View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Elsevier - Publisher Connector Current Biology, Vol. 12, R50–R52, January 22, 2002, ©2002 Elsevier Science Ltd. All rights reserved. PII S0960-9822(01)00671-6 RNA Processing: Splicing and the Dispatch Cytoplasmic Localisation of mRNA Isabel M. Palacios Mago is a Component of the Exon–Exon Junction Complex There is now substantial evidence that post-transcrip- An unexpected link has been discovered between tional events in the nucleus and the cytoplasm can be pre-mRNA splicing in the nucleus and mRNA linked [10]. Pre-messenger RNA (mRNA) splicing can localisation in the cytoplasm. The new findings affect the structure and composition of the ribonucle- suggest that recruitment of the Mago Nashi and oprotein particles (mRNPs), and thereby influence Y14 proteins upon splicing of oskar mRNA is an downstream processing events. For example, the essential step in the localisation of the RNA to the presence of an intron can enhance the efficiency of posterior pole of the Drosophila oocyte. both the nuclear export and translation of mRNA. It has also been shown that mRNAs containing an exon–exon junction more than 50 nucleotides down- Intracellular localisation of mRNA is a common way of stream of an in-frame termination codon are targeted targeting proteins to the regions of the cell where they for nonsense-mediated mRNA decay. This implies that are required. Some of the best characterised exam- mRNAs may carry a ‘mark’ that indicates where an ples of localised mRNAs are found in the Drosophila intron was located in their precursors. In human cell oocyte, where their localisation is essential to define extracts, the spliceosome has been shown to deposit the anterior-posterior and the dorsal-ventral body a multiprotein complex at a conserved position 20–24 axes [1]. For example, the microtubule-dependent nucleotides upstream of an exon–exon junction [11]. localisation of oskar mRNA to the posterior pole of the The first five components of this exon–exon junction oocyte specifies the formation of the pole plasm, a complex to be identified were SRm160, DEK, RNPS1, specialised cytoplasm that contains the determinants Aly/REF and Y14 [11–14]. required for the development of the abdomen and the Y14 is an RNA-binding protein that binds preferen- germline lineage. tially to spliced mRNAs immediately upstream of the The Drosophila egg chamber is composed of 16 exon–exon junctions, and remains bound to the mRNA germ cells, including 15 nurse cells and one oocyte, after nuclear export. Y14 thus has the expected char- surrounded by a layer of somatic follicle cells. The acteristics of an ‘intron marker’ [13,15]. Furthermore, oocyte grows as the nurse cells transfer their cyto- Y14 interacts with the nonsense-mediated mRNA plasmic contents into it. A transcript that localises at decay factor hUpf3, a further component of the the posterior pole of the oocyte, such as oskar mRNA, exon–exon junction complex [14,16], and RNPS1 has is therefore transcribed in the nurse cell nuclei, carried recently been shown to recruit other factors that into the oocyte and finally transported from the ante- mediate nonsense-mediated mRNA decay [17]. Finally, rior to the posterior pole of the oocyte. In mago nashi the exon–exon junction complex facilitates recruit- (mago) mutant oocytes, oskar mRNA remains at the ment of the heterodimeric nuclear export receptor anterior of the oocyte and never reaches the posterior TAP/p15 to spliced mRNAs, probably by interact- pole (Figure 1) [2–5]. ing with Aly/REF [14,18]. The splicing-dependent Thus, Mago protein has a specific function in oskar exon–exon junction complex thus has a function in mRNA localization and co-localizes with the transcript loading the mRNA with the right sort of proteins for at the posterior of the oocyte. However, Mago is an nuclear export, as well as for nonsense-mediated essential, highly conserved and predominantly nuclear mRNA decay. protein that also plays a role in the polarisation of the The human homologue of Mago (Magoh) has oocyte. These observations suggest functions beyond recently been shown to interact with Y14, forming a oskar mRNA localisation, and the discovery that Mago stable heterodimeric complex [19,20]. This suggested is a component of the exon–exon junction complex that Mago may be a functional component of the may begin to answer the remaining questions about exon–exon junction complex, and recent studies [8,9] Mago function. indicate that this is indeed so. Drosophila Mago Recent studies [6–9] — one published recently (DmMago) has been found to bind avidly and specifi- in Current Biology [6] — indicate that the recruitment cally to Drosophila Y14 (also called Tsunagi), forming of Mago and interacting proteins upon splicing of a stable complex that localises in the nucleus, with oskar mRNA is an essential step in the localization nucleolar exclusion. DmMago and its human homo- of the mRNA to the posterior pole of the Drosophila logue are both components of the exon–exon junction oocyte. complex, and like Y14, accompany spliced mRNAs to the cytoplasm. Mago, a protein essential for the pos- Wellcome/CRC Institute, Department of Genetics, University terior localisation of oskar mRNA, is thus a component of Cambridge, Tennis Court Road, CB2 1QR Cambridge, UK. of the exon–exon junction complex that binds directly E-mail: [email protected] to Y14 [8,9]. Current Biology R51 Figure 1. Drosophila oogenesis. A Anterior Posterior (A) The Drosophila ovariole is composed Germarium Stage Stage Stage 9 Stage 13 of the germarium and a series of egg 2-4 5-7 chambers. A single cystoblast, situated at the anterior tip of the germarium, under- Oocyte goes four incomplete divisions to form a Nurse cyst of 16 germline cells interconnected cells by specialised structures called ring canals. Only one cell within the cyst adopts the oocyte fate, and localises to oskar mRNA the posterior of the egg chamber, while B Follicle cells the other 15 cells undergo DNA endo- reduplication and develop as nurse cells. (B) The oocyte grows as it receives cyto- plasmic contents (orange) from the tran- scriptionally active nurse cells, which Oocyte ultimately undergo apoptosis at the end Nurse cells of oogenesis. A layer of somatic follicle Mago/Y14 cells surrounds the germ line cells. At stage 8/9 of oogenesis, oskar mRNA C (i) (ii) (blue) is transported from the nurse cells through the ring canals into the oocyte and, within the oocyte, from the anterior to the posterior pole. At this stage, Mago and Y14 proteins (red) are localised in the nuclei and at the posterior pole. (C) Local- isation of oskar mRNA at stage 9 of ooge- Current Biology nesis in (i) wild-type and (ii) mago nashi mutant egg chambers. In wild-type egg chambers, oskar mRNA localises to the posterior pole of the oocyte. This localisation is completely abolished in mago nashi mutant oocytes, and oskar mRNA is only detected at the anterior pole. In both panels, oskar mRNA is detected by in situ hybridisation. Interestingly, Magoh also binds to the nuclear posterior pole of the oocyte. Last but not least, in y14 export factor TAP, though not to other constituents of mutants, oskar mRNA localisation to the posterior the complex [8]. This observation, together with the pole of the oocyte is abolished, without affecting the interaction of Y14 with Aly/REF and TAP, suggested a nuclear export and translocation of the mRNA from possible role for the Magoh–Y14 complex in mRNA the nurse cells into the oocyte [6,7]. nuclear export. This seems not to be the case, Mago and Y14, two proteins required for oskar mRNA however, as depletion of DmMago–Y14 by double- localization to the posterior pole in the Drosophila stranded RNA-mediated interference (RNAi) in oocyte, are thus components of the exon–exon junction Drosophila cultured cells was found to have no effect complex (Figure 2). These findings suggest that recruit- on nuclear export of bulk mRNAs, although involve- ment of the Mago–Y14 complex upon splicing of oskar ment in the export of specific mRNAs cannot be ruled mRNA is an essential step in the assembly of the poste- out. RNAi against DmMago and Y14 was found to rior localization machinery. Several questions remain to cause a similar inhibition of cell growth to that be answered. For example, is splicing of oskar mRNA observed on depletion of Upf1, suggesting that the essential for its localisation to the posterior pole of the effect may be due to the deficiency of nonsense- oocyte, or could the Y14–Mago complex be deposited mediated mRNA decay [9]. on the transcript by some other means? Is the machin- ery of nonsense-mediated mRNA decay required for Y14 is Required for Localisation of oskar mRNA mRNA localisation, and are the other components of the to the Posterior Pole of the Oocyte oskar mRNA localization machinery reciprocally The identification of the heterodimeric Y14–Mago required for nonsense-mediated mRNA decay? Finally, complex in Drosophila also raises the possibility that do Mago and Y14 function in the post-splicing metabo- the exon–exon junction complex plays a role in the lism of other RNAs? posterior localisation of oskar mRNA in the oocyte. Two groups have recently shown that this is indeed References the case for Y14 [6,7]. First, the heterodimer 1. Palacios, I.M., and St. Johnston, D. (2001) Getting the message across: the intracellular localization of mRNAs in DmMago–Y14 was found to localise in the nuclei of all higher eukaryotes. Annu. Rev. Cell Dev. Biol. 17, 569–614. the cells in the egg chamber.