Phosphorylation of VapB antitoxins affects intermolecular interactions to regulate VapC toxin activity inMycobacterium tuberculosis

Abstract

AbstractToxin-antitoxin modules are present in many bacterial pathogens. The VapBC family is particularly abundant in members of theMycobacterium tuberculosiscomplex, with 50 modules present in theM. tuberculosisgenome. In type IIA modules the VapB antitoxin protein binds to and inhibits the activity of the co-expressed cognate VapC toxin protein. VapB proteins also bind to promoter region sequences and repress expression of thevapB-vapCoperon. Though VapB-VapC interactions can control the amount of free VapC toxin in the bacterial cell, the mechanisms that affect this interaction are poorly understood. Based on our recent finding of Ser/Thr phosphorylation of VapB proteins inM. tuberculosis, we substituted phosphomimetic or phosphoablative amino acids at the phosphorylation sites of two VapB proteins. We found that phosphomimetic substitution of VapB27 and VapB46 resulted in decreased interaction with their respective cognate VapC proteins, whereas phosphoablative substitution did not alter binding. Similarly, we determined that phosphomimetic substitution interfered with VapB binding to promoter region DNA sequences. Both decreased VapB-VapC interaction and decreased VapB repression ofvapB-vapCoperon transcription would result in increased free VapC in theM. tuberculosiscell.M. tuberculosisstrains expressingvapB46-vapC46constructs containing a phosphoablativevapBmutation resulted in lower toxicity compared to a strain expressing nativevapB46, whereas similar or greater toxicity was observed in the strain expressing the phosphomimeticvapBmutation. These results identify a novel mechanism by which VapC toxicity activity can be regulated by VapB phosphorylation, potentially in response to extracytoplasmic as well as intracellular signals.ImportanceIntracellular bacterial toxins are present in many bacterial pathogens and have been linked to bacterial survival in response to stresses encountered during infection. The activity of many toxins is regulated by a co-expressed antitoxin protein that binds to and sequesters the toxin protein. The mechanisms by which an antitoxin may respond to stresses to alter toxin activity are poorly understood. Here we show that antitoxin interactions with its cognate toxin, and with promoter DNA required for antitoxin and toxin expression, can be altered by Ser/Thr phosphorylation of the antitoxin, and thus affect toxin activity. This reversible modification may play an important role in regulating toxin activity within the bacterial cell in response to signals generated during infection.