Induction of beta-methylcrotonyl-coenzyme A carboxylase (MCCase) activity was observed during carbohydrate starvation in sycamore cells. In mitochondria isolated from starved cells, we noticed a marked accumulation of the biotinylated subunit of MCCase, of which the apparent molecular weight of 74000 was similar to that of the polypeptide from mitochondria of potato tubers. Our results provide evidence for a role of MCCase in the catabolic pathway of leucine, a branched-chain amino acid which transiently accumulates in carbon-starved cells in relation to a massive breakdown of proteins. Furthermore, when control sycamore cells were incubated in the presence of exogenous leucine, this amino acid accumulated in the cells and no induction or accumulation of MCCase was observed, indicating that leucine is not responsible for the induction of its catabolic machinery. Finally, MCCase is proposed as a new biochemical marker of the autophagic process triggered by carbohydrate starvation.