Activation of SPARDA defense system by filament assembly reveals beta-relay signaling mechanism widespread in prokaryotic Argonautes

Present in all three domains of life, Argonaute (Ago) proteins use short oligonucleotides as guides to recognize complementary nucleic acid targets and are involved in RNA silencing (eukaryotes) or host defense against invading DNA (prokaryotes). Here, we show that the SPARDA (short prokaryotic Argonaute, DNase associated) system functions in anti-plasmid defense. Upon detection of invading plasmid DNA, it degrades both the invader and host DNA, inducing cell death and preventing further spread of the invader. Upon activation, the recognition signal of the bound guide/target duplex is relayed to other functional SPARDA sites through a structural region, which we termed beta-relay. The associated dramatic conformational changes trigger the formation of a filament, in which the DREN nuclease domains form tetramers poised to cleave dsDNA. Furthermore, we identified the presence of beta-relay in all pAgo clades, providing new insights into the structural mechanisms of pAgo proteins.