All viral pathogens utilize cellular protein synthesis machinery for translation of their own genes and evolved to adapt to tissue-specific variations in the components of this machinery. Furthermore, modification of translational components caused by the host innate defense response or viral infection contributes to either resolution of viral infection or success of the viral pathogen. This is of particular importance for those viruses whose mRNA translation is mediated by an Internal Ribosome Entry Segment (IRES), such as Hepatitis C virus (HCV) and Hepatitis A virus (HAV). We have demonstrated for the first time that the 20S proteasome endoproteolytically cleaves two critical translation factors, elF3 and elF4F. Cleavage of elF3 or elF4F differentially affects the entry of the small ribosomal subunit, the rate-limiting step of protein synthesis, on different viral and cellular mRNAs in vitro. We report further that 26S proteasomes also endoproteolytically cleave elFs via ubiquitin- and ATP-independent processes, and differentially affect the ribosome entry on viral mRNAs. These observations suggest a novel mechanism of host cell translational control of viral mRNAs by proteasomes. These novel activities were observed in vivo where they appear to differentially regulate translation of different viral IRES-containing mRNAs in living cells: the endoproteolytic activity of proteasomes is beneficial for translation of some viral RNAs (e.g. HCV) and detrimental for others (e.g., HAV). The abundance, composition, and endoproteolytic activity of proteasomes vary significantly between organ tissues and in response to environmental cues. We propose that elFs cleavage and, consequently, translation of different viral mRNAs, is significantly modulated by proteasomes in specific tissues and under different conditions. Further, we propose that these factors affect the pathogenesis of virus-induced disease. To test these hypotheses, three specific aims will investigate the mechanism of cleavage of elFs by proteasomes in vitro and in various host tissues as well as the role of proteasomes in translational regulation of different viral mRNAs in vivo. The penultimate goal of this research is the development of novel therapeutic strategies to combat viral diseases which is of critical importance to public health. Coordinated regulation of protein synthesis and degradation is critical for proper cellular and organism function and disturbances in this regulation largely contribute to the development of multiple human disorders including cancers and Alzheimer's disease. Proteasomes are responsible for the majority of non-lysosomal protein degradation in eukaryotic cells and our finding about their involvement in translational control establishes an interesting link between protein synthesis and degradation. Hence, our research may also aid to the development of new approaches for therapeutic intervention of medically important non-viral human diseases.