INDUCTION OF BETA-METHYLCROTONYL-COENZYME-A CARBOXYLASE IN HIGHER-PLANT CELLS DURING CARBOHYDRATE STARVATION - EVIDENCE FOR A ROLE OF MCCASE IN LEUCINE CATABOLISM

Induction of beta-methylcrotonyl-coenzyme A carboxylase (MCCase) activ ity was observed during carbohydrate starvation in sycamore cells. In mitochondria isolated from starved cells, we noticed a marked accumula tion of the biotinylated subunit of MCCase, of which the apparent mole cular weight of 74 000 was similar to that of the polypeptide from mit ochondria of potato tubers. Our results provide evidence for a role of MCCase in the catabolic pathway of leucine, a branched-chain amino ac id which transiently accumulates in carbon-starved cells in relation t o a massive breakdown of proteins. Furthermore, when control sycamore cells mere incubated in the presence of exogenous leucine, this amino acid accumulated in the cells and no induction or accumulation of MCCa se was observed, indicating that leucine is not responsible for the in duction of its catabolic machinery. Finally, MCCase is proposed as a n ew biochemical marker of the autophagic process triggered by carbohydr ate starvation.