SummaryMammalian Staufen 1 is recruited to stress granules and impairs their assembly Journal of Cell Science 564 is considered to be a global mRNA-binding factor, as it was shown to bind to different RNA motifs, including G-quartets, the β-actin zip code, the Myc 3ЈUTR; the FMR1 3ЈUTR, the HIV transactivating response region and the so-called Staufen-binding motif (Rackham and Brown, 2004;Dugré-Brisson et al., 2005; Kim, Y. K. et al., 2005). Recently, at least 7% of cellular mRNAs were shown to be present in Stau1 ribonucleoparticles (RNPs) (Furic et al., 2008). Consistently with this broad spectrum of targets, Staufen was associated with a variety of cytosolic functions. Staufen is involved in mRNA transport in both oocytes and somatic cells in vertebrates, as well as in invertebrates (Ferrandon et al., 1994;Broadus et al., 1998; Kiebler et al., 1999;Micklem et al., 2000;Tang et al., 2001;Belanger et al., 2003;Yoon and Mowry, 2004;Gautrey et al., 2005; Lebeau et al., 2008;Vessey et al., 2008). When bound to the 3ЈUTR, mammalian Stau1 triggers mRNA decay by recruitment of UPF1, a key molecule for mRNA degradation (Kim, Y. K. et al., 2005). By contrast, it enhances CAP-dependent translation when tethered to the 5ЈUTR (Dugré-Brisson et al., 2005). In addition to these cytoplasmic functions, nuclear roles for mammalian Stau1 molecules have begun to emerge (Kiebler et al., 2005). Whether Stau1 functions contribute to stress granule physiology is unknown.In this work we show that, although mammalian Stau1 is always present in stress granules, either induced by cellular stress or by pharmacological inhibition of translation initiation, is not an essential component of these foci. Moreover, we found that Stau1 depletion facilitated stress granule assembly, whereas transfection of Stau1 constructs inhibited their formation. Processing body integrity was moderately affected by Stau1, according to our finding that Stau1 is scarcely recruited to these structures. We show that Stau1 associates with polysomes, protecting them from stressinduced breakdown. We propose that downstream of the stress signaling, the breakdown of polysomes and concomitant stress granule formation are regulated by Stau1. Results Presence of Stau1 in stress granules and processing bodiesWe have previously shown that Stau1 is recruited to stress granules in brain primary cell cultures upon induction of oxidative stress . Here, we analyzed the presence of Stau1 in stress granules induced in transformed and non-transformed cell lines of distinct origin. Using a specific antibody against Stau1 (Craig et al., 2005), we found that this molecule was present in stress granules induced in NIH 3T3, HeLa, BHK, COS-7, WI-38, H1299 and U2OS cells, upon exposure to the calcium-pump inhibitor thapsigargin -a known inductor of endoplasmic reticulum stress -as well as in stress granules induced by arsenite, which induces oxidative stress ( Fig. 1 and M.G.T., L.J.M.T. and G.L.B., unpublished data). Stau1 was observed to localize in every stress granule identified by...

INTRODUCTIONFrom the early steps of mRNA transport to the latest events of degradation, cytoplasmic RNA granules are highly relevant to the physiology of mRNA, including silencing and activation (reviewed in Wickens and Goldstrohm, 2003). Granules packaging targeted mRNAs appear in oligodendrocytes and other polarized vertebrate cells as dense structures, containing also ribosomes and with an average diameter of 1 m ( Barbarese et al., 1995;Barry et al., 1996;Ainger et al., 1997;Knowles and Kosik, 1997;Carson et al., 2001;Krichevsky and Kosik, 2001). A different type of RNA granules known as stress granules (SGs) appears transiently upon induction of cellular stress. SGs are large ribonucleoparticles (RNPs) and are thought to be in dynamic equilibrium with translating polysomes (Kedersha et al., 2000Anderson and Kedersha, 2002).The double-stranded RNA-binding protein Staufen emerges as a relatively ubiquitous RNA granule-forming factor (Ferrandon et al., 1994;Duchaine et al., 2000; Kiebler and DesGrosseillers, 2000;Micklem et al., 2000). This protein was initially described in Drosophila oocytes, where it is found in granules involved in microtubule-dependent localization of maternal mRNAs to define the anterior-posterior axis of the embryo (Lasko, 1999;Kloc et al., 2002). Staufen is recruited into granules upon its interaction with bicoid mRNA 3ЈUTR sequences that mediate targeting of the messenger, and it is strictly required for the formation of these granules (Ferrandon et al., 1994(Ferrandon et al., , 1997. Likewise, the positioning of oskar mRNA at the oocyte posterior pole involves the formation of Staufen-containing granular structures known as polar bodies (Lasko, 1999;Kloc et al., 2002). Staufen also participates in actin-dependent segregation of prospero mRNA during asymmetric division of embryonic CNS cells (reviewed in Lasko, 1999; Kiebler and DesGrosseillers, 2000;Kloc et al., 2002). Moreover, Drosophila Staufen is essential for long-term memory acquisition, a phenomenon known to require mRNA targeting followed by local translation at the synapse (Dubnau et al., 2003).Two homologous genes, Staufen 1 and Staufen 2, were reported in mammalians and amphibians (Kiebler and DesGrosseillers, 2000;Monshausen et al., 2001;Tang et al., 2001;Kress et al., 2004;Yoon and Mowry, 2004 et al., , Kohrmann et al., 1999Macchi et al., 2003). Rat Staufen 1 binds to the dendrite targeting element (DTE) of MAP2 mRNA (Monshausen et al., 2001) and, in addition, Staufen 1 RNPs isolated from brain and cortical neurons contain localized RNAs and associate to motor molecules (Krichevsky and Kosik, 2001;Ohashi et al., 2002;Mallardo et al., 2003;Kanai et al., 2004). Furthermore, overexpression of a truncated form of Staufen 2 leads to a reduction of the dendritic RNA content (Tang et al., 2001). Likewise, interference strategies in amphibian oocytes indicates that Xenopus Staufen 1 is involved in the late localization pathway to the vegetal pole (Kress et al., 2004;Yoon and Mowry, 2004). In this study, we investigated the dist...