Structural basis for effector recognition by an antibacterial type IV secretion system

Area of Science:

• Bacteriology
• Molecular Biology
• Structural Biology

Background:

• Soil, water, and plant bacteria use type IV secretion systems (T4SS) to translocate effector proteins (X-Tfes) into target cells, causing cell death.
• X-Tfes possess a carboxyl-terminal domain (XVIPCD) essential for interaction with the T4SS coupling protein VirD4 and for translocation.
• The structural basis of the XVIPCD-VirD4 interaction remains uncharacterized.

Purpose of the Study:

• To elucidate the structural basis of the interaction between the XVIPCD and the VirD4 coupling protein.
• To determine the functional regions within the XVIPCD domain responsible for VirD4 interaction and effector translocation.

Main Methods:

• Solution Nuclear Magnetic Resonance (NMR) spectroscopy to determine the structure of the XVIPCD.
• Mapping the interaction surface between XVIPCD and the VirD4 central all-alpha domain (VirD4_AAD).
• Isothermal titration calorimetry and in vivo competition assays to assess functional interactions.

Main Results:

• The XVIPCD directly interacts with the VirD4_AAD.
• The N-terminal region of XVIPCD, containing conserved motifs, mediates VirD4_AAD binding.
• Both N- and carboxyl-terminal regions of XVIPCD are necessary for efficient X-Tfe translocation.
• The N-terminal region's stability decreases at pH 7.0 and below, potentially aiding translocation.

Conclusions:

• The XVIPCD-VirD4 interaction is structurally defined by the XVIPCD's N-terminal region binding to VirD4_AAD.
• Distinct regions of XVIPCD have specialized roles in VirD4 interaction and translocation.
• pH-dependent conformational changes in XVIPCD may facilitate effector translocation into the periplasm.