Paradoxical Activation of a Type VI Secretion System Phospholipase Effector by Its Cognate Immunity Protein

Abstract

Gram-negative bacteria use type VI secretion systems deliver toxic effector proteins directly into neighboring competitors. Secreting cells also produce specific immunity proteins that neutralize effector activities to prevent autointoxication.

Location, Location, Location: an Antidote That Both Activates and Neutralizes a Toxin Used in Bacterial Warfare

“…In this issue of the J Bacteriol, Jensen et al ( 10 ) report findings that extend our current understanding of T6SS immunity protein function through their analysis of the Tle-Tli effector-immunity pair from Enterobacter cloacae . Using a series of mutagenesis experiments and coculture competition assays, the authors reveal that the Tli immunity protein exhibits dual functionality, depending on its subcellular localization.…”

“…Interest in characterizing Tle and Tli came from the results of a genetic screen that Jensen et al ( 10 ) designed to study another interbacterial antagonism pathway called contact-dependent inhibition (CDI) ( 11 , 12 ). The CDI pathway of E. cloacae is inactive when bacteria are grown under standard culturing conditions.…”

“…Tli possesses an N-terminal lipoprotein signal sequence that directs its localization to the periplasm so that it can protect against incoming Tle injections by neighboring cells, which is a common property of immunity proteins for periplasm-targeting effectors ( 7 – 9 ). The majority of the transposon insertions that were identified in tli by Jensen et al ( 10 ) were within this lipoprotein signal sequence region, and the authors showed that, in contrast to the wild-type protein, these transposon-disrupted variants of Tli are not exported across the inner membrane to the periplasm and thus cannot confer protection against periplasm-delivered Tle. In addition to the insertion mutants that were present within the N-terminal lipoprotein region of Tli, Jensen et al ( 10 ) notably observed that mutants with insertion sites downstream of the lipoprotein region exhibited more modest permeability to X-gluc.…”

“…The majority of the transposon insertions that were identified in tli by Jensen et al ( 10 ) were within this lipoprotein signal sequence region, and the authors showed that, in contrast to the wild-type protein, these transposon-disrupted variants of Tli are not exported across the inner membrane to the periplasm and thus cannot confer protection against periplasm-delivered Tle. In addition to the insertion mutants that were present within the N-terminal lipoprotein region of Tli, Jensen et al ( 10 ) notably observed that mutants with insertion sites downstream of the lipoprotein region exhibited more modest permeability to X-gluc. This is particularly striking, as one would expect insertions in the tli gene that disrupt the mature form of the protein to be equally detrimental, if not more detrimental, to immunity function.…”

“…These findings led the authors to suspect that Tli may be required for the recruitment of Tle to the T6SS apparatus and/or may be required to activate Tle prior to export. Whereas the former was not addressed directly, Jensen et al ( 10 ) fused the lipase domain of Tle to the secreted T6SS spike protein, VgrG2, to ensure that the lipase would be delivered into the target cells. Notably, coculture competition assays with chimeric VgrG2-lipase showed that this fusion is only toxic if coexpressed with Tli.…”