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.
Summary
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.
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.
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