The peroxisomal ABC transporter， Comatose (CTS)， a full length transporter from Arabidopsis has intrinsic acyl-CoA thioesterase (ACOT) activity， important for physiological function. We used molecular modelling， mutagenesis and biochemical analysis to identify amino acid residues important for ACOT activity. D863， Q864 and T867 lie within transmembrane helix 9. These residues are orientated such that they might plausibly contribute to a catalytic triad similar to type II Hotdog fold thioesterases. When expressed in Saccharomyces cerevisiae， mutation of these residues to alanine resulted in defective of β-oxidation. All CTS mutants were expressed and targeted to peroxisomes and retained substrate-stimulated ATPase activity. When expressed in insect cell membranes， Q864A and S810N had similar ATPase activity to wild type but greatly reduced ACOT activity， whereas the Walker A mutant K487A had greatly reduced ATPase and no ATP-dependent ACOT activity. In wild type CTS， ATPase but not ACOT was stimulated by non-cleavable C14 ether-CoA. ACOT activity was stimulated by ATP but not by non-hydrolysable AMPPNP. Thus， ACOT activity depends on functional ATPase activity but not vice versa， and these two activities can be separated by mutagenesis. Whether D863， Q864 and T867 have a catalytic role or play a more indirect role in NBD-TMD communication is discussed.