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Stress Granules (SG) and Processing Bodies (PB) are cytoplasmic mRNA silencing foci in dynamic equilibrium with translating polysomes. Whereas SG form transiently during the integrated stress response, PB are constitutive and can be additionally induced upon stress, being the two foci in close contact. Staufen 1 is a double-stranded RNA binding protein associated to polysomes and present in RNA granules involved in microfilament- and microtubule-dependent transport in insect and mammalian cells. Using specific antibodies against mammalian Staufen 1 (Craig et al., Proteins 2005), we found that this molecule is recruited to both SG and PB upon induction of endoplasmic reticulum or oxidative stress in mammalian cells (Thomas et al., MBC 2005; Baez and Boccaccio JBC, 2005). However, we found that PB and SG lacking Staufen 1 form in cells that had been depleted of this molecule, indicating that Staufen 1 is not an essential component of these foci. Moreover, Staufen 1 depletion facilitates SG formation upon oxidative or ER-stress induction. Conversely, transient transfection of Staufen 1 impairs the growth of SG and PB upon stress. Downregulation of foci formation by Staufen 1 does not involve inhibition of the stress-induced phosphorylation of eIF2a, a key event for SG assembly. We have mapped the Staufen 1 inhibitory domain to the amino-terminal half of the molecule known to bind to polysomes. In addition, we found that Staufen 1 associates to a subset of stress-resistant polysomes. By the other hand, the aggregation of perinuclear SG implies an active mRNP transport and we found that microtubules and microfilaments are required at distinct steps during SG formation. Our results suggest that Staufen 1 modulates SG formation downstream of the stress signaling by: 1) preventing the stress-induced polysome disassembly; 2) medianting an active, motor-dependent transport of mRNPs out of the foci