Global protein synthesis is emerging as an important player in the context of aging and age-related diseases. However, the intricate molecular networks that regulate protein synthesis are poorly understood. Here, we report that SIRT6, a nuclear-localized histone deacetylase represses global protein synthesis by transcriptionally regulating mTOR signalling via the transcription factor Sp1, independent of its deacetylase activity. Our results suggest that SIRT6 deficiency increases protein synthesis in mice. Further, multiple lines of in vitro evidence suggest that SIRT6 negatively regulates protein synthesis in a cell-autonomous fashion and independent of its catalytic activity. Mechanistically, SIRT6 binds to the zinc finger DNA binding domain of Sp1 and represses its activity. SIRT6 deficiency increased the occupancy of Sp1 at key mTOR signalling gene promoters resulting in enhanced expression of these genes and activation of the mTOR signalling pathway. Interestingly, inhibition of either mTOR or Sp1 abrogated the increased protein synthesis observed under SIRT6 deficient conditions. Moreover, pharmacological inhibition of mTOR restored cardiac function in muscle-specific SIRT6 knockout mice, which spontaneously develop cardiac hypertrophy. Overall, these findings have unravelled a new layer of regulation of global protein synthesis by SIRT6, which can be potentially targeted to combat aging-associated diseases like cardiac hypertrophy.

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The 5′ UTR of Coxsackievirus B3 (CVB3) contains internal ribosome entry site (IRES), which allows cap-independent translation of the viral RNA and a 5′-terminal cloverleaf structure that regulates viral replication, translation and stability. Here, we demonstrate that host protein PSF (PTB associated splicing factor) interacts with the cloverleaf RNA as well as the IRES element. PSF was found to be an important IRES trans acting factor (ITAF) for efficient translation of CVB3 RNA. Interestingly, cytoplasmic abundance of PSF protein increased during CVB3 infection and this is regulated by phosphorylation status at two different amino acid positions. Further, PSF protein was up-regulated in CVB3 infection. The expression of CVB3–2A protease alone could also induce increased PSF protein levels. Furthermore, we observed the presence of an IRES element in the 5′UTR of PSF mRNA, which is activated during CVB3 infection and might contribute to the elevated levels of PSF. It appears that PSF IRES is also positively regulated by PTB, which is known to regulate CVB3 IRES. Taken together, the results suggest for the first time a novel mechanism of regulations of ITAFs during viral infection, where an ITAF undergoes IRES mediated translation, sustaining its protein levels under condition of translation shut-off.

Coxsackievirus B3 is an enterovirus, with positive-sense single-stranded RNA genome containing 'Internal Ribosome Entry Site' (IRES) in the 5ʹUTR. Once sufficient viral proteins are synthesized in the cell from the input RNA, viral template switches from translation to replication to synthesize negativestrand RNA. Inhibition of translation is a key step in regulating this switch as the positive-strand RNA template should be free of ribosomes to enable polymerase movement. In this study, we show how a host protein hnRNP C1/C2 inhibits viral RNA translation. hnRNP C1/C2 interacts with stem-loop V in the IRES and displaces poly-pyrimidine tract binding protein, a positive regulator of translation. We further demonstrate that hnRNP C1/C2 induces translation to replication switch, independently from the already known role of the ternary complex (PCBP2-3CD-cloverleaf RNA). These results suggest a novel function of hnRNP C1/C2 in template switching of positive-strand from translation to replication by a new mechanism. Using mathematical modelling, we show that the differential affinity of hnRNP C1/C2 for positive and negative-strand RNAs guides the final ± RNA ratio, providing first insight in the regulation of the positive to negative-strand RNA ratio in enteroviruses.

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