The structural basis of human SAMD9 and the functional separation of its dual immune activities
摘要
Sterile alpha motif domain-containing protein 9 (SAMD9) is an interferon-inducible protein that has been implicated in both host antiviral defense and inherited human diseases. However, the structural basis governing full-length SAMD9 organization and innate immune signaling remains unclear. Here, we determined the atomic structure of human SAMD9 by cryo-electron microscopy and combined mutagenesis, biochemical, and cellular analyses to define structure-function relationship. SAMD9 forms an integrated multidomain core stabilized by extensive intramolecular contacts, several of which coincide with disease-associated variants. The C-terminal oligonucleotide-binding fold domain adopts a self-inhibitory state through interactions with the NTPase and tetratricopeptide repeat domains. We also found that the majority of SAMD9 forms a homodimer mediated primarily by charged residues within the NTPase domain. Disruption of this interface shifts SAMD9 toward a monomeric conformation and significantly attenuates induction of interferon-stimulated genes. By contrast, although we identified a well-defined ATP-binding pocket, purified SAMD9 shows minimal ATP hydrolysis activity. Mutations of ATP-contacting residues within SAMD9 reduced phenylalanine tRNA cleavage but did not affect interferon activation. Together, our results establish the oligonucleotide-binding fold as a structural determinant of SAMD9 auto-inhibition and provide a structural framework for the functional separation of interferon signaling from the previously established tRNA ribonuclease effector function.
参考文献
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