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Phosphorylation of the α-subunit of the eukaryotic initiation factor 2 (eIF2) on Ser51 is an early event associated with the down-regulation of protein synthesis at the level of translation and initiation of a transcriptional program. This constitutes a potent mechanism to overcome various stress conditions. In mammals, four eIF2α-kinases [PKR-like endoplasmic reticulum kinase (PERK), dsRNA-activated protein kinase (PKR), heme regulated inhibitor (HRI) and general control nonderepressible-2 (GCN2)], activated following specific stresses, have been shown to be involved in this process. In this article, we report that the ubiquitously expressed adaptor protein Nck, composed only of Src homology domains and classically implicated in cell signaling by activated plasma membrane receptor tyrosine kinases, modulates eIF2α-kinase-mediated eIF2αSer51 phosphorylation in a specific manner. Our results show that Nck not only prevents eIF2α phosphorylation upon PERK activation, as reported previously, but also reduces eIF2α phosphorylation in conditions leading to PKR and HRI activation. By contrast, the overexpression of Nck in mammalian cells fails to attenuate eIF2αSer51 phosphorylation in response to amino acid starvation, a stress well known to activate GCN2. This observation is further confirmed by showing that Nck fails to alter eIF2αSer51 phosphorylation in Saccharomyces cerevisiae, for which the sole eIF2α-kinase is Gcn2p. Our results suggest the existence of a novel mechanism that specifically modulates the phosphorylation of eIF2α on Ser51 under various stress conditions.