ABSTRACT
Legionella pneumophila, a bacterial pathogen that causes a severe pneumonia known as Legionnaires' disease, extensively exploits the ubiquitin (Ub) pathway in the infected host cells through certain virulence effectors excreted by the Dot/Icm system. To date, several Dot/Icm effectors have been found to act as Ub ligases, and four effectors, including LotA, LotB, LotC, and Ceg7, have been identified as deubiquitinases (DUBs) from the ovarian tumor (OTU) domain family. LotA is unique among other OTU DUBs because it possesses two distinct DUB domains and exclusively exhibits catalytic activity against K6-linked diUb and polyUb chains. However, the structure of LotA and the molecular mechanism for the dual DUB activity remains elusive. In this study, we solved the structure of LotA in complex with proximally bound Ub and distal covalently bound Ub. Both Ub molecules are bound to the DUB1 domain and mimic a K6-linked diUb. Structural analysis reveals that the DUB1 domain utilizes a distinct mechanism for recognition of the K6-linked diUb within a large S1' binding site that is uncommon to OTU DUBs. Structural fold of the LotA DUB2 domain closely resembles LotB and LotC, similarly containing an extra α-helix lobe that has been demonstrated to play an important role in Ub binding. Collectively, our study uncovers the structural basis for the dual catalytic activity of the unique OTU family DUB LotA.
Subject(s)
Bacterial Proteins , Deubiquitinating Enzymes , Legionella pneumophila , Bacterial Proteins/chemistry , Deubiquitinating Enzymes/chemistry , Legionella pneumophila/enzymology , Ubiquitin/metabolism , Catalysis , Protein Domains , Protein Conformation, alpha-HelicalABSTRACT
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 (HipTLp) and the antitoxin Lpg2369 (HipSLp) correspond to the C-terminus and N-terminus of HipA from HipBA TA, respectively. By determining crystal structures of autophosphorylated HipTLp, its complex with AMP-PNP, and the structure of HipTLp-HipSLp complex, we identify residues in HipTLp critical for ATP binding and those contributing to its interactions with HipSLp. Structural analysis reveals that HipSLp binding induces a loop-to-helix shift in the P-loop of HipTLp, 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.
