Thioredoxin links redox to the regulation of fundamental processes of plant mitochondria

Abstract

Mitochondria contain thioredoxin (Trx), a regulatory disulfide protein, and an associated flavoenzyme, NADP/Trx reductase, which provide a link to NADPH in the organelle. Unlike animal and yeast counterparts, the function of Trx in plant mitochondria is largely unknown. Accordingly, we have applied recently devised proteomic approaches to identify soluble Trx-linked proteins in mitochondria isolated from photosynthetic (pea and spinach leaves) and heterotrophic (potato tubers) sources. Application of the mitochondrial extracts to mutant Trx affinity columns in conjunction with proteomics led to the identification of 50 potential Trx-linked proteins functional in 12 processes: photorespiration, citric acid cycle and associated reactions, lipid metabolism, electron transport, ATP synthesis/transformation, membrane transport, translation, protein assembly/folding, nitrogen metabolism, sulfur metabolism, hormone synthesis, and stress-related reactions. Almost all of these targets were also identified by a fluorescent gel electrophoresis procedure in which reduction by Trx can be observed directly. In some cases, the processes targeted by Trx depended on the source of the mitochondria. The results support the view that Trx acts as a sensor and enables mitochondria to adjust key reactions in accord with prevailing redox state. These and earlier findings further suggest that, by sensing redox in chloroplasts and mitochondria, Trx enables the two organelles of photosynthetic tissues to communicate by means of a network of transportable metabolites such as dihydroxyacetone phosphate, malate, and glycolate. In this way, light absorbed and processed by means of chlorophyll can be perceived and function in regulating fundamental mitochondrial processes akin to its mode of action in chloroplasts.

Fig. 1.

Reduction of proteins of pea leaf mitochondria by Trx after treatment with N-ethylmaleimide to remove background SH groups. The mutant affinity column eluate was either not treated (A) or reduced by Trx (B) using E. coli Trx, NADPH, and NTR. After reduction, the proteins were derivatized with monobromobimane and subjected to 2D electrophoresis. (C) As in B, but stained for protein.

Fig. 2.

Role of Trx as a regulatory link between chloroplasts and mitochondria. Reactions linking Trx to reproduction and apoptosis are not included. The mitochondrial pools of NADH and NADPH would be equilibrated by transhydrogenase activities (45). Chl, chlorophyll; Fd, ferredoxin.