Architecture of the outer-membrane core complex from a conjugative type IV secretion system

Area of Science:

• Microbiology
• Structural Biology
• Molecular Biology

Background:

• Conjugation is a primary mechanism for the dissemination of antibiotic resistance genes among bacteria.
• The type 4 secretion system (T4SS) is a large, double membrane-spanning nanomachine essential for bacterial conjugation.
• The F plasmid-encoded T4SS is a well-studied system, but its atomic-level structural details remain largely unknown.

Purpose of the Study:

• To determine the high-resolution structure of a complete conjugative outer membrane core complex (OMCC).
• To elucidate the structural basis for the function of the F-plasmid encoded T4SS in bacterial conjugation.

Main Methods:

• Cryo-electron microscopy (cryo-EM) was employed to solve the structure of the OMCC.
• The structure was determined at an unprecedented resolution of 3.3 Å.

Main Results:

• The 2.1 MDa F-OMCC complex exhibits a unique architecture with two radial concentric rings of differing symmetry.
• This unique arrangement leads to distinct protein stoichiometry and flexibility compared to other known OMCCs.
• The determined structure reveals the F-OMCC as a highly dynamic molecular machine.

Conclusions:

• The high-resolution structure provides critical insights into the mechanism of the F-plasmid T4SS.
• The dynamic nature of the F-OMCC has significant implications for pilus extension and retraction during DNA transfer.
• Understanding this structure is crucial for developing strategies to combat antibiotic resistance spread.