The -- complex is essential for activating the antiviral response during infection.
G3BP1 directly interacts with inactive PKR, facilitating its activation during cellular stress.
Caprin1 also interacts with PKR and is necessary for efficient PKR activation at .
The activation of PKR by this complex does not require the recognition of foreign double-stranded RNA.
This mechanism connects stress responses and innate immune activation through PKR.
Antiviral activity of G3BP1 and PKR is contingent upon their complex formation with Caprin1.
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UNLABELLED: (SGs) are dynamic cytoplasmic repositories containing translationally silenced mRNAs that assemble upon cellular stress. We recently reported that the SG nucleating protein promotes antiviral activity and is essential in (PKR) recruitment to stress granules, thereby driving phosphorylation of the α subunit of eukaryotic initiation factor 2 (eIF2α). Here, we delineate the mechanism for SG-dependent PKR activation. We show that G3BP1 and inactive PKR directly interact with each other, dependent on both the NTF2-like and PXXP domains of G3BP1. The G3BP1-interacting protein also directly interacts with PKR, regulates efficient PKR activation at the stress granule, and is also integral for the release of active PKR into the cytoplasm to engage in substrate recognition. The G3BP1-Caprin1-PKR complex represents a new mode of PKR activation and is important for antiviral activity of G3BP1 and PKR during infection with mengovirus. Our data links stress responses and their resultant SGs with innate immune activation through PKR without a requirement for foreign double-stranded RNA (dsRNA) pattern recognition.
IMPORTANCE: Our previous work indicates that stress granules have antiviral activity and mediate innate immunity through functions of G3BP1; however, the mechanistic details of these functions were not resolved. We show that much of the antiviral activity of stress granules is contingent on the function of PKR in a complex with G3BP1 and Caprin1. The PKR-G3BP1-Caprin1 complex undergoes dynamic transitioning within and outside stress granules to accomplish PKR activation and translational repression. This mechanism appears to function distinctly from canonical pattern recognition of double-stranded RNA by PKR. Therefore, this mechanism bridges the stress response with innate immunity, allowing the cell to respond to many cellular stressors and amplify the pathogen pattern recognition systems of innate immunity.
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