A Small Family of Chloroplast Atypical Thioredoxins

Inbal Dangoor,Alexander Levitan,Ohad Pasand,Avihai Danon,Hadas Peled-Zehavi

doi:10.1104/pp.108.128314

Inbal Dangoor, Alexander Levitan + Show 3 more

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https://doi.org/10.1104/pp.108.128314

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Journal: Plant Physiology	Publication Date: Dec 24, 2008
Citations: 67	License type: CC BY 4.0

Affiliation: Weizmann Institute of Science

Abstract

The reduction and the formation of regulatory disulfide bonds serve as a key signaling element in chloroplasts. Members of the thioredoxin (Trx) superfamily of oxidoreductases play a major role in these processes. We have characterized a small family of plant-specific Trxs in Arabidopsis (Arabidopsis thaliana) that are rich in cysteine and histidine residues and are typified by a variable noncanonical redox active site. We found that the redox midpoint potential of three selected family members is significantly less reducing than that of the classic Trxs. Assays of subcellular localization demonstrated that all proteins are localized to the chloroplast. Selected members showed high activity, contingent on a dithiol electron donor, toward the chloroplast 2-cysteine peroxiredoxin A and poor activity toward the chloroplast NADP-malate dehydrogenase. The expression profile of the family members suggests that they have distinct roles. The intermediate redox midpoint potential value of the atypical Trxs might imply adaptability to function in modulating the redox state of chloroplast proteins with regulatory disulfides.

Highlights

The reduction and the formation of regulatory disulfide bonds serve as a key signaling element in chloroplasts
The first protein with a noncanonical active site that emerged from a National Center for Biotechnology Information (NCBI) BLASTP search using a query of AtTrx-f1 sequence was AtACHT1
The AtACHTs constitute a small family of chloroplast Trx-like proteins that display a redox midpoint potential that is significantly less reducing compared to the classic Trxs (Fig. 3), a redox active site with a different central dipeptide than the canonical sequence, and several additional conserved Cys and His residues outside of the active site (Fig. 1)

Summary

Introduction

The reduction and the formation of regulatory disulfide bonds serve as a key signaling element in chloroplasts. Trxs share a similar fold (Trx-fold) and a common active site motif comprised of two vicinal Cys separated by two variable amino acids, the CXXC motif (Martin, 1995) Despite these common features, members of the Trx superfamily have diverse functions ranging from protein disulfide reductases to disulfide isomerases or to disulfide transferases. The classic chloroplast Trxs are implicated in the light-dependent activation by reduction of different enzymes, including several metabolic enzymes such as NADP-malate dehydrogenase (MDH) and Fru-1,6-bisP, as well as in protection against oxidative stress (Schurmann and Buchanan, 2008) Additional roles, such as in oxidative-type reactions, are inferred indirectly by studies demonstrating that protein disulfide bonds are formed correctly in the chloroplast (Staub et al, 2000; Bally et al, 2008; Wittenberg and Danon, 2008). It was found that HCF164 is essential for the assembly of the chloroplast cytochrome b6f (Lennartz et al, 2001)

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