Functional plasticity of antibacterial EndoU toxins

Michalska, Karolina; Nhan, Dinh Quan; Willett, Julia L. E.; Stols, Lucy M.; Eschenfeldt, William H.; Jones, Allison M.; Nguyen, Josephine Y.; Koskiniemi, Sanna; Low, David A.; Goulding, Celia W.; Joachimiak, Andrzej; Hayes, Christopher S.

Bacteria use several different secretion systems to deliver toxic EndoU ribonucleases into neighboring cells. Here, we present the first structure of a prokaryotic EndoU toxin in complex with its cognate immunity protein. The contact-dependent growth inhibition toxin CdiA-CT(STECO31) from Escherichia coli STEC_O31 adopts the eukaryotic EndoU fold and shares greatest structural homology with the nuclease domain of coronavirus Nsp15. The toxin contains a canonical His-His-Lys catalytic triad in the same arrangement as eukaryotic EndoU domains, but lacks the uridylate-specific ribonuclease activity that characterizes the superfamily. Comparative sequence analysis indicates that bacterial EndoU domains segregate into at least three major clades based on structural variations in the N-terminal subdomain. Representative EndoU nucleases from clades I and II degrade tRNA molecules with little specificity. In contrast, CdiA-CT(STECO31) and other clade III toxins are specific anticodon nucleases that cleave tRNA(Glu) between nucleotides C37 and m(2)A38. These findings suggest that the EndoU fold is a versatile scaffold for the evolution of novel substrate specificities. Such functional plasticity may account for the widespread use of EndoU effectors by diverse inter-bacterial toxin delivery systems.