Argonaute (Ago) proteins are ubiquitous across all kingdoms of life. Eukaryotic Agos (eAgos) use small RNAs to recognize transcripts for RNA silencing in eukaryotes. In contrast, the functions of prokaryotic counterparts (pAgo) are less well known. Recently, short pAgos in conjunction with the associated TIR or Sir2 (SPARTA or SPARSA) were found to serve as antiviral systems to combat phage infections. Zhen et at (Nat Comms, 2024, 15, 450) presented the cryo-EM structures of nicotinamide adenine dinucleotide (NAD+)-bound SPARSA with and without nucleic acids at resolutions of 3.1 Å and 3.6 Å, respectively. Structural biology results revealed that the APAZ (Analogue of PAZ) domain and the short pAgo form a featured architecture similar to the long pAgo to accommodate nucleic acids. The authors identified the key residues for NAD+ binding and elucidated the structural basis for guide RNA and target DNA recognition. Using structural comparisons, molecular dynamics simulations, and biochemical experiments, the paper proposed a putative mechanism for NAD+ hydrolysis in which an H186 loop mediates nucleophilic attack by catalytic water molecules. Overall, this study provides mechanistic insight into the antiphage role of the SPARSA system. Here you can see the cryo-EM structure of the prokaryotic SPARSA system complex (PDB code: 8JL0)

#molecularart ... #sparsa ... #complex ... #antiphage ... #cryoem

Structure rendered with @proteinimaging and depicted with @corelphotopaint