Molecular mechanism of toxin neutralization in the HipBST toxin-antitoxin system of Legionella pneumophila.

Toxin-antitoxin (TA) systems are ubiquitous genetic modules in bacteria and archaea. Here, we perform structural and biochemical characterization of the Legionella pneumophila effector Lpg2370, demonstrating that it is a Ser/Thr kinase. Together with two upstream genes, lpg2370 constitutes the tripartite HipBST TA. Notably, the toxin Lpg2370 (HipT Lp ) and the antitoxin Lpg2369 (HipS Lp ) correspond to the C-terminus and N-terminus of HipA from HipBA TA, respectively. By determining crystal structures of autophosphorylated HipT Lp , its complex with AMP-PNP, and the structure of HipT Lp -HipS Lp complex, we identify residues in HipT Lp critical for ATP binding and those contributing to its interactions with HipS Lp . Structural analysis reveals that HipS Lp binding induces a loop-to-helix shift in the P-loop of HipT Lp , leading to the blockage of ATP binding and inhibition of the kinase activity. These findings establish the L. pneumophila effector Lpg2370 as the HipBST TA toxin and elucidate the molecular basis for HipT neutralization in HipBST TA.