Palmitoylation of the three isoforms of human endothelin-converting enzyme-1

Endothelin-converting enzyme-1 (ECE-1) is a membrane-bound metalloprotease that catalyses the conversion of inactive big endothelins into active endothelins. Here we have examined whether the three isoforms of human ECE-1 (ECE-1a, ECE-1b and ECE-1c) are modified by the covalent attachment of the fatty acid palmitate and have evaluated a potential functional role of this modification. To do this, wild-type and mutant enzymes were expressed and analysed by metabolic labelling with [3H]palmitate, immunoprecipitation and SDS/PAGE. All three ECE-1 isoforms were found to be palmitoylated via hydroxylamine-sensitive thioester bonds. In addition, the isoforms showed similar levels of acylation. Cys46 in ECE-1a, Cys58 in ECE-1b and Cys42 in ECE-1c were identified as sites of palmitoylation and each of these cysteines accounted for all the palmitoylation that occured in the corresponding isoform. Immunofluorescence analysis demonstrated further that palmitoylated and non-palmitoylated ECE-1 isoforms had the same subcellular localizations. Moreover, complete solubility of the three isoforms in Triton X-100 revealed that palmitoylation does not target ECE-1 to cholesterol and sphingolipid-rich membrane domains or caveolae. The enzymic activities of ECE-1a, ECE-1b and ECE-1c were also not significantly affected by the absence of palmitoylation.