Background: SAMHD1, a dGTP-activated dNTPase, inhibits retrovirus infection at the reverse transcription step in monocytes and quiescent T lymphocytes.Results: dGTP-induced SAMHD1 tetramerization correlates with its functional activation.Conclusion: SAMHD1 tetramers are the biologically active form of this dNTPase.Significance: Learning how SAMHD1 function is regulated is important for understanding innate and anti-viral immunity.

Background: Human SAMHD1 protein restricts HIV/SIV infection of myeloid cells and is targeted for proteasomal degradation by HIV-2 Vpx protein.Results: Vpx binds the divergent C terminus of human SAMHD1 and loads it onto DCAF1 substrate receptor of CRL4 E3 ubiquitin ligase.Conclusion: Vpx programs SAMHD1 degradation by loading it onto CRL4DCAF1.Significance: Learning how viruses overcome innate anti-viral mechanisms is critical for the conception of new antiviral therapeutics.

Gasdermin D (GSDMD) is an effector molecule for pyroptosis downstream of canonical and noncanonical inflammasome signaling pathways. Cleavage of GSDMD by inflammatory caspases triggers the oligomerization and lipid binding by its N-terminal domain, which assembles membrane pores, whereas its C-terminal domain binds the N-terminal domain to inhibit pyroptosis. Despite recent progress in our understanding of the structure and function of the murine gasdermin A3 (mGSDMA3), the molecular mechanisms of GSDMD activation and regulation remain poorly characterized. Here, we report the crystal structures of the full-length murine and human GSDMDs, which reveal the architecture of the GSDMD N-terminal domains and demonstrate distinct and common features of autoinhibition among gasdermin family members utilizing their b1-b2 loops. Disruption of the intramolecular domain interface enhanced pyroptosis, whereas mutations at the predicted lipid-binding or oligomerization surface reduced cytolysis. Our study provides a framework for understanding the autoinhibition, lipid binding, and oligomerization of GSDMD by using overlapping interfaces.

Regulation of cell volume is of great importance because persistent swelling or shrinkage leads to cell death. Tissues experience hypertonicity in both physiological (kidney medullar cells) and pathological states (hypernatremia). Hypertonicity induces an adaptive gene expression program that leads to cell volume recovery or apoptosis under persistent stress. We show that the commitment to apoptosis is controlled by phosphorylation of the translation initiation factor eIF2␣, the master regulator of the stress response. Studies with cultured mouse fibroblasts and cortical neurons show that mutants deficient in eIF2␣ phosphorylation are protected from hypertonicity-induced apoptosis. A novel link is revealed between eIF2␣ phosphorylation and the subcellular distribution of the RNA-binding protein heterogeneous nuclear ribonucleoprotein A1 (hnRNP A1). Stress-induced phosphorylation of eIF2␣ promotes apoptosis by inducing the cytoplasmic accumulation of hnRNP A1, which attenuates internal ribosome entry site-mediated translation of anti-apoptotic mRNAs, including Bcl-xL that was studied here. Hypertonic stress induced the eIF2␣ phosphorylation-independent formation of cytoplasmic stress granules (SGs, structures that harbor translationally arrested mRNAs) and the eIF2␣ phosphorylation-dependent accumulation of hnRNP A1 in SGs. The importance of hnRNP A1 was demonstrated by induction of apoptosis in eIF2␣ phosphorylation-deficient cells that express exogenous cytoplasmic hnRNP A1. We propose that eIF2␣ phosphorylation during hypertonic stress promotes apoptosis by sequestration of specific mRNAs in SGs in a process mediated by the cytoplasmic accumulation of hnRNP A1.