Type VI secretion system translocates a phage tail spike-like protein into target cells where it cross-links actin

Abstract

Genes encoding type VI secretion systems (T6SS) are widely distributed in pathogenic Gram-negative bacterial species. In Vibrio cholerae, T6SS have been found to secrete three related proteins extracellularly, VgrG-1, VgrG-2, and VgrG-3. VgrG-1 can covalently cross-link actin in vitro, and this activity was used to demonstrate that V. cholerae can translocate VgrG-1 into macrophages by a T6SS-dependent mechanism. Protein structure search algorithms predict that VgrG-related proteins likely assemble into a trimeric complex that is analogous to that formed by the two trimeric proteins gp27 and gp5 that make up the baseplate ''tail spike'' of Escherichia coli bacteriophage T4. VgrG-1 was shown to interact with itself, VgrG-2, and VgrG-3, suggesting that such a complex does form. Because the phage tail spike protein complex acts as a membrane-penetrating structure as well as a conduit for the passage of DNA into phage-infected cells, we propose that the VgrG components of the T6SS apparatus may assemble a ''cellpuncturing device'' analogous to phage tail spikes to deliver effector protein domains through membranes of target host cells.bacteriophage ͉ cytotoxicity ͉ Vibrio cholerae ͉ virulence

Cryo‐ EM reconstruction of Type VI secretion system baseplate and sheath distal end

“…This is consistent with sizes of known effectors: 129‐kDa VgrG‐1, 113‐kDa VgrG‐3, or 155‐kDa pair VasX‐VasW (Dong et al , 2013; Joshi et al , 2017). Since 77‐kDa VgrG‐2 with no extension domain is required for T6SS assembly (Appendix Fig S3; Pukatzki et al , 2007), there is also enough space for effectors interacting with VgrGs, such as 72‐kDa TseL (Dong et al , 2013), in agreement with the model that many effectors bind to the VgrG/PAAR spike (Fig EV6; Shneider et al , 2013). However, the repertoire of secreted effectors is large (Durand et al , 2014; Alcoforado Diniz et al , 2015; Hachani et al , 2016), and therefore, it is likely that the overall shape of T6SS baseplate will vary between organisms.…”

Evaluation of the roles played by Hcp and VgrG type 6 secretion system effectors in Aeromonas hydrophila SSU pathogenesis

“…We previously showed that VgrG-1 in A. hydrophila ATCC 7966 T carries a C-terminal extension domain with ADPribosylation activity, and, through its T6SS, it targets host actin and promotes host cell apoptosis (Suarez et al, 2010a). VgrGs with C-terminal extension domains have been termed 'evolved VgrGs', and another such VgrG-1 with an actin cross-linking activity domain has been reported in Vibrio cholerae (Pukatzki et al, 2007). Interestingly, each effector has multiple copies, presumably paralogues of each other.…”

VgrG C terminus confers the type VI effector transport specificity and is required for binding with PAAR and adaptor–effector complex

“…It may feature the effector function at the C-terminal domain. Such "evolved VgrGs" include Vibrio cholerae VgrG-1, with an actin cross-linking domain; Aeromonas hydrophila VgrG1, with an actin-ADP ribosylating VIP-2 domain; and V. cholerae VgrG-3, with glycoside hydrolase activity to target peptidoglycan (9,(25)(26)(27). Furthermore, VgrG may function as a carrier for effector delivery by binding directly with specific effectors carrying the proline-alanine-alanine-arginine (PAAR) domain (28,29).…”