Compartmentation of the branched-chain amino acid biosynthetic pathways of S. cerevisiae. The boxed characters refer to the genes involved directly or indirectly in the pathways. Their protein products are as follows: ILV1, threonine deaminase; ILV2, acetohydroxy acid synthase catalytic subunit; ILV5, acetohydroxy acid reductoisomerase; ILV3, dihydroxy acid dehydratase; BAT1, mitochondrial branched-chain amino acid aminotransferase; BAT2, cytosolic branched-chain amino acid aminotransferase; LEU4l, α-isopropylmalate synthase I, long form; LEU4s, α-isopropylmalate synthase I, short form; LEU9, α-isopropylmalate synthase II; X, hypothetical α-isopropylmalate transporter; LEU1, isopropylmalate isomerase; LEU2, β-isopropylmalate dehydrogenase; LEU5, protein necessary for the accumulation of CoA within the mitochondria; presumably an importer of CoA or precursor thereof; ATM1, ABC transporter involved in exporting Fe-S clusters to the cytosol; the implied interaction between Bat1p and Atmp is hypothetical (58, 66). Abbreviations: KB, α-ketobutyrate; AALD, active acetaldehyde; PYR, pyruvate; AL, acetolactate; AHB, α-aceto-α-hydroxybutyrate; DHIV, α,β-dihydroxyisovalerate; DHMV, α,β-dihydroxy-β-methylvalerate; KIV, α-ketoisovalerate; KMV, α-keto-β-methylvalerate; α-IPM, α-isopropylmalate; β-IPM, β-isopropylmalate; KIC, α-ketoisocaproate; Fe/S, iron-sulfur cluster. Not shown for reasons of clarity: transport of leucine and/or KIC back into the mitochondria.