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Type III secretion system effector proteins are mechanically labile.

Marc-André LeBlanc1,2, Morgan R Fink1, Thomas T Perkins3,4

  • 1Department of Biochemistry, University of Colorado, Boulder, CO 80309.

Proceedings of the National Academy of Sciences of the United States of America
|March 16, 2021
PubMed
Summary
This summary is machine-generated.

Type III secretion system (T3SS) effectors are mechanically labile, unfolding easily under low force. This mechanical property, not low stability, facilitates their secretion into host cells.

Keywords:
atomic force microscopyprotein stabilitysingle-molecule force spectroscopytype III secretion systemunfolding free energy

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Area of Science:

  • Microbiology
  • Molecular Biology
  • Biophysics

Background:

  • Gram-negative bacteria utilize type III secretion systems (T3SS) to inject effector proteins into host cells for colonization.
  • Effector secretion requires partial unfolding to navigate the narrow T3SS channel, but the mechanism remains unclear.

Purpose of the Study:

  • To investigate the thermodynamic stability and mechanical properties of Salmonella effector proteins SptP and SopE2.
  • To determine if mechanical lability, rather than low thermodynamic stability, facilitates T3SS effector secretion.

Main Methods:

  • Calculated unfolding free energy ([Formula: see text]) for SptP and SopE2.
  • Utilized atomic force microscopy (AFM)-based force spectroscopy to mechanically unfold individual SptP and SopE2 molecules.
  • Measured unfolding force (F_unfold) and distance to the transition state (Δx‡).

Main Results:

  • SptP and SopE2 exhibit typical globular protein stability ([Formula: see text] ≈ 6-7 kcal/mol).
  • SptP and SopE2 are mechanically labile, unfolding at low forces (≤17 pN) with large compliance (Δx‡ ≈ 1.5-1.6 nm).
  • These mechanical properties contrast sharply with robust, brittle proteins like GFP, ubiquitin, and DHFR.

Conclusions:

  • T3SS effector secretion is likely a mechanical unfolding process.
  • Mechanical lability, characterized by low unfolding force and high compliance, is crucial for efficient effector protein secretion via T3SS.