Abstract
In eukaryotic organisms, cysteine palmitoylation is an important reversible modification that impacts protein targeting, folding, stability, and interactions with partners. Evidence suggests that protein palmitoylation contributes to key biological processes in Apicomplexa with the recent palmitome of the malaria parasite Plasmodium falciparum reporting over 400 substrates that are modified with palmitate by a broad range of protein S-acyl transferases. Dynamic palmitoylation cycles require the action of an acyl-protein thioesterase (APT) that cleaves palmitate from substrates and conveys reversibility to this posttranslational modification. In this work, we identified candidates for APT activity in Toxoplasma gondii. Treatment of parasites with low micromolar concentrations of β-lactone- or triazole urea-based inhibitors that target human APT1 showed varied detrimental effects at multiple steps of the parasite lytic cycle. The use of an activity-based probe in combination with these inhibitors revealed the existence of several serine hydrolases that are targeted by APT1 inhibitors. The active serine hydrolase, TgASH1, identified as the homologue closest to human APT1 and APT2, was characterized further. Biochemical analysis of TgASH1 indicated that this enzyme cleaves substrates with a specificity similar to APTs, and homology modeling points toward an APT-like enzyme. TgASH1 is dispensable for parasite survival, which indicates that the severe effects observed with the β-lactone inhibitors are caused by the inhibition of non-TgASH1 targets. Other ASH candidates for APT activity were functionally characterized, and one of them was found to be resistant to gene disruption due to the potential essential nature of the protein.
Highlights
S-Palmitoylation is an important reversible modification that involves the action of an acyl-protein thioesterase (APT)
Evidence suggests that protein palmitoylation contributes to key biological processes in Apicomplexa with the recent palmitome of the malaria parasite Plasmodium falciparum reporting over 400 substrates that are modified with palmitate by a broad range of protein S-acyl transferases
Dynamic palmitoylation cycles require the action of an acyl-protein thioesterase (APT) that cleaves palmitate from substrates and conveys reversibility to this posttranslational modification
Summary
S-Palmitoylation is an important reversible modification that involves the action of an acyl-protein thioesterase (APT). As a member of the superfamily of ␣/-hydrolases, the serine hydrolase class of enzymes, APT1 was initially classified as a lysophospholipase [25] until the favored substrates were identified as thioacylated proteins [26] This enzyme is potentially responsible for the depalmitoylation of a number of proteins that go through a palmitoylation cycle, only a few candidates for this activity have been identified. We report severe effects caused by inhibitors of the hAPTs on the lytic cycle of T. gondii and P. falciparum By using these inhibitors in combination with a serine hydrolase probe, we detected several enzymes potentially responsible for APT activity. A gene closely homologous to hAPT1 encodes one target of these inhibitors This active serine hydrolase 1 (TgASH1) is conserved across the coccidian subgroup of the Apicomplexa and is dispensable for T. gondii survival in tissue culture. We investigate the properties of TgASH1 and three distantly related putative serine hydrolases that may contribute to APT activity and compensate for the loss of TgASH1
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