Roles of the Hcp family proteins in the pathogenicity of <i>Salmonella typhimurium</i> 14028s

The type VI secretion system (T6SS) is a new secretion system that is widely distributed among Gram-negative bacteria. The core component hemolysin-coregulated protein (Hcp) can be used as both its structural protein and secretory protein or chaperone protein. Studies on Hcp are important to elucidate the overall virulence mechanism of T6SS. <i>Salmonella typhimurium</i> is an important foodborne pathogen. There are three copies of <i>hcp</i> genes identified in <i>S</i>. Typhimurium 14028s. This study aimed to characterize the functions of the three Hcp family proteins and to elucidate the interactions among them. The <i>hcp</i> gene deletion mutants were constructed by λ Red-based recombination system. Effects of <i>hcp</i> mutation on the pathogenicity of 14028s were studied by bacterial competition assays, <i>Dictyostelium discoideum</i> assays and mouse model. The three Hcp family proteins were found to play different roles. Hcp1 can affect the transcription of <i>rpoS</i> and type 2 flagellar gene and influence the motility of 14028s. It is also involved in the intracellular survival of 14028s in <i>Dictyostelium discoideum</i>; Hcp2 is involved in the early proliferative capacity of 14028s in mice and can prevent its excessive proliferation; Hcp3 did not show direct functions in these assays. Hcp1 can interact with Hcp2 and Hcp3. Deletion of one <i>hcp</i> gene can result in a transcription level variation in the other two <i>hcp</i> genes. Our findings elucidated the functions of the three Hcp family proteins in <i>S</i>.Typhimurium and illustrated that there are interactions between different Hcp proteins. This study will be helpful to fully understand how T6SS actions in an organism.