Potential role for Human KCTD9 in the autophagy pathway: A novel autophagosome-associated protein
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Date
2014-04-16
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Johns Hopkins University
Abstract
KCTD9 belongs to the human potassium channel tetramerization domain (KCTD) family and is of unknown biological function. Our studies in yeast showed that the yeast homologue of KCTD, Whi2, is required to reduce TOR (target of rapamycin) kinase activity, thereby slowing down cell growth and inducing autophagy. Therefore, it was thought that KCTD9 may be associated to autophagosomes and playing a role in inducing autophagy.
For the first time, we were able to confirm that KCTD9’s subcellular localization. KCTD9 forms cytoplasmic puncta that fall within the range of the size of autophagosomes. In order to identify these structures, KCTD9 was co-expressed with numerous cellular markers including endosomal and autophagy proteins. KCTD9 puncta did not co-localize with endosomal markers, but co-localized with LC3, a canonical marker of autophagosomes. Furthermore, KCTD9 co-localized with autophagy proteins involved in the nucleation and elongation of the autophagosome - Vps34, Beclin-1, Atg5, and Atg12. While KCTD9 puncta re-localize to LC3 marked structures under starvation conditions, it induces the formation of Vps34 and Atg5 puncta in complete media. Furthermore, KCTD9 associated with the endoplasmic reticulum (ER) suggesting it could be localizing to subdomains of the ER from which autophagosomes derive from.
Knockdown of KCTD9 leads to a decrease in the conversion of LC3-I to LC3-II indicating a stall in the autophagy pathway. KCTD9 could be playing a role in the initiation of this pathway by recruiting early autophagy proteins on the ER. Moreover, since it co-localized with E3-ligase Cullin-3 and acts a substrate adaptor, KCTD9 could be recruiting selective autophagic cargo.
These results suggest that KCTD9 is a novel autophagosome-associated protein playing a potential role in the induction of the autophagic pathway.
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Keywords
KCTD9, Autophagy