the Drosophila embryonic CNS.Int. J. Dev. Biol. 1998; 42: 379-383PubMed Google Scholar, 63Oleynikov Y Singer R.H RNA localization different zipcodes, same postman?.Trends Cell Biol. 1998; 8: 381-383Abstract Full Text Full Text PDF PubMed Scopus (108) Google Scholar). During the generation of the Drosophila embryonic CNS, neuroblasts undergo an asymmetric cell division that produces another neuroblast, and a ganglion mother cell (GMC), which gives rise to neurons and glia. Localization of Prospero, a transcription factor, to the GMC daughter is a key event in this cell fate choice. Both Prospero and prospero mRNA are partitioned to the GMC. Staufen, a dsRNA-binding protein required for localizing maternal RNAs in the oocyte (reviewed in 73St. Johnston D The intracellular localization of messenger RNAs.Cell. 1995; 81: 161-170Abstract Full Text PDF PubMed Scopus (510) Google Scholar), binds to prospero mRNA and is required for its localization to the GMC (10Broadus J Fuerstenberg S Doe C.Q Staufen-dependent localization of prospero mRNA contributes to neuroblast daughter-cell fate.Nature. 1998; 391: 792-795Crossref PubMed Scopus (221) Google Scholar, 67Schuldt A.J Adams J.H Davidson C.M Micklem D.R Haseloff J Johnston D.S Brand A.H Miranda mediates asymmetric protein and RNA localization in the developing nervous system.Genes Dev. 1998; 12: 1847-1857Crossref PubMed Scopus (202) Google Scholar). Another protein, Miranda, is required to localize both Prospero and Staufen to the GMC. Since Prospero is independently localized to the GMC, is prospero mRNA localization gratuitous? The answer is no. While staufen mutants alone are not defective in GMC differentation, staufen is important for GMC fate, since staufen mutations enhance defects in GMC fate caused by hypomorphic prospero alleles. Thus, this binary cell fate decision appears to be controlled redundantly by localization of both prospero mRNA and Prospero to the GMC daughter cell. Several mRNAs are localized to the animal or vegetal poles of the Xenopus oocyte, and some are implicated in axial patterning of the embryo (reviewed in 66Schnapp B.J Arn E.A Deshler J.O Highett M.I RNA localization in Xenopus oocytes.Semin. Cell Dev. Biol. 1997; 8: 529-540Crossref PubMed Scopus (24) Google Scholar). Mary Lou King (University of Miami Medical School) presented definitive evidence for an essential role of one vegetally localized mRNA, VegT mRNA, in early embryogenesis. VegT encodes a T box transcription factor (86Zhang J King M.L Xenopus VegT RNA is localized to the vegetal cortex during oogenesis and encodes a novel T-box transcription factor involved in mesodermal patterning.Development. 1996; 122: 4119-4129PubMed Google Scholar). Since VegT is expressed both maternally and zygotically, the contributions of maternal and zygotic transcripts had to be separated to determine the role of the vegetally localized oocyte mRNA. This was accomplished by injecting antisense oligonucleotides into the vegetal pole of oocytes. Depletion of maternal VegT had global effects on cell fates in all regions of the embryo (87Zhang J Houston D.W King M.L Payne C Wylie C Heasman J The role of maternal VegT in establishing the primary germ layers in Xenopus embryos.Cell. 1998; 94: 515-524Abstract Full Text Full Text PDF PubMed Scopus (376) Google Scholar). The nature of the developmental defects indicated that maternal VegT is required to establish the three primary germ layers of the embryo. Ascidian eggs contain localized determinants that are important for the early embryonic development of these simple chordates. Billie Swalla (Pennsylvania State University) reported on the characterization of some of these localized maternal factors, which include both RNAs and proteins (see 79Swalla B.J Jeffery W.R Localization of ribosomal protein L5 mRNA in myoplasm during ascidian development.Dev. Genet. 1996; 19: 258-267Crossref PubMed Scopus (12) Google Scholar). RNAs localized in distinct regions of the ascidian egg include ScYC (which is likely to be the ascidian mitochondrial 16S rRNA) and L5 (which encodes a ribosomal protein), both localized in the myoplasm, and PCNA RNA, localized to the ectoplasm. Swalla described a localized protein, p58, and experiments using closely related species that suggest that p58 is required for establishing the dorsal/ventral embryonic axis. The establishment of germ cells is one of the earliest cell fate decisions made in organisms with distinct soma and germlines. Germ cell fate often correlates with the inheritance of specialized maternal egg cytoplasm, the germ plasm, enriched for mitochondria and polar granules, organelles containing proteins and RNAs (reviewed in 36Ikenishi K Germ plasm in Caenorhabditis elegans, Drosophila and Xenopus.Dev. Growth Differ. 1998; 40: 1-10Crossref PubMed Scopus (127) Google Scholar). One RNA localized to Drosophila germ plasm is the mitochondrial encoded large ribosomal RNA, mtlrRNA. Because of its mitochondrial source, classical genetic approaches were not useful to determine the functional significance of its localization. Satoru Kobayashi (University of Tsukuba, Japan) showed, by inactivating mtlrRNA with injected targeted ribozymes, that it is required for the formation of pole cells (35Iida T Kobayashi S Essential role of mitochondrially encoded large rRNA for germ-line formation in Drosophila embryos.Proc. Natl. Acad. Sci. USA. 1998; 95: 11274-11278Crossref PubMed Scopus (78) Google Scholar). Ultrastructural analysis revealed that mtlrRNA moves from its site of synthesis in mitochondria to the surface of nearby polar granules. Since mtlrRNA disappears when pole cells form, its function, while necessary, is transitory. It is not immediately apparent why it is associated with the polar granules, since mtlrRNA is normally a component of mitochondrial ribosomes. Ruth Lehmann (New York University Medical School, Skirball Institute) reported on the germ cell functions of Drosophila Nanos, which is restricted to the posterior pole by localization of its mRNA (reviewed in 30Gavis E Expeditions to the pole RNA localization in Xenopus and Drosophila.Trends Cell Biol. 1997; 7: 485-492Abstract Full Text PDF PubMed Scopus (39) Google Scholar). In its role in abdomen formation, Nanos is required, along with Pumilio, to negatively regulate the translation of maternal hunchback mRNA (reviewed in 55Macdonald P.M Smibert C.A Translational regulation of maternal mRNAs.Curr. Opin. Genet. Dev. 1996; 6: 403-407Crossref PubMed Scopus (55) Google Scholar). By genetic manipulations, embryos were produced that are missing the lethal abdominal defects normally associated with loss of maternal nanos function. Pole cells, the germ cell precursors, form in these embryos but do not migrate correctly, leading to agametic gonads. Kobayashi and Miho Asaoka (University of Tsukuba) also presented results that suggest that both Pumilio and Nanos act together to regulate gene expression in pole cells. Thus, Nanos, while not required for germ cell formation, is required for germ cell function (45Kobayashi S Yamada M Asaoka M Kitamura T Essential role of the posterior morphogen nanos for germline development in Drosophila.Nature. 1996; 380: 708-711Crossref PubMed Scopus (253) Google Scholar, 24Forbes A Lehmann R Nanos and Pumilio have critical roles in the development and function of Drosophila germline stem cells.Development. 1998; 125: 679-690Crossref PubMed Google Scholar). In nematodes, germ cell fate also correlates with the inheritance of specialized cytoplasm containing electron-dense granules called P granules. These granules, like their counterparts in flies and frogs, contain both protein and RNA. Susan Strome (Indiana University) reported on the role of a P granule–associated protein, the product of the pgl-1 gene (37Kawasaki I Shim Y.H Kirchner J Kaminker J Wood W.B Strome S PGL-1, a predicted RNA-binding component of germ granules, is essential for fertility in C. elegans.Cell. 1998; 94: 635-645Abstract Full Text Full Text PDF PubMed Scopus (285) Google Scholar). The Pgl-1 protein is predicted to be an RNA-binding protein due to the presence of an RGG motif. She showed that Pgl-1 is required for function of the germline, but not for its initial determination. P granules are present in pgl-1 mutants but are defective. Pgl-1 could function through its RNA binding capacity to localize RNA to P granules, or possibly to regulate translation of RNAs associated with P granules. Translational regulation of localized mRNAs can further refine the cellular distribution of their protein products. Several mRNAs localized in the Drosophila oocyte are subject to localization-dependent translational control: only localized transcripts are translated, while nonlocalized transcripts are not (reviewed in 55Macdonald P.M Smibert C.A Translational regulation of maternal mRNAs.Curr. Opin. Genet. Dev. 1996; 6: 403-407Crossref PubMed Scopus (55) Google Scholar). Elizabeth Gavis (Princeton University) presented surprising results that emphasize the biological importance of localization-dependent translation. The levels of nanos (nos) mRNA and oskar (osk) mRNA in bisected anterior and posterior halves of embryos were measured by Northern blots, revealing that much more of these mRNAs are unlocalized than was suspected. Gavis estimated that 96% of nos mRNA, and 82% of osk mRNA, are unlocalized. The biological significance of localization-dependent translation is underscored by these results. How is posterior translational activation achieved? Anne Ephrussi (EMBL, Heidelberg) reported that translational activation of osk mRNA requires interaction between 5′ and 3′ elements in the mRNA. The Bruno translational repressor binds specifically to osk 3′ UTR RNA sequences called BREs (Bruno response elements) (38Kim-Ha J Kerr K Macdonald P.M Translational regulation of oskar mRNA by Bruno, an ovarian RNA-binding protein, is essential.Cell. 1995; 81: 403-412Abstract Full Text PDF PubMed Scopus (378) Google Scholar). A derepressor/activator element was identified between two translation start codons in the osk 5′ UTR, along with two proteins, p50 and p68, that bind this RNA sequence (32Gunkel N Yano T Markussen F.H Olsen L.C Ephrussi A Localization-dependent translation requires a functional interaction between the 5′ and 3′ ends of oskar mRNA.Genes Dev. 1998; 12: 1652-1664Crossref PubMed Scopus (122) Google Scholar). This element functions only in the context of RNAs that undergo BRE-mediated repression. p50 is able to bind both the 5′ derepressor element as well as the BRE elements in the 3′ UTR and may function as a corepressor with Bruno when it is bound at the BRE. The function of the p68 protein is currently being investigated. Elizabeth Gavis (Princeton University) presented evidence that in the case of nos mRNA, activation of translation at the posterior pole does not require special localized activation factors. Gavis previously identified a 90 NT element in the nos 3′ UTR, the translation control element (TCE), that is required for translational repression of unlocalized nos mRNA (31Gavis E.R Lunsford L Bergsten S.E Lehmann R A conserved 90 nucleotide element mediates translational repression of nanos RNA.Development. 1996; 122: 2791-2800Crossref PubMed Google Scholar). At this meeting, Gavis showed that a region of the 3′ UTR, designated the "+2" element, which contains the TCE plus an adjacent 89 NT, can independently direct RNA localization and can also interact with translational repressors. She proposed that the +2 element can bind to either translational repressors or localization factors, but not to both. Thus, when localization factors are bound to nos mRNA at the posterior pole, translational repressors cannot bind, and translation ensues. Gurken, a TGFα homolog, is required for a set of precisely localized signaling events between the oocyte and overlying follicles cells that establish first the anterior-posterior and then the dorsal-ventral axes of the oocyte (reviewed in 48Lehmann R Cell-cell signaling, microtubules, and the loss of symmetry in the Drosophila oocyte.Cell. 1995; 83: 353-356Abstract Full Text PDF PubMed Scopus (43) Google Scholar). Restriction of Gurken to the sites in the oocyte where signaling occurs is accomplished by localization of gurken (grk) mRNA. At this meeting, localization-dependent translation of grk mRNA was also shown to regulate its expression. In early oogenesis the oocyte nucleus is positioned at the posterior pole of the oocyte along with grk mRNA, and in mid-oogenesis both are repositioned to the anterior/dorsal corner of the oocyte. Paul Lasko (McGill University) reported that Vasa, an RNA-binding protein with similarity to the DEAD box family of RNA helicases, is required for efficient translation of posteriorly localized grk mRNA (77Styhler S Nakamura A Swan A Suter B Lasko P vasa is required for GURKEN accumulation in the oocyte, and is involved in oocyte differentiation and germline cyst development.Development. 1998; 125: 1569-1578PubMed Google Scholar, 82Tomancak P Guichet A Zavorszky P Ephrussi A Oocyte polarity depends on regulation of gurken by Vasa.Development. 1998; 125: 1723-1732Crossref PubMed Google Scholar). Robert Cohen (University of Kansas) reported that translational repression also restricts the spatial distribution of Gurken. He showed that while some grk mRNA comigrates with the oocyte nucleus as it moves from the posterior to the anterior of the oocyte, some grk mRNA is left behind at the posterior pole and subsequently moves independently to the anterior cortex, where it localizes transiently to the ventral surface. This ventrally positioned grk mRNA is translationally repressed. Cohen showed that repression requires the action of the K10 gene, which probably acts indirectly since K10 protein is restricted to the oocyte nucleus. In some cases, expression of a localized mRNA may be spatially restricted within its localized domain. Joseph Yost (University of Utah) reported that in the case of Xenopus Xwnt11 expression, subcellular localization of the mRNA does not alone predict the final protein distribution. Although Xwnt11 mRNA is localized to the entire vegetal cortex of the oocyte, Xwnt11 protein is more abundant in prospective dorsal embryonic cells. Yost showed that this dorsal/ventral difference in protein levels correlated with increased polyadenylation and polysome loading of Xwnt11 mRNA in dorsal cells. Localized RNAs contain cis-acting signals that target them to domains within cells. These signals have been termed RNA "zipcodes" (39Kislauskis E.H Singer R.H Determinants of mRNA localization.Curr. Opin. Cell Biol. 1992; 6: 975-978Crossref Scopus (108) Google Scholar), since they direct an RNA to its final address within a cell. The following picture emerges from considerable genetic and molecular dissection of these RNA sequences. RNA localization signals usually lie in the 3′ UTRs of mRNAs, in regions predicted to be rich in secondary structures. Genetic studies support the in vivo existence of these secondary structures. In several instances redundant localization elements are dispersed throughout a large region of the 3′ UTR. In some cases localization elements are closely linked to cis-acting sequences that regulate translation. Paul Macdonald (Stanford University) presented results from his extensive analysis of the cis-acting RNA sequences required for bicoid (bcd) mRNA localization to the anterior pole of the Drosophila oocyte. A large segment of the bcd 3′ UTR was originally shown to contain its localization signal (56Macdonald P.M Struhl G cis-acting sequences responsible for anterior localization of bicoid mRNA in Drosophila embryos.Nature. 1988; 336: 595-598Crossref PubMed Scopus (254) Google Scholar). Recently Macdonald has shown that elements in the bcd 3′ UTR mediate two RNA localization programs; one is initiated by an RNA recognition event designated "event A," and the other is initiated by an RNA recognition event designated "event B" (53Macdonald P.M Kerr K Redundant RNA recognition events in bicoid mRNA localization.RNA. 1997; 12: 1413-1420Google Scholar). During oogenesis, event A– and event B–initiated localization programs overlap (i.e., are redundant) from stage 6 onward, but only event A is active during stages 4–6. Macdonald described a detailed analysis of the RNA sequence requirements for event A–mediated RNA localization (54Macdonald P.M Kerr K Mutational analysis of an RNA recognition element that mediates localization of bicoid mRNA.Mol. Cell Biol. 1998; 18: 3788-3795Crossref PubMed Scopus (70) Google Scholar). These sequences lie within a stem/loop (SLV) of the proposed secondary structure of the bcd 3′ UTR. The effects of point mutations in this region identified two short nucleotide bulges and adjacent nucleotides where identities are constrained, indicating that these sites constitute recognition sites for an RNA-binding protein or proteins, as well as an adjacent helical stem that may have a structural role. The existence of redundancy in the localization elements implies that proteins that bind RNA and mediate localization could be functionally redundant. Macdonald has created RNA reporters stripped of redundancy, which undergo only event A– or event B–initiated localization (53Macdonald P.M Kerr K Redundant RNA recognition events in bicoid mRNA localization.RNA. 1997; 12: 1413-1420Google Scholar). These should provide important tools for both the biochemical and genetic analysis of RNA-binding proteins that function in these redundant RNA localization pathways. The Xenopus Vg1 mRNA localization signal also consists of redundant elements spread out over a large segment of the 3′ UTR termed the Vg1 localization element (LE) (60Mowry K.L Melton D.A Vegetal messenger RNA localization directed by a 340-nt RNA sequence element in Xenopus oocytes.Science. 1992; 255: 991-994Crossref PubMed Scopus (217) Google Scholar). Several laboratories have contributed to a detailed analysis of the Vg1 LE, using complementary approaches: Kim Mowry (Brown University), Bruce Schnapp (Harvard University), and Joel Yisraeli (Hebrew University Medical School). Both the 5′ and 3′ ends of the Vg1 LE are required for localization, and several short repeated motifs, termed E1–E4, and VM1, contribute to RNA localization in quantitative and redundant fashions (17Deshler J.O Highett M.I Schnapp B.J Localization of Xenopus Vg1 mRNA by Vera protein and the endoplasmic reticulum.Science. 1997; 276: 1128-1131Crossref PubMed Scopus (234) Google Scholar and 18Deshler J.O Highett M.I Abramson T Schnapp B.J A highly conserved RNA-binding protein for cytoplasmic mRNA localization in vertebrates.Curr. Biol. 1998; 8: 489-496Abstract Full Text Full Text PDF PubMed Scopus (197) Google Scholar, 29Gautreau D Cote C.A Mowry K.L Two copies of a subelement from the Vg1 RNA localization sequence are sufficient to direct vegetal localization in Xenopus oocytes.Development. 1997; 124: 5013-5020PubMed Google Scholar, 33Havin L Git A Elisha Z Oberman F Yaniv K Schwartz S.P Standart N Yisraeli J.K RNA-binding protein conserved in both microtubule- and microfilament-based RNA localization.Genes Dev. 1998; 12: 1593-1598Crossref PubMed Scopus (183) Google Scholar). The E2 element is reported to be the binding site for Vera (see below; 18Deshler J.O Highett M.I Abramson T Schnapp B.J A highly conserved RNA-binding protein for cytoplasmic mRNA localization in vertebrates.Curr. Biol. 1998; 8: 489-496Abstract Full Text Full Text PDF PubMed Scopus (197) Google Scholar), and Mowry reported that the VM1 element is required for VgRBP60 binding (see below). Mary Lou King (University of Miami Medical School) defined further the sequence requirements for localizing Xenopus Xcat 2 and Veg T mRNAs. The Xcat 2 localization signal, located in the 3′ UTR, is relatively short, only 57 NT long, and contains E1 and E2-like repeats, the significance of which is being tested. In contrast, the VegT 3′ UTR localization signal is more similar to the Vg1 LE, in that it is long and bipartite, with sequence requirements at both ends which contain E2 repeats. Robert Cohen (University of Kansas) reported an exception to the rule that localization signals lie in the 3′ UTR: he mapped the Drosophila gurken mRNA localization signal to the first 100 NT of the transcript. John Carson (University of Connecticut Health Sciences Center) described two elements within the 3′ UTR of myelin basic protein (MBP) mRNA, required for distinct steps in its localization to the processes of oligodendrocytes: the 21 nucleotide RNA transport sequence (RTS) and a longer RNA localization region (RLR) (2Ainger K Avossa D Diana A.S Barry C Barbarese E Carson J.H Transport and localization elements in myelin basic protein mRNA.J. Cell Biol. 1997; 138: 1077-1087Crossref PubMed Scopus (259) Google Scholar). Carson showed that the RTS sequence is bifunctional, conferring both transport and translation enhancement to heterologous reporter RNAs. In many cases, localized RNAs are transported as large RNA/protein complexes that can be seen by light microscopy. Staufen, a ds RNA-binding protein that can form particles in association with bcd mRNA (23Ferrandon D Elphick L Nusslein-Volhard C St. Johnston D Staufen protein associates with the 3′UTR of bicoid mRNA to form particles that move in a microtubule-dependent manner.Cell. 1994; 79: 1221-1232Abstract Full Text PDF PubMed Scopus (351) Google Scholar), is known to function in RNA localization pathways because of the effects of staufen mutations (reviewed in 73St. Johnston D The intracellular localization of messenger RNAs.Cell. 1995; 81: 161-170Abstract Full Text PDF PubMed Scopus (510) Google Scholar; see also 10Broadus J Fuerstenberg S Doe C.Q Staufen-dependent localization of prospero mRNA contributes to neuroblast daughter-cell fate.Nature. 1998; 391: 792-795Crossref PubMed Scopus (221) Google Scholar, 67Schuldt A.J Adams J.H Davidson C.M Micklem D.R Haseloff J Johnston D.S Brand A.H Miranda mediates asymmetric protein and RNA localization in the developing nervous system.Genes Dev. 1998; 12: 1847-1857Crossref PubMed Scopus (202) Google Scholar). Biochemical approaches have recently been used to identify components of RNA transport particles by searching for proteins that bind cis-acting RNA localization elements. Exl protein, which binds a bcd RNA localization element (57MacDonald P.M Leask A Kerr K exl protein specifically binds BLE1, a bicoid mRNA localization element, and is required for one phase of its activity.Proc. Natl. Acad. Sci. USA. 1995; 92: 10787-10791Crossref PubMed Scopus (37) Google Scholar), was discovered in this manner. At this meeting, several RNA-binding proteins implicated in RNA localization were reported. A particularly interesting theme that is emerging from these studies is that the same RNA-binding protein may mediate RNA localization in different cell types, and in different species, via pathways with different cytoskeletal requirements (63Oleynikov Y Singer R.H RNA localization different zipcodes, same postman?.Trends Cell Biol. 1998; 8: 381-383Abstract Full Text Full Text PDF PubMed Scopus (108) Google Scholar). Robert Singer (Albert Einstein College of Medicine) described the characterization of zipcode-binding protein 1 (ZBP-1), a protein isolated from chick fibroblasts (65Ross A.F Oleynikov Y Kislauskis E.H Taneja K.L Singer R.H Characterization of a beta-actin mRNA zipcode-binding protein.Mol. Cell Biol. 1997; 17: 2158-2165Crossref PubMed Google Scholar). ZBP-1 binds specifically to the proximal portion of the 54 NT β-actin RNA localization element (the "zipcode"), which targets the RNA to the leading edge of motile fibroblasts (40Kislauskis E.H Zhu X Singer R.H Sequences responsible for intracellular localization of beta-actin messenger RNA also affect cell phenotype.J. Cell Biol. 1994; 127: 441-451Crossref PubMed Scopus (416) Google Scholar). ZBP-1 contains five RNA-binding domains: an RRM-type RNA-binding domain and four KH domains, as well as nuclear import and export signals. Singer presented recent results of Yuri Oleynikov that show that ZBP-1 colocalizes with β-actin mRNA, and that overexpression of ZBP-1 causes increased localization of β-actin mRNA. To identify RNA-binding proteins required for localization of Vg1 mRNA in Xenopus oocytes, three labs working independently used biochemical approaches to identify several proteins that bind specifically to the Vg1 RNA localization element, Vg1 LE (68Schwartz S.P Aisenthal L Elisha Z Oberman F Yisraeli J.K A 69-kDa RNA-binding protein from Xenopus oocytes recognizes a common motif in two vegetally localized maternal mRNAs.Proc. Natl. Acad. Sci. USA. 1992; 89: 11895-11899Crossref PubMed Scopus (79) Google Scholar, 59Mowry K.L Complex formation between stage-specific oocyte factors and a Xenopus mRNA localization element.Proc. Natl. Acad. Sci. USA. 1996; 93: 14608-14613Crossref PubMed Scopus (60) Google Scholar, 17Deshler J.O Highett M.I Schnapp B.J Localization of Xenopus Vg1 mRNA by Vera protein and the endoplasmic reticulum.Science. 1997; 276: 1128-1131Crossref PubMed Scopus (234) Google Scholar). Bruce Schnapp (Harvard University) and Joel Yisraeli (Hebrew University Medical School) reported that proteins their labs had identified as Vera (Schnapp) or Vg1 RBP (Yisraeli) are in fact the same protein, and that this protein is the Xenopus homolog of chicken ZBP-1 (18Deshler J.O Highett M.I Abramson T Schnapp B.J A highly conserved RNA-binding protein for cytoplasmic mRNA localization in vertebrates.Curr. Biol. 1998; 8: 489-496Abstract Full Text Full Text PDF PubMed Scopus (197) Google Scholar, 33Havin L Git A Elisha Z Oberman F Yaniv K Schwartz S.P Standart N Yisraeli J.K RNA-binding protein conserved in both microtubule- and microfilament-based RNA localization.Genes Dev. 1998; 12: 1593-1598Crossref PubMed Scopus (183) Google Scholar). There is also a human homolog of unknown function, KOC (KH-domain protein overexpressed in cancer cells). A role for these proteins in localizing β-actin and Vg1 mRNAs is strongly suggested by the fact that mutant versions of the β-actin or Vg1 mRNA localization elements that perturb RNA localization also fail to bind ZBP-1 or Vera/Vg1 RBP, respectively. In addition, Yisraeli showed that the distribution of Vg1 RBP in the cytoplasm precisely mirrors that of Vg1 mRNA. This unexpected result, that the homologous protein in Xenopus and chicken is implicated in RNA localization, suggests that RNA localization pathways in vertebrates may share common, essential features. While several mRNAs selectively localize to dendrites in adult neurons, but are excluded from axons, the situation is different in cultured developing neurons, where some mRNAs are targeted to the growth cones of both immature dendrites and axons (reviewed in 71Steward O mRNA localization in neurons a multipurpose mechanism?.Neuron. 1997; 18: 9-12Abstract Full Text Full Text PDF PubMed Scopus (213) Google Scholar). Gary Bassell (Albert Einstein College of Medicine) described the case of β-actin mRNA, which is localized to granules that are sorted to dendritic and axonal growth cones (8Bassell G.J Zhang H Byrd A.L Femino A.M Singer R.H Taneja K
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