Fig. 1: Class II microcins interact with TonB-dependent transporters.
From: Structural insights into Cir-mediated killing by the antimicrobial protein Microcin V
A Schematic representation of Class II microcin domains. SS signal sequence, RBD receptor binding domain. B Schematic overview of class II microcin function. Immature pre-microcins are produced in the cytoplasm of producer strains and subsequently exported by Type I secretion (T1SS). This process is accomplished by a C39 peptidase-containing ATP-binding cassette transporter (CvaB, orange), a membrane fusion protein (CvaA, blue), and the outer-membrane efflux pump TolC (magenta). During secretion, CvaB cleaves the signal peptide of pre-microcins, releasing the mature protein for export. Mature microcins in supernatant then localize to the surface of prey bacteria and are imported by binding and exploiting receptors from the Ton (and Tol) system, which harness the proton motive force via the inner membrane Ton motor to energize transport across the outer membrane. Once they have entered the periplasm, microcins are free to interact with their cognate targets at the inner membrane or cytoplasm. Producer strains are protected from auto-bactericidal activity via production of an immunity protein. OM outer membrane, PG peptidoglycan, IM inner membrane, TDBT TonB-dependent transporter. C Flow cytometry histograms showing dansyl fluorescence shifts. WT E. coli cells treated with dansyl-MccV RBD (blue peaks) exhibit a shift relative to the untreated population (red peaks), and a greater shift is observed when Cir is overexpressed from a plasmid (right graph). This shift is lost when the Cir receptors are absent in a cirA::kan mutant (middle graph). D Zone of inhibition assays demonstrating the sensitivity of E. coli to spotted predator strain secreting WT MccV. The WT E. coli lawn demonstrates clearance, indicating sensitivity to MccV. This activity is lost when cirA is knocked out. Activity is restored when a plasmid expressing Cir is used to transform the cirA::kan strain, while presence of an empty plasmid lacking a complementary cirA gene remains resistant.
