?Why does zDHHC9 require an accessory protein (GCP16) for S-acylation activity

Abstract

acylation is a reversible post-translational protein modification regulating key cellular functions and physiological processes, including membrane targeting of proteins, channel and receptor function, vesicle trafficking, cell growth and adhesion, and synaptic plasticity. S-acylation is catalysed by the zDHHC family of palmitoyl acyltransferases, mutations in which have been implicated in numerous human pathologies. Disruption to zDHHC9 leads to intellectual disability, seizures, and language problems. Additionally, zDHHC9 mutations are associated with some cancers, via palmitoylation of the Ras proto-oncogenes. The zDHHC9 appears to be unique in the zDHHC family as it requires an obligatory accessory protein, GCP16. However, the role of the zDHHC9-GCP16 interaction remains to be clarified. The aim of this project was to critically analyse the evidence on the function of the GCP16 in regulating and modulating zDHHC9 activity. Experiments using human zDHHC9-GCP16 and the yeast orthologues Erf2-Erf4 provide evidence supporting the hypothesis that GCP16 functions to regulate zDHHC9 S-acylation activity by stabilizing the auto acylated enzyme intermediate. Yeast Erf4 is required for the stable formation of the palmitoyl-Erf2 intermediate and for palmitate transfer from Erf2 to Ras2 substrate. Furthermore, a potential secondary role of GCP16 in stabilising the zDHHC9 enzyme was revealed, with studies on Erf2-Erf4 suggesting that the accessory protein protects the zDHHC enzyme from ubiquitin-mediated degradation. Finally, a potential role of GCP16 in protein trafficking of zDHHC5 and non-apoptotic cell death was revealed, pointing to its potential broader cellular functions.