Genetic Evidence for SecY Translocon-Mediated Import of Two Contact-Dependent Growth Inhibition (CDI) Toxins

Abstract

The C-terminal (CT) toxin domains of contact-dependent growth inhibition (CDI) CdiA proteins target Gram-negative bacteria and must breach both the outer and inner membranes of target cells to exert growth inhibitory activity. Here, we examine two CdiA-CT toxins that exploit the bacterial general protein secretion machinery after delivery into the periplasm. A Ser281Phe amino acid substitution in transmembrane segment 7 of SecY, the universally conserved channel-forming subunit of the Sec translocon, decreases the cytotoxicity of the membrane depolarizing orphan10 toxin from enterohemorrhagic Escherichia coli EC869. Target cells expressing secYS281F and lacking either PpiD or YfgM, two SecY auxiliary factors, are fully protected from CDI-mediated inhibition either by CdiA-CTo10EC869 or by CdiA-CTGN05224, the latter being an EndoU RNase CdiA toxin from Klebsiella aerogenes GN05224 that has a related cytoplasm entry domain. RNase activity of CdiA-CTGN05224 was reduced in secYS281F target cells and absent in secYS281F ΔppiD or secYS281F ΔyfgM target cells during competition co-cultures. Importantly, an allele-specific mutation in secY (secYG313W) renders ΔppiD or ΔyfgM target cells specifically resistant to CdiA-CTGN05224 but not to CdiA-CTo10EC869, further suggesting a direct interaction between SecY and the CDI toxins. Our results provide genetic evidence of a unique confluence between the primary cellular export route for unfolded polypeptides and the import pathways of two CDI toxins. IMPORTANCE Many bacterial species interact via direct cell-to-cell contact using CDI systems, which provide a mechanism to inject toxins that inhibit bacterial growth into one another. Here, we find that two CDI toxins, one that depolarizes membranes and another that degrades RNA, exploit the universally conserved SecY translocon machinery used to export proteins for target cell entry. Mutations in genes coding for members of the Sec translocon render cells resistant to these CDI toxins by blocking their movement into and through target cell membranes. This work lays the foundation for understanding how CDI toxins interact with the protein export machinery and has direct relevance to development of new antibiotics that can penetrate bacterial cell envelopes.

Lipidation of Class IV CdiA Effector Proteins Promotes Target Cell Recognition during Contact-Dependent Growth Inhibition

“…FHA-2 is thought to form a translocation conduit to transfer the toxin-containing CdiA-CT region into the target cell periplasm ( 19 ). Once inside the periplasm, the CdiA-CT is cleaved from the effector, and the released fragment hijacks integral membrane proteins to enter the target cell cytoplasm ( 19 , 25 , 26 ).…”

Burkholderia multivorans requires species‐specific GltJK for entry of a contact‐dependent growth inhibition system protein

“…Perhaps interactions of BcpA‐CT with membrane‐embedded or channel‐exposed portions of the GltK transmembrane helix or cytoplasmic region are necessary for translocation. Consistent with this hypothesis, a transmembrane helix of SecY was recently found to contribute to cytoplasmic transport of an E. coli CdiA‐CT (Jones et al., 2021 ).…”

Proteolytic processing induces a conformational switch required for antibacterial toxin delivery

“…1b , step 5). In the Enterobacteriaceae, entry domains that exploit MetI, GltJK, RbsC, FtsH, PtsG, SecY, and YciB have been described 18 , 22 . A similar import strategy is utilized in other phylogenetic clades.…”