Identification of a post-translationally myristoylated autophagy-inducing domain released by caspase cleavage of huntingtin

Abstract

Huntingtondisease(HD) isadebilitatingneurodegenerativediseasecharacterizedbythelossofmotorcontroland cognitive ability that ultimately leads todeath. It iscausedbythe expansionofapolyglutamine tract in thehuntingtin (HTT) protein, which leads to aggregation of the protein and eventually cellular death. Both the wild-type and mutant form of the protein are highly regulated by post-translational modifications including proteolysis, palmitoylation and phosphorylation. We now demonstrate the existence of a new post-translational modification of HTT: the addition of the 14 carbon fatty acidmyristate to a glycine residue exposed on a caspase-3-cleaved fragment (post-translationalmyristoylation)andthatmyristoylationof thisfragment isalteredinaphysiologically relevant model of mutant HTT. Myristoylated HTT553-585-EGFP, but not its non-myristoylated variant, initially localized to the ER, induced the formation of autophagosomes and accumulated in abnormally large autophagolysosomal/lysosomal structures in a variety of cell types, including neuronal cell lines under nutrient-rich conditions. Our results suggest thataccumulationofmyristoylatedHTT553-586 incellsmay alter the rateof productionof autophagosomes and/or their clearance through the heterotypic autophagosomal/lysosomal fusion process. Overall, our novel observations establish a role for the post-translational myristoylation of a caspase-3-cleaved fragment of HTT, highly similar to the Barkor/ATG14L autophagosome-targeting sequence domain thought to sense,maintainand/or promotemembrane curvature in the regulationofautophagy.Abnormalprocessingorproduction of thismyristoylated HTT fragmentmight be involved in the pathophysiology of HD. © The Author 2014. Published by Oxford University Press. All rights reserved.