Abstract
ABSTRACTThe 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 enteropathogenic Escherichia coli (EPEC) autotransporter, and YopH, a protein detected on the surface of Yersinia, 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.
Highlights
The type 3 secretion system (T3SS) is essential for bacterial virulence through delivering effector proteins directly into the host cytosol
Histidinetagged segment proteins (42 kDa) and His-EPEC secreted protein A (EspA) were purified in nickel-nitrilotriacetic acid (NTA) columns, while the whole EspC passenger domain was purified from supernatants by filtration through 100-kDa-cutoff membranes (Fig. 1B)
Interactions of these purified proteins with His-EspA were analyzed by blot overlay; proteins were run in SDS-PAGE, transferred to polyvinylidene difluoride (PVDF) membranes, and probed with His-EspA
Summary
The type 3 secretion system (T3SS) is essential for bacterial virulence through delivering effector proteins directly into the host cytosol. 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 enteropathogenic Escherichia coli (EPEC) autotransporter, and YopH, a protein detected on the surface of Yersinia, 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. All these effectors hamper different aspects of the cell physiology, including subverting innate immune pathways, those involved in phagocytosis, host cell survival, apoptotic cell death, and inflammatory signaling, which are all required to cause disease [20, 22]
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