24 KEYWORDS25 Salmonella, SseK, glycosyltransferase, death receptor signaling 26 2 27 ABSTRACT 28 Strains of Salmonella utilise two distinct type three secretion systems to deliver effector proteins 29 directly into host cells. The Salmonella effectors SseK1 and SseK3 are arginine 30 glycosyltransferases that modify mammalian death domain containing proteins with N-acetyl 31 glucosamine (GlcNAc) when overexpressed ectopically or as recombinant protein fusions. Here, we 32 combined Arg-GlcNAc glycopeptide immunoprecipitation and mass spectrometry to identify host 33 proteins GlcNAcylated by endogenous levels of SseK1 and SseK3 during Salmonella infection. We 34 observed that SseK1 modified the mammalian signaling protein TRADD, but not FADD as 35 previously reported. Overexpression of SseK1 greatly broadened substrate specificity, while ectopic 36 co-expression of SseK1 and TRADD increased the range of modified arginine residues within the 37 death domain of TRADD. In contrast, endogenous levels of SseK3 resulted in modification of the 38 death domains of receptors of the mammalian TNF superfamily, TNFR1 and TRAILR, at residues 39 Arg 376 and Arg 293 respectively. Structural studies on SseK3 showed that the enzyme displays a 40 classic GT-A glycosyltransferase fold and binds UDP-GlcNAc in a narrow and deep cleft with the 41 GlcNAc facing the surface. Together our data suggests that Salmonellae carrying sseK1 and sseK3 42 employ the glycosyltransferase effectors to antagonise different components of death receptor 43 signaling. 44 45 3 46 AUTHOR SUMMARY 47 Many Gram-negative pathogens employ type three secretion systems to translocate specialised 48 bacterial effector proteins into the host cell. The activities of many Salmonella effectors are not well 49 understood, and identifying the host targets of these effectors will lead to a better understanding of 50 Salmonella pathogenesis. The overexpression of effectors in vitro can provide useful insights into 51 their function, but these results may not represent the true biological role of these effectors during 52 infection. In this study, we showed that endogenous levels of two Salmonella effectors target 53 different host death domain containing signaling proteins that are required for inflammatory and 54 cell death signaling. The study developed new tools to study cognate effector interactions and 55 established the important of effector expression levels in defining natural and unnatural interactions.
56 INTRODUCTION57 Pathogenic serovars of Salmonella utilise two type three secretion systems (T3SS), encoded by 58 Salmonella pathogenicity island-1 and -2 (SPI-1 and SPI-2) to deliver distinct cohorts of effector 59 proteins into host cells during infection [1, 2]. These effector proteins subvert normal cellular 60 processes and collectively enable the bacteria to invade and persist within host cells, partially 61 through the manipulation of inflammatory cell signaling and programmed cell death (reviewed in 62 [3, 4]). While the importance of effector translocatio...