Quantifying the role of chaperones in protein translocation by computational modeling.

Salvatore Assenza,Paolo De Los Rios,Alessandro Barducci

doi:10.3389/fmolb.2015.00008

Salvatore Assenza, Paolo De Los Rios + Show 1 more

Open Access

https://doi.org/10.3389/fmolb.2015.00008

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Abstract

The molecular chaperone Hsp70 plays a central role in the import of cytoplasmic proteins into organelles, driving their translocation by binding them from the organellar interior. Starting from the experimentally-determined structure of the E. coli Hsp70, we computed, by means of molecular simulations, the effective free-energy profile for substrate translocation upon chaperone binding. We then used the resulting free energy to quantitatively characterize the kinetics of the import process, whose comparison with unassisted translocation highlights the essential role played by Hsp70 in importing cytoplasmic proteins.

Highlights

Molecular chaperones are protein machines that assist other proteins in various cellular processes. 70-kDa Heat Shock Proteins (Hsp70s) are possibly the most versatile chaperones, supervising a wide variety of cellular tasks (Mayer and Bukau, 2005) that range from disaggregation of stable protein aggregates (Diamant et al, 2000) to driving post-translational import of cytoplasmic proteins into organelles (Matlack et al, 1999; Neupert and Brunner, 2002; Liu et al, 2014)
In the mitochondrial matrix chaperone-substrate dissociation is accelerated by the presence of nucleotide exchange factors, that catalyze the release of ADP and the ensuing rebinding of ATP
For the present purposes the number n of substrate residues that have been imported into the mitochondrial matrix is a convenient coordinate to describe the system, whose dynamics can be modeled as a diffusion process on the corresponding free-energy landscape

Summary

Introduction

Molecular chaperones are protein machines that assist other proteins in various cellular processes. 70-kDa Heat Shock Proteins (Hsp70s) are possibly the most versatile chaperones, supervising a wide variety of cellular tasks (Mayer and Bukau, 2005) that range from disaggregation of stable protein aggregates (Diamant et al, 2000) to driving post-translational import of cytoplasmic proteins into organelles (Matlack et al, 1999; Neupert and Brunner, 2002; Liu et al, 2014). According to the current view, an ATP-consuming import motor located into the mitochondrial matrix drives the inward translocation of nuclear-encoded proteins. Mitochondrial Hsp (mtHsp70) is the central element of this motor: it is recruited by the TIM complex on the matrix side through interactions with the TIM44 protein, which is part of the pore, and with the pore-associated PAM16/18 proteins. The latter contain a J domain, whose role is to dramatically enhance the ATP-hydrolysis rate of Hsp, increasing by orders of magnitude its affinity for substrates. The ATP-driven binding of the chaperones to incoming proteins drives their translocation

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