The mechanisms regulating initiation of mRNA translation for the generation of protein products that mediate interferon (IFN) responses are largely unknown. We have previously shown that both Type I and II IFNs engage the mammalian target of rapamycin (mTOR), resulting in downstream phosphorylation and deactivation of the translational repressor 4E-BP1 (eIF4E-binding protein 1). In the current study, we provide direct evidence that such regulation of 4E-BP1 by IFN␣ or IFN␥ results in sequential dissociation of 4E-BP1 from eukaryotic initiation factor-4E and subsequent formation of a functional complex between eukaryotic initiation factor-4E and eukaryotic initiation factor-4G, to allow initiation of mRNA translation. We also demonstrate that the induction of key IFN␣-or IFN␥-inducible proteins (ISG15 (interferon-stimulated Type I (␣, , ␦, ⑀, , and ) and II (␥) interferons (IFNs) 2 are pleiotropic cytokines that exhibit important antiviral, immunomodulatory, and growth inhibitory properties via engagement of widely expressed cell surface-specific receptors (1-5). The IFNs constitute the first line of the immune antiviral defense and are key components of the immune surveillance against tumors (1-5). In addition, because of their important biological properties, different recombinant IFNs (␣, , and ␥) have been used extensively as therapeutic agents in a variety of clinical syndromes in humans, and certain IFN-subtypes are approved for the treatment of human malignancies, viral syndromes, and neurologic diseases (6 -10).Over the years, the mechanisms of IFN signal transduction have been the focus of attention by many research groups and have come under extensive investigation. Since the demonstration of the existence of STAT pathways that regulate transcription of interferon-inducible genes (2, 3), there has been gradual accumulation of evidence pointing toward a complicated network of multiple signaling pathways, whose coordinated function is necessary for the generation of IFN biological responses (4, 5). For instance, it has now become apparent that, in addition to different combinations of IFN-activated JAKs and STATs, the coordinated function of the phosphatidylinositol (PI) 3-kinase and the p38 mitogen-activated protein kinase pathways are essential for optimal IFN-dependent transcriptional regulation (5). The PI 3Ј-kinase pathway appears to be required for IFN-inducible transcriptional activation by regulating phosphorylation of STAT1 on serine 727 (11, 12), probably via intermediate engagement and activation of protein kinase C-␦ (13-16). Activation of the p38 mitogen-activated protein kinase appears to be also necessary for Type I, but not II, * This work was supported by National Institutes of Health Grants CA77816, CA100579, and CA94079 (to L. C. P.), by a grant from the Department of Veterans Affairs (to L. C. P.), and Canadian Institutes of Health Research Grant MOP15094 (to E. N. F.). The costs of publication of this article were defrayed in part by the payment of page charges. This article mus...