title: A Novel Mechanism for Protein Delivery by the Type 3 Secretion System for Extracellularly Secreted Proteins.
abstract: The type 3 secretion system (T3SS) is essential for bacterial virulence through delivering effector proteins directly into the host cytosol. Here, we identified an alternative delivery mechanism of virulence factors mediated by the T3SS, which consists of the association of extracellularly secreted proteins from bacteria with the T3SS to gain access to the host cytosol. Both EspC, a protein secreted as an enteropathogenicEscherichia coli(EPEC) autotransporter, and YopH, a protein detected on the surface ofYersinia, require a functional T3SS for host cell internalization; here we provide biophysical and molecular evidence to support the concept of the EspC translocation mechanism, which requires (i) an interaction between EspA and an EspC middle segment, (ii) an EspC translocation motif (21 residues that are shared with the YopH translocation motif), (iii) increases in the association and dissociation rates of EspC mediated by EspA interacting with EspD, and (iv) an interaction of EspC with the EspD/EspB translocon pore. Interestingly, this novel mechanism does not exclude the injection model (i.e., EspF) operating through the T3SS conduit; therefore, T3SS can be functioning as an internal conduit or as an external railway, which can be used to reach the translocator pore, and this mechanism appears to be conserved among different T3SS-dependent pathogens.IMPORTANCEThe type 3 secretion system is essential for injection of virulence factors, which are delivered directly into the cytosol of the host cells for usurping and subverting host processes. Recent studies have shown that these effectors proteins indeed travel inside an "injectisome" conduit through a single step of translocation by connecting the bacterium and host cell cytoplasms. However, all findings are not compatible with this model. For example, both YopH, a protein detected on the surface ofYersinia, and EspC, an autotransporter protein secreted by enteropathogenicE. coli, require a functional T3SS for host cell translocation. Both proteins have an intermediate extracellular step before their T3SS-dependent translocation. Here, we show an alternative delivery mechanism for these extracellularly secreted virulence factors that are then incorporated into the T3SS to enter the cells; this novel mechanism coexists with but diverges from the canonical injection model that involves the passage of the protein inside the injectisome.
authors.affiliation: Department of Cell Biology, Centro de Investigación y de Estudios Avanzados del IPN (CINVESTAV-IPN), Mexico City, Mexico.
authors.forename: Farid
authors.initials: F
authors.lastname: Tejeda-Dominguez
authors.affiliation: Department of Cell Biology, Centro de Investigación y de Estudios Avanzados del IPN (CINVESTAV-IPN), Mexico City, Mexico.
authors.forename: Jazmin
authors.initials: J
authors.lastname: Huerta-Cantillo
authors.affiliation: Department of Cell Biology, Centro de Investigación y de Estudios Avanzados del IPN (CINVESTAV-IPN), Mexico City, Mexico.
authors.forename: Lucia
authors.initials: L
authors.lastname: Chavez-Dueñas
authors.affiliation: Department of Cell Biology, Centro de Investigación y de Estudios Avanzados del IPN (CINVESTAV-IPN), Mexico City, Mexico fnavarro@cell.cinvestav.mx.
authors.forename: Fernando
authors.initials: F
authors.lastname: Navarro-Garcia
mesh_terms: D001425:Bacterial Outer Membrane Proteins / Q000378:metabolism*; D004926:Escherichia coli / Q000378:metabolism*; D029968:Escherichia coli Proteins / Q000378:metabolism*; D021381:Protein Transport; D017027:Protein Tyrosine Phosphatases / Q000378:metabolism*; D000069296:Type III Secretion Systems / Q000378:metabolism*; D015007:Yersinia / Q000378:metabolism*
chemical_list: D001425:Bacterial Outer Membrane Proteins; D029968:Escherichia coli Proteins; C102541:EspC protein, E coli; D000069296:Type III Secretion Systems; D017027:Protein Tyrosine Phosphatases; C067571:yopH protein, Yersinia
keywords: EspA filament; EspB-EspD pore; EspC; YopH; autotransporter proteins; enteropathogenic E. coli; protein binding; protein translocation; secretion systems; translocator proteins