A Superfamily of T6SS Antibacterial Effectors Displaying L,D-carboxypeptidase Activity Towards Peptidoglycan

Abstract

SummaryType VI secretion systems (T6SSs) are contractile nanomachines used by bacteria to inject toxic effectors into competitors. The identity and mechanism of many effectors remain unknown. We characterized a Salmonella SPI-6 T6SS antibacterial effector called Tae5STM (type VI amidase effector 5). Tae5STM is toxic in target-cell periplasm and is neutralized by its cognate immunity protein (Tai5STM). Microscopy analysis revealed that cells expressing the effector stop dividing and lose cell envelope integrity. Bioinformatic analysis uncovered similarities between Tae5STM and the catalytic domain of L,D-transpeptidase. Point mutations on conserved catalytic histidine and cysteine residues abrogated toxicity. Biochemical assays revealed that Tae5STM displays L,D-carboxypeptidase activity, cleaving peptidoglycan tetrapeptides between meso-diaminopimelic acid3 and D-alanine4. Phylogenetic analysis showed that Tae5STM homologs constitutes a new superfamily of T6SS-associated amidase effectors distributed among α-, β- and γ-proteobacteria. This work expands our current knowledge about bacterial effectors used in interbacterial competition.

Identification of A Putative T6SS Immunity Islet in Salmonella Typhi

“…Finally, a t0734 homolog was found to reside downstream of a protein with a conserved L,D-transpeptidase domain, suggesting it is a peptidoglycan-active protein. Although this activity in itself is not generally bacteriolytic, a recent report identifies a novel class of peptidoglycan amidase T6SS effectors with L,D-carboxypeptidase activity that exhibits significant similarity to L,D-transpeptidases [ 56 ]. Coupled with its location immediately downstream of hcp , this suggests that this putative L,D-transpeptidase-domain protein is also likely to encode an effector protein.…”