Francisella tularensis virulence relies on a conserved putative catalytic triad within the Type VI secretion system...

Gram-negative bacteria utilize type VI secretion systems (T6SS) for microbial competition and host interaction. While most pathogens rely on the canonical T6SSⁱ, Francisella species uniquely possess T6SSⁱⁱ. The highly virulent human pathogen Francisella tularensis harbors a distinct T6SSⁱⁱ variant that includes pdpC, encoding a putative effector protein. Bioinformatic analysis revealed a conserved amino acid triad in PdpC, homologous to motifs found in Make Caterpillars Floppy family toxins. To investigate the functional relevance of this triad, site-directed mutagenesis was performed in the live vaccine strain of F. tularensis, substituting each residue with alanine. Mutants showed impaired phagosomal escape, reduced intracellular replication, and marked attenuation in the mouse infection model. Equivalent mutations introduced into F. novicida, a model for T6SS-mediated secretion, confirmed the triad's importance. Mass spectrometry analysis demonstrated that PdpC is secreted in a T6SS-dependent manner. Importantly, the mutations did not affect secretion, and deletion of pdpC did not alter the overall secretion profile. These findings indicate that the conserved triad is essential for PdpC's effector function but dispensable for its secretion. This study highlights a critical motif required for Francisella virulence and provides new insights into the specialized mechanisms of T6SSⁱⁱ effectors.