Date Published: August 13, 2018
Publisher: Public Library of Science
Author(s): Thorsten Brink, Veronika Leiss, Peter Siegert, Doris Jehle, Julia K. Ebner, Carsten Schwan, Aliaksei Shymanets, Sebastian Wiese, Bernd Nürnberg, Michael Hensel, Klaus Aktories, Joachim H. C. Orth, Renée M. Tsolis.
Salmonella enterica serotype Typhimurium (S. Typhimurium) is one of the most frequent causes of food-borne illness in humans and usually associated with acute self-limiting gastroenteritis. However, in immunocompromised patients, the pathogen can disseminate and lead to severe systemic diseases. S. Typhimurium are facultative intracellular bacteria. For uptake and intracellular life, Salmonella translocate numerous effector proteins into host cells using two type-III secretion systems (T3SS), which are encoded within Salmonella pathogenicity islands 1 (SPI-1) and 2 (SPI-2). While SPI-1 effectors mainly promote initial invasion, SPI-2 effectors control intracellular survival and proliferation. Here, we elucidate the mode of action of Salmonella SPI-2 effector SseI, which is involved in control of systemic dissemination of S. Typhimurium. SseI deamidates a specific glutamine residue of heterotrimeric G proteins of the Gαi family, resulting in persistent activation of the G protein. Gi activation inhibits cAMP production and stimulates PI3-kinase γ by Gαi-released Gβγ subunits, resulting in activation of survival pathways by phosphorylation of Akt and mTOR. Moreover, SseI-induced deamidation leads to non-polarized activation of Gαi and, thereby, to loss of directed migration of dendritic cells.
Salmonella enterica serovars are pathogenic bacteria that cause severe diseases ranging from enteric fever (e.g. by Salmonella Typhi) to gastroenteritis and bacteraemia caused by non-typhoidal Salmonella (NTS). Salmonella Typhimurium, the model organism of NTS infection, has a broad host spectrum and is one of the most frequent causes of food-borne illness in humans and other vertebrates including food-producing animals. S. Typhimurium infection is usually associated with acute self-limiting gastroenteritis in immunocompetent individuals. However, in immunocompromised patients, S. Typhimurium can disseminate and lead to severe systemic diseases [1–4].
Our studies elucidate the molecular mechanism of the S. Typhimurium SPI-2 T3SS effector SseI. This effector protein plays a crucial role in S. Typhimurium infection after invasion of the pathogen and modulates the immune responses of the host . Here, we show that SseI deamidates a specific glutamine residue (Gln205 in Gαi2) in the α-subunits of heterotrimeric G proteins, which is involved in GTP hydrolysis, thereby the G protein is persistently activated. Thus, T3SS effector SseI exhibits the same mode of action as exotoxin PMT from Pasteurella multocida. This finding is in line with the structural similarity of both toxins, although the primary sequence identity of the catalytic domains is only ~ 20% . Exchange of the conserved catalytic amino acid residues C178, H216 and D231 inhibited the deamidase activity of SseI. Because SseI is per se not able to enter target cells, we constructed a chimeric toxin (PMT-SseIC), consisting of the N-terminal binding and translocation domain of PMT and the deamidating domain of SseI. Additionally, we transfected mammalian cells with a GFP-SseI construct. These experiments confirmed our in vitro results in intact cells, showing that SseI acts as a deamidase on heterotrimeric G proteins.