Type III Secretion Effectors with Arginine N-Glycosyltransferase Activity

Abstract

Type III secretion systems are used by many Gram-negative bacterial pathogens to inject proteins, known as effectors, into the cytosol of host cells. These virulence factors interfere with a diverse array of host signal transduction pathways and cellular processes. Many effectors have catalytic activities to promote post-translational modifications of host proteins. This review focuses on a family of effectors with glycosyltransferase activity that catalyze addition of N-acetyl-d-glucosamine to specific arginine residues in target proteins, leading to reduced NF-κB pathway activation and impaired host cell death. This family includes NleB from Citrobacter rodentium, NleB1 and NleB2 from enteropathogenic and enterohemorrhagic Escherichia coli, and SseK1, SseK2, and SseK3 from Salmonella enterica. First, we place these effectors in the general framework of the glycosyltransferase superfamily and in the particular context of the role of glycosylation in bacterial pathogenesis. Then, we provide detailed information about currently known members of this family, their role in virulence, and their targets.

Keywords: Citrobacter; Escherichia; NleB; Salmonella; SseK; death domains; effectors; glycosyltransferases; type III secretion.

Figure 1

Genetic structure of Salmonella pathogenicity island 1 (SPI1) and Salmonella pathogenicity island 2 (SPI2) of S. enterica serovar Typhimurium strain 14028, and the locus of enterocyte effacement (LEE) island of E. coli O127:H6 strain E2348/69. Genes are colored according to their functional categories [22,28].

Figure 2

Glycosyltransferase mechanisms. An S_N2 (substitution, nucleophilic, bimolecular) process is accepted for inverting glycosyltransferases (GTs). Several mechanisms have been proposed for retaining enzymes. The orthogonal mechanism is depicted here. ‡: Transition state. Image by Dr. Brock Schumann (Wikimedia Commons, CC BY-SA 3.0, https://creativecommons.org/licenses/by-sa/3.0/deed.en).

Figure 3

Glycosyltransferase folds. (a) Diagram of the GT-A fold protein SpsA from Bacillus subtilis, belonging to family GT2 (PDB ID: 1QG8) [51]; (b) diagram of the GT-B fold-type protein GtfB from Amycolaptosis orientalis, belonging to family GT1 (PDB ID: 1IIR) [52]; (c) diagram of the GT-C fold protein PglB from Campylobacter lari, belonging to family GT66 (PDB ID: 3RCE) [46]. Drawings created with NGL [53].

Figure 4

Phylogenetic analysis of NleB/SseK proteins. Alignment and phylogenetic reconstructions were performed using the function “build” of ETE3 v3.1.1 [118] as implemented on the GenomeNet (https://www.genome.jp/tools/ete/). The tree was constructed using FastTree v2.1.8 with default parameters [119]. Values at nodes are SH-like local support.

Figure 5

Structure of NleB/SseK family members. (a) Crystal structure of SseK3 from S. enterica serovar Typhimurium strain SL1344 (PDB ID: 6CGI) [147]; (b) crystal structure of NleB1 from E. coli O127:H6 (PDB ID: 6E66) [146]. Drawings created with NGL [53].

Figure 6

Summary of NleB/SseK host protein targets.