Abstract
Bacterial type IV secretion systems (T4SSs) are largely responsible for the proliferation of multi-drug resistance. We solved the structure of the outer-membrane core complex (OMCCF) of a T4SS encoded by a conjugative F plasmid at <3.0 Å resolution by cryoelectron microscopy. The OMCCF consists of a 13-fold symmetrical outer ring complex (ORC) built from 26 copies of TraK and TraV C-terminal domains, and a 17-fold symmetrical central cone (CC) composed of 17 copies of TraB β-barrels. Domains of TraV and TraB also bind the CC and ORC substructures, establishing that these proteins undergo an intraprotein symmetry alteration to accommodate the C13:C17 symmetry mismatch. We present evidence that other pED208-encoded factors stabilize the C13:C17 architecture and define the importance of TraK, TraV and TraB domains to T4SSF function. This work identifies OMCCF structural motifs of proposed importance for structural transitions associated with F plasmid dissemination and F pilus biogenesis.
Highlights
Bacterial type IV secretion systems (T4SSs) are largely responsible for the proliferation of multi-drug resistance
The T4SSs are organized as cell-envelope-spanning nanomachines with distinct architectural features that include an outermembrane core complex (OMCC), an inner membrane complex (IMC), a connecting periplasmic stalk or cylinder, and two or three hexameric ATPases positioned at the cytoplasmic face of the IMC1,7
The flexible linker that was not detected in the cryolectron microscopy (CryoEM) map, but connects the two TraB domains involved in mismatch accommodation, does not contribute to OMCCF function
Summary
Bacterial type IV secretion systems (T4SSs) are largely responsible for the proliferation of multi-drug resistance. D,e,f Atomic models of the (d) entire structural unit, (e) the upper TraKCT/TraVCT complex showing residues whose interactions are predicted to stabilize the ORC, and (f) the lower TraKNT dimer with associated TraB linker segments.
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