Staphylococcus aureus is a notorious and globally distributed pathogenic bacterium. New strategies to develop novel antibiotics based on intrinsic bacterial toxin–antitoxin (TA) systems have been recently reported. Because TA systems are present only in bacteria and not in humans, these distinctive systems are attractive targets for developing antibiotics with new modes of action. S. aureus PemIK is a type II TA system, comprising the toxin protein PemK and the labile antitoxin protein PemI. Here, we determined the crystal structures of both PemK and the PemIK complex, in which PemK is neutralized by PemI. Our biochemical approaches, including fluorescence quenching and polarization assays, identified Glu20, Arg25, Thr48, Thr49, and Arg84 of PemK as being important for RNase function. Our study indicates that the active site and RNA-binding residues of PemK are covered by PemI, leading to unique conformational changes in PemK accompanied by repositioning of the loop between β1 and β2. These changes can interfere with RNA binding by PemK. Overall, PemK adopts particular open and closed forms for precise neutralization by PemI. This structural and functional information on PemIK will contribute to the discovery and development of novel antibiotics in the form of peptides or small molecules inhibiting direct binding between PemI and PemK.

Type II toxin–antitoxin (TA) systems encode two proteins: a toxin that inhibits cell growth and an antitoxin that neutralizes the toxin by direct intermolecular protein–protein interactions. The bacterial HipBA TA system is implicated in persister formation. The Haemophilus influenzae HipBA TA system consists of a HipB antitoxin and a HipA toxin, the latter of which is split into two fragments, and here we investigate this novel three-component regulatory HipBA system. Structural and functional analysis revealed that HipAN corresponds to the N-terminal part of HipA from other bacteria and toxic HipAC is inactivated by HipAN, not HipB. This study will be helpful in understanding the detailed regulatory mechanism of the HipBAN+C system, as well as why it is constructed as a three-component system.

Polyketide metabolism-associated proteins in Mycobacterium tuberculosis play an essential role in the survival of the bacterium, which makes them potential drug targets for the treatment of tuberculosis (TB). The novel ribonuclease protein Rv1546 is predicted to be a member of the steroidogenic acute regulatory protein-related lipid-transfer (START) domain superfamily, which comprises bacterial polyketide aromatase/cyclases (ARO/CYCs). Here, we determined the crystal structure of Rv1546 in a V-shaped dimer. The Rv1546 monomer consists of four α-helices and seven antiparallel β-strands. Interestingly, in the dimeric state, Rv1546 forms a helix-grip fold, which is present in START domain proteins, via three-dimensional domain swapping. Structural analysis revealed that the conformational change of the C-terminal α-helix of Rv1546 might contribute to the unique dimer structure. Site-directed mutagenesis followed by in vitro ribonuclease activity assays was performed to identify catalytic sites of the protein. This experiment suggested that surface residues R63, K84, K88, and R113 are important in the ribonuclease function of Rv1546. In summary, this study presents the structural and functional characterization of Rv1546 and supplies new perspectives for exploiting Rv1546 as a novel drug target for TB treatment.