Caspase cleavage of Atg4D stimulates GABARAP-L1 processing and triggers mitochondrial targeting and apoptosis.
Autophagy is an important catabolic process with roles in cell survival and cell death. It sequesters cytosol and organelles within double-membrane autophagosomes that deliver their contents to lysosomes for degradation. Autophagosome biogenesis is coordinated by the autophagy-related protein 4 (Atg4) family of C54 endopeptidases (Atg4A-Atg4D). These enzymes prime and then later delipidate the autophagosome marker, Atg8. Here, we show that one family member, Atg4D, is cleaved by caspase-3 in vitro and in apoptotic cells. Atg4D is a poor priming and delipidation enzyme in vitro, but truncated DeltaN63 Atg4D displays increased activity against the Atg8 paralogue, gamma-aminobutyric acid receptor-associated protein-like 1 (GABARAP-L1). In living cells, DeltaN63 Atg4D stimulates the delipidation of GABARAP-L1, whereas siRNA silencing of the gene expressing Atg4D abrogates GABARAP-L1 autophagosome formation and sensitises cells to starvation and staurosporine-induced cell death. Interestingly, Atg4D overexpression induces apoptosis, which is preceded by the caspase-independent recruitment of Atg4D to mitochondria and is facilitated by a putative C-terminal Bcl-2 homology 3 (BH3) domain. Atg4D also acquires affinity for damaged mitochondria in cells treated with hydrogen peroxide. These data suggest that Atg4D is an autophagy regulator that links mitochondrial dysfunction with apoptosis.
Betin
Virginie M S
VM
Cell Biology Laboratories, Department of Biochemistry, School of Medical Sciences, University of Bristol, University Walk, Bristol BS8 1TD, UK.
Lane
Jon D
JD
D048868:Adaptor Proteins, Signal Transducing / Q000235:genetics / Q000378:metabolism*; D000595:Amino Acid Sequence; D017209:Apoptosis / Q000187:drug effects; D001343:Autophagy / Q000187:drug effects; D000071183:Autophagy-Related Proteins; D053148:Caspase 3 / Q000378:metabolism*; D003546:Cysteine Endopeptidases / Q000235:genetics / Q000378:metabolism*; D006367:HeLa Cells; D006801:Humans; D006861:Hydrogen Peroxide / Q000494:pharmacology; D050356:Lipid Metabolism; D008869:Microtubule-Associated Proteins / Q000235:genetics / Q000378:metabolism*; D008928:Mitochondria / Q000187:drug effects / Q000201:enzymology* / Q000473:pathology; D008969:Molecular Sequence Data; D009154:Mutation; D018384:Oxidative Stress; D017434:Protein Structure, Tertiary; D034622:RNA Interference; D015398:Signal Transduction / Q000187:drug effects; D019311:Staurosporine / Q000494:pharmacology; D013312:Stress, Physiological / Q000187:drug effects; D013997:Time Factors; D014162:Transfection
D048868:Adaptor Proteins, Signal Transducing; D000071183:Autophagy-Related Proteins; C430503:GABARAPL1 protein, human; D008869:Microtubule-Associated Proteins; D006861:Hydrogen Peroxide; C507524:ATG4B protein, human; C543490:ATG4D protein, human; C505407:CASP3 protein, human; D053148:Caspase 3; D003546:Cysteine Endopeptidases; D019311:Staurosporine