Tracking the Unfolding Pathway of a Multirepeat Protein via Tryptophan Scanning: EVIDENCE OF LOCALIZED INSTABILITY IN THE MITOCHONDRIAL IMPORT RECEPTOR Tom70

Simon R Bushell,Stephen P Bottomley,Jamie Rossjohn,Travis Beddoe

doi:10.1074/jbc.m602966200

Simon R Bushell, Stephen P Bottomley + Show 2 more

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https://doi.org/10.1074/jbc.m602966200

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Abstract

The tetratricopeptide repeat (TPR) is a degenerate 34-amino acid repeating motif that forms a repeating helix-turn-helix structure and is a well characterized mediator of protein-protein interactions. Recently, a biophysical investigation on one naturally occurring TPR protein, Tom70, found that the mitochondrial receptor displayed an unusual three-state unfolding pathway, distinct from the two-state model usually displayed by TPR proteins. To investigate this unusual behavior, we undertook a tryptophan-scanning analysis of Tom70, where both native and engineered tryptophan residues are used as fluorescent reporters to monitor the range of local and global unfolding events that comprise the unfolding pathway of Tom70. Specifically, seven Tom70 variants were constructed, each with a single tryptophan residue in each of the seven TPR repeats of Tom70. By combining equilibrium and kinetic fluorescent unfolding assays, with circular dichroism experiments, our study reveals that the unusual folding pathway of Tom70 is a consequence of the unfolding of two separate, autonomous TPR arrays, with the less stable region appearing to account for the low structural stability of Tom70.

Highlights

The tetratricopeptide repeat (TPR) is a degenerate 34-amino acid repeating motif that forms a repeating helix-turn-helix structure and is a well characterized mediator of protein-protein interactions
By combining equilibrium and kinetic fluorescent unfolding assays, with circular dichroism experiments, our study reveals that the unusual folding pathway of Tom70 is a consequence of the unfolding of two separate, autonomous TPR arrays, with the less stable region appearing to account for the low structural stability of Tom70
Using site-directed mutagenesis, Trp124 was conservatively mutated to a phenylalanine, ensuring that any emission signal detected when the protein is excited at 295 nm will arise from a single tryptophan introduced into each TPR motif

Summary

Introduction

The tetratricopeptide repeat (TPR) is a degenerate 34-amino acid repeating motif that forms a repeating helix-turn-helix structure and is a well characterized mediator of protein-protein interactions. By combining equilibrium and kinetic fluorescent unfolding assays, with circular dichroism experiments, our study reveals that the unusual folding pathway of Tom is a consequence of the unfolding of two separate, autonomous TPR arrays, with the less stable region appearing to account for the low structural stability of Tom. By combining equilibrium and kinetic fluorescent unfolding assays, with circular dichroism experiments, our study reveals that the unusual folding pathway of Tom is a consequence of the unfolding of two separate, autonomous TPR arrays, with the less stable region appearing to account for the low structural stability of Tom70 Since it was first identified in 1990 [1], tetratricopeptide repeat (TPR) motifs have been found in over 300 different proteins [2, 3]. Bioinformatic analysis of the primary structure of Tom yields suggests that its TPR repeats are organized in a 2:5 arrangement [15], an unusual composition, considering most other TPR-containing proteins have just a single array

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