Molecular basis for the targeting process of injectisome substrate proteins in Salmonella enterica

A major virulence factor of many pathogenic gram-negative bacteria, including Salmonella enterica Typhimurium, is the injectisome, a needle-like structure that injects toxins into host cells during infection. The injectisome employs a type-III secretion system (T3SS) for the export of toxin proteins. Before initiating toxin secretion, it secretes its own structural components through a highly controlled self-assembly process. However, because T3SS substrates do not contain a typical cleavable secretion signal in the aminoterminal region of the peptide sequence [1], it remains unknown how these substrates are selected from thousands of cytoplasmic proteins and specifically targeted to the T3SS. This project explores the hypothesis that substrate gene transcription and translation occur in close proximity to the T3SS, enabling efficient recognition and secretion. This concept is inspired a process known as transertion in which transcription, translation, and membrane insertion of proteins are spatially coordinated. Recent findings suggest that Vibrio parahaemolyticus utilizes transertion for T3SS substrate secretion [2], but its role in Salmonella remains unclear. To test this hypothesis, co-localization analyses of chromosomal substrate gene loci and the T3SS, as well as spatial studies on membrane proximity will be conducted. In summary, this project will provide critical insights into the type III secretion process, shedding light on the assembly mechanism of the injectisome and the secretion of toxins during infection. Ultimately, it will elucidate the potential role of local transcription and translation of substrate genes in these processes.

[1] Michiels T, Wattiau P, Brasseur R, Ruysschaert JM, Cornelis G (1990). Secretion of Yop proteins by Yersiniae. Infect Immun 58(9), 2840–2849
[2] Kaval KG, Chimalapati S, Siegel SD, Nalleli G, Jananee J, Ankur BD, Kim O (2023). Membrane-localized expression, production and assembly of Vibrio parahaemolyticus T3SS2 provides evidence for transertion. Nat Commun 14, 1178