Acyl protein thioesterases hydrolyze fatty acid thioesters on cysteine residues of proteins. The two protein depalmitoylases APT1 and APT2 have a very high degree of similarity and show substantial overlap in substrate utility. Potent, selective, and cell-permeable activity-based probes are needed to study the role of these enzymes. Here, we employ solid-phase synthesis to create a library of covalent probes based on a triazole urea-reactive electrophile, leading to several potent and cell-permeable probes of human APT1/2. We demonstrate that inhibition of APT1/2 in cells does not have an effect on steady-state levels of protein palmitoylation, implying that substrates hydrolyzed by APT1/2 can also be hydrolyzed by other protein depalmitoylases.
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