Abstract
Macroautophagy requires membrane trafficking and remodelling to form the autophagosome and deliver its contents to lysosomes for degradation. We have previously identified the TBC domain-containing protein, TBC1D14, as a negative regulator of autophagy that controls delivery of membranes from RAB11-positive recycling endosomes to forming autophagosomes. In this study, we identify the TRAPP complex, a multi-subunit tethering complex and GEF for RAB1, as an interactor of TBC1D14. TBC1D14 binds to the TRAPP complex via an N-terminal 103 amino acid region, and overexpression of this region inhibits both autophagy and secretory traffic. TRAPPC8, the mammalian orthologue of a yeast autophagy-specific TRAPP subunit, forms part of a mammalian TRAPPIII-like complex and both this complex and TBC1D14 are needed for RAB1 activation. TRAPPC8 modulates autophagy and secretory trafficking and is required for TBC1D14 to bind TRAPPIII. Importantly, TBC1D14 and TRAPPIII regulate ATG9 trafficking independently of ULK1. We propose a model whereby TBC1D14 and TRAPPIII regulate a constitutive trafficking step from peripheral recycling endosomes to the early Golgi, maintaining the cycling pool of ATG9 required for initiation of autophagy.
Highlights
To remain healthy, cells must clear their cytosol of misfolded proteins, dysfunctional organelles and intracellular pathogens
We have previously demonstrated that endogenous TBC1D14 is localised to the Golgi stack (Longatti et al, 2012), and we observed that the core TRAPP subunit TRAPPC4 and its GEF target RAB1B both partially localise to the Golgi with TBC1D14 in HEK293A cells (Fig 1F)
We show here that the mammalian version of the TRAPPIII complex binds to TBC1D14, and that this interaction is mediated by an N-terminal 103 amino acid stretch in TBC1D14, which we call the TRAPPbinding region (TBR), and TRAPPC8, the mammalian orthologue of yeast Trs85 (Scrivens et al, 2011)
Summary
Cells must clear their cytosol of misfolded proteins, dysfunctional organelles and intracellular pathogens To this end, eukaryotes employ the evolutionarily conserved autophagy pathways (Mizushima et al, 2008). The completed autophagosome matures by fusing with the lysosome, allowing the contents of the autophagosome to be degraded and returned to the cytosol. This recycling function is crucial for cells and organisms to survive periods of stress, such as amino acid starvation, growth factor withdrawal and hypoxia (Lamb et al, 2013b), and dysregulation of the autophagy pathways plays a role in pathological states including ageing, bacterial infection, neurodegeneration and cancer (Mizushima et al, 2008)
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