The Swarts and Jinek labs could elucidate the structure of the prokaryotic immune system SPARTA in its monomeric inactive form and tetrameric active form, and could deduce mechanistically how binding to the DNA target leads to this activations. The multimerization is essential for that active SPARTA can fundtion as an NADase. Their findings have been published in the article" Target DNA-dependent activation mechanism of the prokaryotic immune system SPARTA" in Nucleic Acids Research.
Abstract
In both prokaryotic and eukaryotic innate immune systems, TIR domains function as NADases that degrade the key metabolite NAD+ or generate signaling molecules. Catalytic activation of TIR domains requires oligomerization, but how this is achieved varies in distinct immune systems. In the Short prokaryotic Argonaute (pAgo)/TIR-APAZ (SPARTA) immune system, TIR NADase activity is triggered upon guide RNA-mediated recognition of invading DNA by an unknown mechanism. Here, we describe cryo-EM structures of SPARTA in the inactive monomeric and target DNA-activated tetrameric states. The monomeric SPARTA structure reveals that in the absence of target DNA, a C-terminal tail of TIR-APAZ occupies the nucleic acid binding cleft formed by the pAgo and TIR-APAZ subunits, inhibiting SPARTA activation. In the active tetrameric SPARTA complex, guide RNA-mediated target DNA binding displaces the C-terminal tail and induces conformational changes in pAgo that facilitate SPARTA-SPARTA dimerization. Concurrent release and rotation of one TIR domain allow it to form a composite NADase catalytic site with the other TIR domain within the dimer, and generate a self-complementary interface that mediates cooperative tetramerization. Combined, this study provides critical insights into the structural architecture of SPARTA and the molecular mechanism underlying target DNA-dependent oligomerization and catalytic activation.
Read the Publication in Nucleic Acids Research (Open Access)
Abstract, figure and title from Finocchio, Koopal et al (2024) Nucleic Acids Res published under a CC BY-NC 4.0 license.