Enrichment of Zα domains at cytoplasmic stress granules is due to their innate ability to bind nucleic acids
Z domains are a subfamily of winged Helix-Turn-Helix (wHTH) domains found exclusively in proteins involved in the nucleic acids sensory pathway of vertebrate innate immune system and host evasion by viral pathogens. Interestingly, they are the only known protein domains that recognise the left-handed helical conformation of both dsDNA and dsRNA, known as Z-DNA and Z-RNA. Previously, it has been demonstrated that ADAR1 and ZBP1, two proteins possessing the Z domains, localize to cytosolic stress granules. It was further speculated that such localization is principally mediated by Z domains. To characterize and better understand such distinct and specific localization, we characterised the in vivo interactions and localization pattern for the amino terminal region of human DAI harbouring two Z domains (Z{beta}DAI). Using immunoprecipitation and mass spectrometry, we identified several interacting partners that were components of the complex formed by Z domains and RNAs. Differential interacting partners to wild-type Z, relative to mutant proteins, demonstrated that most of the physiologically relevant interactions are mediated by the nucleic acid binding ability of the Z{beta}. Further, we also show enrichment of selected complex components in cytoplasmic stress granules under conditions of stress. This ability is mostly lost in the mutants of Z{beta}DAI (Z{beta}DAI 4xmut) that lack nucleic-acid binding ability. Thus, we posit that the mechanism for the translocation of Z domain-containing proteins to stress granules is mainly mediated by the nucleic acid binding ability of their Z domains. Finally, we demonstrate that FUS and PSF/p54nrb, two RNA binding proteins with established roles in stress granules, interact with Z, which provides strong evidence for a role of these proteins in the innate immune system.