The Type VI Secretion System Antifungal Effector Tfe2 Inhibits Protein Translation and Drives Hyperactivation of TORC1.

Type VI Secretion Systems (T6SS) are utilised by many bacteria to deliver toxic effectors into neighbouring bacterial, fungal or host cells. Whilst many antibacterial effectors are well characterised, much less is known regarding the identity or mode-of-action of antifungal effectors. Here we combine structural modelling with proteomics and in vivo approaches, to show that the Serratia marcescens antifungal effector Tfe2 adopts a novel fold and functions as a potent inhibitor of protein translation leading to hyperactivation of the TORC1 kinase. We show that Tfe2 expression in Saccharomyces cerevisiae, or treatment with the protein translation inhibitor cycloheximide, drive identical increases in free intracellular amino acids and hyperactivation of TORC1. This, in turn, triggers the Tfe2 and cycloheximide-mediated rapid turnover of amino acid transporters through stimulating substrate-independent endocytosis. Polysome profiling, however, revealed differences in Tfe2 and cycloheximide-mediated protein translation inhibition, with Tfe2 inhibiting initiation of translation. Tfe2-mediated hyperactivation of TORC1 may also underpin adaptive responses to this effector which include significant remodelling of the lipidome and notable alterations in organelle and cell wall structures. Collectively this study has provided new insight into the mode-of-action of a structurally novel antifungal effector Tfe2.