eIF2B-capturing viral protein NSs suppresses the integrated stress response

Abstract

Various stressors such as viral infection lead to the suppression of cap-dependent translation and the activation of the integrated stress response (ISR), since the stress-induced phosphorylated eukaryotic translation initiation factor 2 [eIF2(αP)] tightly binds to eIF2B to prevent it from exchanging guanine nucleotide molecules on its substrate, unphosphorylated eIF2. Sandfly fever Sicilian virus (SFSV) evades this cap-dependent translation suppression through the interaction between its nonstructural protein NSs and host eIF2B. However, its precise mechanism has remained unclear. Here, our cryo-electron microscopy (cryo-EM) analysis reveals that SFSV NSs binds to the α-subunit of eIF2B in a competitive manner with eIF2(αP). Together with SFSV NSs, eIF2B retains nucleotide exchange activity even in the presence of eIF2(αP), in line with the cryo-EM structures of the eIF2B•SFSV NSs•unphosphorylated eIF2 complex. A genome-wide ribosome profiling analysis clarified that SFSV NSs expressed in cultured human cells attenuates the ISR triggered by thapsigargin, an endoplasmic reticulum stress inducer. Furthermore, SFSV NSs introduced in rat hippocampal neurons and human induced-pluripotent stem (iPS) cell-derived motor neurons exhibits neuroprotective effects against the ISR-inducing stress. Since ISR inhibition is beneficial in various neurological disease models, SFSV NSs may be a promising therapeutic ISR inhibitor.

A point mutation in the nucleotide exchange factor eIF2B constitutively activates the integrated stress response by allosteric modulation

“…eIF2B is a far more dynamic complex than we realized just a year ago. Small molecules (ISRIB and its derivatives), the natural substrate (eIF2), and viral proteins (SFSV NSs) can stabilize eIF2B in its active A-State ( Kashiwagi et al, 2021 ; Schoof et al, 2021b ; Schoof et al, 2021a ; Zyryanova et al, 2021 ). Conversely, binding of the inhibitor (eIF2-P) can compete with these molecules by shifting the decamer to the inhibited I-State ( Schoof et al, 2021a ; Zyryanova et al, 2021 ).…”

Move and countermove: the integrated stress response in picorna- and coronavirus-infected cells

“…Notably, this function is not conserved in NSs from Rift Valley fever virus, another phlebovirus [74] . The SFSV NSs, BW-CoV AcP10 and AiV L proteins do not share any similarity in sequence or predicted structure 12•• , 75•• , 76•• , indicating that they arose independently via convergent evolution. Although they all prevent eIF2B-inactivation by eIF2(p), it is unknown whether they do so via the same molecular mechanism.…”

“…However, whereas binding of eIF2(p) forces the eIF2B heterodecamer into an GEF-inactive conformation, NSs binding preserves GEF activity by maintaining eIF2B's conformation in its active state thus allowing binding of eIF2 and continued GDP/GTP exchange ( Fig. 3 A) 75•• , 76•• . Currently, no structures are available of AiV-L or AcP10-bound eIF2B.…”

Regulation and function of elF2B in neurological and metabolic disorders

“…In yeast, mutations in these sugar phosphate binding sites of eIF2Bα have been shown to disperse eIF2B bodies [100]. Viral proteins counteract translational shutdown by binding to host eIF2B and induce its productive state independently of p-eIF2α [120,121]. While some additional protein factors have been shown to bind to eIF2B [122,123], the contribution of other binding partners to eIF2B activity has not received much attention.…”