Thioesterase; part of the gene clusters that mediate the biosynthesis of AM-toxins, host-selective toxins (HSTs) causing Alternaria blotch on apple, a worldwide distributed disease (PubMed:17990954). AM-toxins are cyclic depsipeptides containing the 3 residues 2-hydroxy-isovaleric acid (2-HIV), dehydroalanine, L-alanine which are common for all 3 AM-toxins I to III. The fourth precursor is L-alpha-amino-methoxyphenyl-valeric acid (L-Amv) for AM-toxin I, L-alpha-amino-phenyl-valeric acid (L-Apv) for AM-toxin II, and L-alpha-amino-hydroxyphenyl-valeric acid (L-Ahv) for AM-toxin III (Probable). AM-toxins have two target sites for affecting susceptible apple cells; they cause invagination of the plasma membrane and electrolyte loss and chloroplast disorganization (PubMed:22846083). The non-ribosomal peptide synthetase AMT1 contains 4 catalytic modules and is responsible for activation of each residue in AM-toxin (PubMed:10875335). The aldo-keto reductase AMT2 catalyzes the conversion of 2-keto-isovaleric acid (2-KIV) to 2-hydroxy-isovaleric acid (2-HIV), one of the precursor residues incorporated by AMT1 during AM-toxin biosynthesis, by reduction of its ketone to an alcohol (PubMed:15066029). The cytochrome P450 monooxygenase AMT3 and the thioesterase AMT4 are also important for AM-toxin production, but their exact function within the AM-toxin biosynthesis are not known yet (PubMed:17990954). Up to 21 proteins (including AMT1 to AMT4) are predicted to be involved in AM-toxin biosynthesis since their expression ishighly up-regulated in AM-toxin-producing cultures (PubMed:17990954). Mycotoxin biosynthesis. Expression is up-regulated more than 10 fold in toxin producing cultures. Produces smaller amounts of AM-toxin than the wild type but still causes lesions on apple leaves. Gene clusters encoding host-selective toxins (HSTs) are localized on conditionally dispensable chromosomes (CDCs), also called supernumerary chromosomes, where they are present in multiple copies (PubMed:17990954). The CDCs are not essential for saprophytic growth but controls host-selective pathogenicity (PubMed:17990954). Belongs to the AMT4 thioesterase family.