Involvement of <i>hcp2</i> on maintaining cell wall integrity and pathogenicity of <i>Vibrio alginolyticus</i> by modulating Sec pathway

Hemolysin co-regulated protein 2 (Hcp2) is a core component of the type VI secretion system 2 in Vibrio alginolyticus, a widespread marine pathogen that infects humans and aquaculture species. Deletion of hcp2 (Δhcp2) significantly attenuated virulence in zebrafish larvae, showing reduced abdominal edema and impaired toll-like receptor (tlr)2 and tlr4-mediated innate immune activation compared with the wildtype strain. Enzyme-linked immunosorbent assay and cell lysis assays revealed that the Δhcp2 strain exhibited decreased levels of lipopolysaccharide (LPS) and peptidoglycan (PGN), along with increased cell permeability and abnormal cell wall structures observed by transmission electron microscopy. Proteomic and transcriptional analyses further demonstrated that expression of the Sec system components SecB, SecD, and SecF was markedly reduced in the Δhcp2 strain and positively regulated by hcp2. Bioinformatic prediction combined with protein-DNA docking analysis suggested that the transcription of secB, secD, and secF was co-regulated by the cAMP and cAMP Receptor Protein (cAMP-CRP) complex and RNA polymerase sigma D factor (RpoD), although rpoD expression itself remained unaffected. Together with previous evidence that Hcp2 positively regulates cAMP-CRP, these findings suggested that Hcp2 modulated LPS and PGN translocation, probably through the cAMP-CRP pathway, thereby maintaining cell wall integrity and virulence in V.alginolyticus.