Autophagy is an important cellular process that maintains homeostasis by degrading and recycling damaged proteins and organelles. Autophagy receptors, such as p62/SQSTM1, recognize these intracellular cargo and mediate their engulfment by the double-membrane autophagosome. The autophagosomes are subsequently targeted to the lysosome for degradation. An early regulatory step in this process is the activation and lipidation of ATG8 related proteins such as microtubule-associated protein-1 light chain 3 (LC3). Conjugation of LC3 to phosphatidlyethanolamine (PE) is needed to localize LC3 to the assembling autophagosome and to recruit the core autophagy machinery. This lipidation step depends on activation by the ATG4 family of proteins, also knowns as autophagins. ATG4 is a cysteine protease that cleaves and activates LC3 and LC3-related proteins at a conserved glycine residue to allow PE conjugation by the ATG12-ATG5-ATG16 protein complex. There are four ATG4 isoforms: ATG4A, ATG4B, ATG4C, and ATG4D. The functions of these various isoforms are currently unclear. A study of ATG4C, also known as autophagin-3, revealed a role in induced autophagy but not in basal autophagy (1). The authors demonstrated wide tissue distribution of ATG4C protein through western blotting with an ATG4C antibody. Generation of an ATG4C knockout mouse allowed the investigation of autophagy in the absence of ATG4C, however the authors saw only a modest decrease in levels of autophagy and even then only under starvation or stressful conditions. This reduction in autophagy was small despite the complete absence of ATG4C as demonstrated through RT-PCR and western blotting with the ATG4C antibody. However, the authors did find an increased susceptibility to cancer in these ATG4C knockout mice. ATG4C antibodies will serve as tools for further studies and may offer insight into ATG4C's role in autophagy. In addition to western blotting, ATG4C antibodies could be useful for immunoprecipitation experiments to identify interactors and substrates. Immunostaining with ATG4C antibodies could also provide useful information on dynamic subcellular localization upon induction of autophagy or under cellular stress.
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