1.C.131.  The VasX Toxin (VasX) Family 

VasX, a unique toxin, requires a functional T6SS for secretion. Deletion of vasX does not affect export or enzymatic function of the structural T6SS proteins Hcp and VgrG-1, showing that VasX is dispensable for the assembly and is not a component of the physical translocon complex. VasX localizes to the bacterial membrane, interacts with membrane lipids, and is a virulence factor, as a V. cholerae mutant lacking vasX exhibits a phenotype of attenuated virulence toward Dictyostelium discoideum.(Miyata et al. 2011).

Miyata et al. 2013 described a dual expression profile of the T6SS immunity protein-encoding genes tsiV1, tsiV2, and tsiV3 that provide pandemic V. cholerae strains with T6SS immunity and allows T6SS-silent strains to maintain immunity against attacks by T6SS-active bacterial neighbors. The dual expression profile allows transcription of the three genes encoding immunity proteins independently of other T6SS proteins encoded within the same operon. One of these immunity proteins, TsiV2, protects against the T6SS effector VasX which is encoded immediately upstream of tsiV2. VasX is a secreted, lipid-binding protein. The presence of an internal promoter in the open reading frame of vasX drives expression of the downstream gene tsiV2. Furthermore, VasX acts in conjunction with VasW, an accessory protein to VasX, to compromise the inner membrane of prokaryotic target cells. The dual regulatory profile of the T6SS immunity protein-encoding genes tsiV1, tsiV2, and tsiV3 permits V. cholerae to tightly control T6SS gene expression while maintaining immunity to T6SS activity (Miyata et al. 2013).

By systemically mutating the active sites of 3 Vibrio cholerae effectors, TseL, VasX, and VgrG3, Liang et al. 2019 showed that their physical presence but not their activities is crucial for T6SS assembly. Catalytic mutants of TseL and VgrG3 and truncated VasX mutations abolished the killing of the effector-cognate immunity mutants. The VasX-mediated antimicrobial activity is solely dependent on the C-terminal colicin domain. Removal of the colicin domain abolished VasX secretion and reduced T6SS assembly, while deletion of the colicin internal loop abolished its toxicity but had little effect on secretion and assembly (Liang et al. 2019).


 

References:

Liang, X., F. Kamal, T.T. Pei, P. Xu, J.J. Mekalanos, and T.G. Dong. (2019). An onboard checking mechanism ensures effector delivery of the type VI secretion system in. Proc. Natl. Acad. Sci. USA 116: 23292-23298.

Miyata, S.T., D. Unterweger, S.P. Rudko, and S. Pukatzki. (2013). Dual expression profile of type VI secretion system immunity genes protects pandemic Vibrio cholerae. PLoS Pathog 9: e1003752.

Miyata, S.T., M. Kitaoka, T.M. Brooks, S.B. McAuley, and S. Pukatzki. (2011). Vibrio cholerae requires the type VI secretion system virulence factor VasX to kill Dictyostelium discoideum. Infect. Immun. 79: 2941-2949.

Examples:

TC#NameOrganismal TypeExample
1.C.131.1.1

A secreted lipid binding protein of 1085 aas and 5 - 6 TMSs, VasX.  Essential for killing of Dictyostelium discoideum (Miyata et al. 2011).  It is a pore-forming toxin (Durand et al. 2014). It is a T6SS effector and may have a structure resembling that of Colicin Ia (Russell et al. 2014). VasW (295 aas, possibly with a C-terminal TMS) is an accessory protein for VasX (Miyata et al. 2013). The TsiV2 immunity protein (242 aas and 3 or 4 TMSs) protects against VasX. Its gene is adjacent to the vasX gene, and its promoter is within the vasX gene, clearly suggesting a common functional role to counteract the function of VasX (Miyata et al. 2013).

Proteobacteria

VasX of Vibrio cholerae

 
1.C.131.1.2

Uncharacterized protein of 1174 aas and 6 putative TMSs.  May be a pore-forming toxin (Durand et al. 2014).

Proteobacteria

UP of Pseudomonas aeruginosa

 
1.C.131.1.3

Uncharacterized protein of 955 aas.

Proteobacteria

UP of Marinobacter manganoxydans

 
1.C.131.1.4

Uncharacterized protein of 1165 aas.

Proteobacteria

UP of Vibrio coralliilyticus

 
1.C.131.1.5

Uncharacterized protein of 1142 aas

Proteobacteria

UP of Pseudomonas syringae

 
Examples:

TC#NameOrganismal TypeExample
1.C.131.2.1

Uncharacterized protein of 1007 aas and 6 putative TMSs.  May form pores in membranes (Durand et al. 2014).

Proteobacteria

UP of Burkholderia thailandensis

 
1.C.131.2.2

Uncharacterized protein of 978 aas and 4 - 6 TMSs.  May be a pore-forming toxin (Durand et al. 2014).

Proteobacteria

UP of Variovorax paradoxus

 
1.C.131.2.3

Uncharacterized protein of 947 aas.  This protein is 80% identical to a close homologue, shown to be a pore forming toxin, a T9SS effector, in Burkholderia thailandensis (Russell et al. 2014).

Proteobacteria

UP of Burkholderia pseudomallei