Localization of maternal mRNAs in many developing organisms provides the basis for both initial polarity during oogenesis and patterning during embryogenesis. Prominent examples of this phenomenon are found in Xenopus laevis, where localized maternal mRNAs generate developmental polarity along the animal/vegetal axis. Targeting of mRNA molecules to specific subcellular regions is a fundamental mechanism for spatial regulation of gene expression, and considerable progress has been made in defining the underlying molecular pathways.

Cytoplasmic localization of mRNA molecules is a powerful mechanism for generating cell polarity. In vertebrates, one paradigm is localization of Vg1 RNA within the Xenopus oocyte, a process directed by recognition of a localization element within the Vg1 3' UTR. We show that specific base changes within the localization element abolish both localization in vivo and binding in vitro by a single protein, VgRBP60. VgRBP60 is homologous to a human hnRNP protein, hnRNP I, and combined immunolocalization and in situ hybridization demonstrate striking colocalization of hnRNP I and Vg1 RNA within the vegetal cytoplasm of the Xenopus oocyte. These results implicate a novel role in cytoplasmic RNA transport for this family of nuclear RNA-binding proteins.

SummaryIntracellular targeting of mRNAs has long been recognized as a means to produce proteins locally, but has only recently emerged as a prevalent mechanism used by a wide variety of polarized cell types. Localization of mRNA molecules within the cytoplasm provides a basis for cell polarization, thus underlying developmental processes such as asymmetric cell division, cell migration, neuronal maturation and embryonic patterning. In this review, we describe and discuss recent advances in our understanding of both the regulation and functions of RNA localization during animal development.Key words: RNA localization, RNA transport, Local translation, Cell polarity, Post-transcriptional gene regulation Introduction Establishment of cell polarity is crucial for the execution of developmental programmes governing key processes, including specification of cell fates, individual or collective cell movements and specialization of somatic cell types. Cell polarization depends on the asymmetric segregation of organelles and various molecules within the cell. Polarized accumulation of RNA molecules was first visualized nearly 30 years ago, when -actin mRNA was found to be asymmetrically localized within ascidian eggs and embryos (Jeffery et al., 1983). Following this, the discovery of the first localized maternal mRNAs in Xenopus (Rebagliati et al., 1985) and Drosophila oocytes (Frigerio et al., 1986;Berleth et al., 1988) provided evidence for the earlier proposal that localized RNA determinants could be responsible for early embryonic patterning (Kandler-Singer and Kalthoff, 1976). mRNAs were soon found to be asymmetrically distributed within differentiated somatic cells, such as fibroblasts (Lawrence and Singer, 1986), oligodendrocytes (Trapp et al., 1987) and neurons (Garner et al., 1988), and to colocalize with their encoded proteins, establishing intracellular transport of mRNAs as a potential mechanism used to target the production of selected proteins to discrete sites.Significant improvements in RNA detection methods led to the identification of a growing number of localized mRNAs. Still, in the early 2000s, the set of described targeted mRNAs was limited to ~100 (reviewed by Bashirullah et al., 1998;Palacios and St Johnston, 2001) and the process of mRNA localization was thought to be restricted to specific cell types. However, recent genome-wide analyses (see Table 1) have changed this view dramatically, and strongly suggest that subcellular targeting of mRNAs is a prevalent mechanism used by polarized cells to establish functionally distinct compartments (Fig. 1). Particularly striking was the discovery that >70% of the 2314 expressed transcripts analysed in a highresolution in situ hybridization screen were subcellularly localized in Drosophila embryos (Lécuyer et al., 2007). Moreover, hundreds to thousands of mRNAs have been detected in cellular compartments as diverse as the mitotic apparatus (Blower et al., 2007;Sharp et al., 2011), pseudopodia (Mili et al., 2008), dendrites (Moccia et al., 2003;Poon et a...

The germ cell lineage in Xenopus is specified by the inheritance of germ plasm, which originates within a distinct "mitochondrial cloud" (MC) in previtellogenic oocytes. Germ plasm contains localized RNAs implicated in germ cell development, including Xcat2 and Xdazl. To understand the mechanism of the early pathway through which RNAs localize to the MC, we applied live confocal imaging and photobleaching analysis to oocytes microinjected with fluorescent Xcat2 and Xdazl RNA constructs. These RNAs dispersed evenly throughout the cytoplasm through diffusion and then became progressively immobilized and formed aggregates in the MC. Entrapment in the MC was not prevented by microtubule disruption and did not require localization to germinal granules. Immobilized RNA constructs codistributed and showed coordinated movement with densely packed endoplasmic reticulum (ER) concentrated in the MC, as revealed with Dil16(3) labeling and immunofluorescence analysis. Vg1RBP/Vera protein, which has been implicated in linking late pathway RNAs to vegetal ER, was shown to bind specifically both wild-type Xcat2 3' untranslated region and localization-defective constructs. We found endogenous Vg1RBP/Vera and Vg1RBP/Vera-green fluorescent protein to be largely excluded from the MC but subsequently to codistribute with Xcat2 and ER at the vegetal cortex. We conclude that germ line RNAs localize into the MC through a diffusion/entrapment mechanism involving Vg1RBP/Vera-independent association with ER.

Cytoplasmic localization of mRNAs is a widespread mechanism for generating cell polarity and can provide the basis for patterning during embryonic development. A prominent example of this is localization of maternal mRNAs in Xenopus oocytes, a process requiring recognition of essential RNA sequences by protein components of the localization machinery. However, it is not yet clear how and when such protein factors associate with localized RNAs to carry out RNA transport. To trace the RNA–protein interactions that mediate RNA localization, we analyzed RNP complexes from the nucleus and cytoplasm. We find that an early step in the localization pathway is recognition of localized RNAs by specific RNA-binding proteins in the nucleus. After transport into the cytoplasm, the RNP complex is remodeled and additional transport factors are recruited. These results suggest that cytoplasmic RNA localization initiates in the nucleus and that binding of specific RNA-binding proteins in the nucleus may act to target RNAs to their appropriate destinations in the cytoplasm.

The heterogeneous nuclear ribonucleoprotein particle (hnRNP) proteins play important roles in mRNA processing in eukaryotes, but little is known about how they are regulated by cellular signaling pathways. The polypyrimidine-tract binding protein (PTB, or hnRNP I) is an important regulator of alternative pre-mRNA splicing, of viral RNA translation, and of mRNA localization. Here we show that the nucleocytoplasmic transport of PTB is regulated by the 3,5-cAMP-dependent protein kinase (PKA). PKA directly phosphorylates PTB on conserved Ser-16, and PKA activation in PC12 cells induces Ser-16 phosphorylation. PTB carrying a Ser-16 to alanine mutation accumulates normally in the nucleus. However, export of this mutant protein from the nucleus is greatly reduced in heterokaryon shuttling assays. Conversely, hyperphosphorylation of PTB by coexpression with the catalytic subunit of PKA results in the accumulation of PTB in the cytoplasm. This accumulation is again specifically blocked by the S16A mutation. Similarly, in Xenopus oocytes, the phospho-Ser-16-PTB is restricted to the cytoplasm, whereas the non-Ser-16-phosphorylated PTB is nuclear. Thus, direct PKA phosphorylation of PTB at Ser-16 modulates the nucleo-cytoplasmic distribution of PTB. This phosphorylation likely plays a role in the cytoplasmic function of PTB. T he heterogeneous nuclear ribonucleoprotein particle (hnRNP) proteins are involved in a variety of processes in mRNA metabolism including pre-mRNA splicing, mRNA transport, and translation (1). These processes are often regulated by cellular signaling pathways, but how this dynamic control is achieved is mostly unknown. Some hnRNP proteins are known to be phosphorylated, but in most cases the particular kinase that modifies the protein is not known. Moreover, it is generally not clear how such modifications affect protein function. HnRNP proteins localize primarily in the nucleus at steady state but some of them engage in nucleo-cytoplasmic shuttling (2, 3). There are examples of hnRNP localization being altered by specific signaling pathways (4, 5).The polypyrimidine tract-binding protein (PTB, or hnRNP I) has both nuclear and cytoplasmic functions. In the nucleus, it is a splicing repressor of a number of alternative exons (6, 7). In the cytoplasm, PTB plays a role in viral RNA translation through internal ribosome entry sites (8-11). Also in the cytoplasm, PTB is implicated in mRNA localization in Xenopus oocytes (12). The protein contains four RNA-recognition-motif type RNA binding domains (RRMs) and a conserved N-terminal domain. The N-terminal 55-aa segment of PTB contains both nuclear import and export signals and is sufficient to allow some nucleocytoplasmic shuttling in heterokaryon assays (13-17). There is an additional sequence within RRM2 that enhances nuclear export (17). At steady state, PTB is highly enriched in the nucleus, but its distribution must be regulated because the protein also has cytoplasmic functions. However, little is known about this regulation and what cellular signaling...

Contained within a single cell, the fertilized egg, is information that will ultimately specify the entire organism. During early embryonic cleavages, cells acquire distinct fates and their differences in developmental potential might be explained by localization of informational molecules in the egg. The mechanisms by which Vg1 RNA, a maternal mRNA, is translocated to the vegetal pole of Xenopus oocytes may indicate how developmental signals are localized. Data presented here show that a 340-nucleotide localization signal present in the 3' untranslated region of Vg1 RNA is sufficient to direct RNA localization to the vegetal pole.

In eukaryotic cells, the conversion of gene transcripts into messenger RNA's involves multiple factors, including the highly abundant small nuclear ribonucleoprotein (snRNP) complexes that mediate the splicing reaction. Separable factors are also required for the 3' end processing of histone pre-mRNA's. The two conserved signals flanking the 3' cleavage site are recognized by discrete components present in active HeLa cell extracts: the upstream stem loop associates with a nuclease-insensitive factor, while binding to the downstream element is mediated by a component having the properties of a snRNP. The sequence of the RNA moiety of the low abundance human U7 snRNP suggests how the relatively degenerate downstream element of mammalian pre-mRNA's could be recognized by RNA base-pairing.