The intrinsically disordered Tarp protein from chlamydia binds actin with a partially preformed helix
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Updated: Feb 15, 2026
Actin Co-Sedimentation Assay; for the Analysis of Protein Binding to F-Actin
Published on: March 28, 2008
James Tolchard1, Samuel J Walpole1, Andrew J Miles2
1Center for Molecular and Structural Biology, School of Chemistry, University of East Anglia, Norwich Research Park, Norwich, NR4 7TJ, UK.
Abstract:
Tarp (translocated actin recruiting phosphoprotein) is an effector protein common to all chlamydial species that functions to remodel the host-actin cytoskeleton during the initial stage of infection. In C. trachomatis, direct binding to actin monomers has been broadly mapped to a 100-residue region (726-825) which is predicted to be predominantly disordered, with the exception of a ~10-residue α-helical patch homologous to other WH2 actin-binding motifs. Biophysical investigations demonstrate that a Tarp726-825 construct behaves as a typical intrinsically disordered protein; within it, NMR relaxation measurements and chemical shift analysis identify the ten residue WH2-homologous region to exhibit partial α-helix formation. Isothermal titration calorimetry experiments on the same construct in the presence of monomeric G-actin show a well defined binding event with a 1:1 stoichiometry and Kd of 102 nM, whilst synchrotron radiation circular dichroism spectroscopy suggests the binding is concomitant with an increase in helical secondary structure. Furthermore, NMR experiments in the presence of G-actin indicate this interaction affects the proposed WH2-like α-helical region, supporting results from in silico docking calculations which suggest that, when folded, this α-helix binds within the actin hydrophobic cleft as seen for other actin-associated proteins.
Insights
Translocated actin recruiting phosphoprotein (Tarp) remodels the host cytoskeleton during chlamydial infections. Biophysical studies reveal Tarp binds actin monomers, increasing its helical structure and facilitating infection.
Area of Science:
- Microbiology
- Structural Biology
- Biochemistry
Background:
- Tarp is a chlamydial effector protein crucial for host cell invasion.
- It remodels the host actin cytoskeleton during early infection stages.
- A specific region of Tarp (726-825) binds actin monomers.
Purpose of the Study:
- To characterize the biophysical properties of the Tarp actin-binding region.
- To elucidate the structural changes in Tarp upon G-actin binding.
- To understand the mechanism of Tarp-mediated actin cytoskeleton remodeling.
Main Methods:
- Nuclear Magnetic Resonance (NMR) spectroscopy for protein structure and dynamics.
- Isothermal Titration Calorimetry (ITC) for binding affinity measurements.
- Synchrotron Radiation Circular Dichroism (SRCD) spectroscopy for secondary structure analysis.
- In silico docking for predicting protein-protein interactions.
Main Results:
- The Tarp 726-825 region exhibits intrinsically disordered protein characteristics with a partially helical WH2-like motif.
- Tarp binds monomeric G-actin with high affinity (Kd = 102 nM) and 1:1 stoichiometry.
- Actin binding induces an increase in helical secondary structure within the Tarp construct.
- NMR and docking data suggest the helical Tarp region binds within the actin hydrophobic cleft.
Conclusions:
- Tarp undergoes a disorder-to-order transition upon actin binding, involving its WH2-like motif.
- This structural change is critical for Tarp's function in actin cytoskeleton manipulation during chlamydial infection.
- The findings provide insights into the molecular mechanisms of bacterial effector protein interactions with the host cytoskeleton.
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