Kazuya Nishio

Deficiencies in mitochondrial protein import are associated with a number of diseases. However, although nonimported mitochondrial proteins are at great risk of aggregation, it remains largely unclear how their accumulation causes cell dysfunction. Here, we show that nonimported citrate synthase is targeted for proteasomal degradation by the ubiquitin ligase SCFUcc1. Unexpectedly, our structural and genetic analyses revealed that nonimported citrate synthase appears to form an enzymatically active conformation in the cytosol. Its excess accumulation caused ectopic citrate synthesis, which, in turn, led to an imbalance in carbon flux of sugar, a reduction of the pool of amino acids and nucleotides, and a growth defect. Under these conditions, translation repression is induced and acts as a protective mechanism that mitigates the growth defect. We propose that the consequence of mitochondrial import failure is not limited to proteotoxic insults, but that the accumulation of a nonimported metabolic enzyme elicits ectopic metabolic stress.

Citrate synthase (CS) catalyzes the conversion of oxaloacetate and acetyl coenzyme A into citrate and coenzyme A in the mitochondrial tricarboxylic acid (TCA) cycle. In plants, mitochondrial metabolism, including the TCA cycle, occurs in interaction with photosynthetic metabolism. The controlled regulation of several enzymes in the TCA cycle, such as CS, is important in plants. Here, the first crystal structure of a plant mitochondrial CS, CSY4 from Arabidopsis thaliana (AtCSY4), has been determined. Structural comparison of AtCSY4 with mitochondrial CSs revealed a high level of similarity. Inhibition analysis showed a similar manner of inhibition as in mitochondrial CSs. The effect of oxidation on one of a pair of cysteine residues in AtCSY4 was speculated upon based on the folded structure.