A Salmonella-specific protein enhances the antimicrobial peptide response of the PhoPQ two-component system

Salmonella enterica, a major global pathogen, has evolved a distinctive pathogenesis that differs from related enteric pathogens. A central regulator of Salmonella-specific virulence gene expression is the broadly conserved PhoP/PhoQ (PhoPQ) two-component system. PhoPQ serves a distinct role in Salmonella compared to other species, suggesting a need to adapt its function for its unique and expanded role in this species. In this study, we identify a novel, Salmonella-specific component of the PhoPQ signaling system, a small putative lipoprotein we name PalA. We show that PalA localizes to the cytoplasmic membrane, where it interacts with PhoQ, leading to PhoP activation and a corresponding reprogramming of the PhoP regulon. PalA's transcriptomic impact is entirely PhoPQ-dependent, indicating strict specificity. Upon cationic antimicrobial peptide exposure, palA expression is induced, and it strongly activates PhoPQ, but palA shows low expression and has a minimal effect under low pH or low Mg²⁺ conditions, indicating its impact on PhoPQ is signal-specific. Notably, PalA does not stimulate PhoPQ systems from closely related species, indicating that Salmonella PhoPQ has adapted to promote its activation by PalA. Collectively, this study unveils an evolutionary adaptation by Salmonella that enhances its virulence gene expression in response to cationic antimicrobial peptide exposure.IMPORTANCESalmonella, a major human pathogen, controls the expression of its virulence genes in part through the PhoP/PhoQ (PhoPQ) two-component system. In this study, we identify and characterize a novel component of this system: a small protein unique to Salmonella, which we name PalA. We show that exposure to cationic antimicrobial peptides (CAMPs), a host-derived signal encountered during infection, induces PalA expression. PalA then binds PhoQ, activating it and thereby enhancing virulence gene expression. This study reveals a previously overlooked component of the Salmonella PhoPQ signaling network, which is among the best-studied bacterial regulatory systems and has served as an important model system for understanding the regulation of bacterial pathogenesis. Furthermore, this work illustrates the important role small "adaptor proteins" can play in conferring new signaling properties to ancestral regulatory systems, in this case by boosting activity and the response to a specific environmental cue.