Autophagy (macroautophagy) is a catabolic process which targets intracellular components such as proteins and organelles for degradation. Originally described as a bulk degradation process, current research supports its selective nature (1). Selective autophagy targets specific cellular components for degradation including the endoplasmic reticulum (2) (ER-phagy), mitochondria (3) (mitophagy), peroxisomes (3) (pexophagy), ribosomes (4) (ribophagy) and bacteria (5) (xenophagy). Autophagy relies on a newly formed phagophore, a membrane structure which elongates, sequesters cellular content, and fuses to form a double membrane vesicle known as the autophagosome. Fusion of autophagosomes with lysosomes gives rise to the autophagolysosome, where cellular components are degraded by lysosome hydrolases (1).
Autophagic flux is supported by autophagy-related proteins (Atgs) initially identified in yeast (6,7). The core autophagy machinery is comprised of 17 Atg proteins that play specific roles in autophagosome formation. Among these Atg proteins, Atg8 is not only involved in autophagosome formation but also functions in cargo selection. In mammals, several Atg8 homologues have been identified including microtubule-associated protein 1 light chain 3 alpha, beta and gamma - LC3A, LC3B, and LC3C (8) respectively, as well as GABA type A receptor-associated protein (GABARAP), GABARAP-Like1, and GABARAP-Like2 (9). LC3 (predicted molecular weight 14kD) is ubiquitously expressed and undergoes posttranslational processing after synthesis. First, the cysteine protease Atg4 cleaves a carboxy terminal sequence to generate the cytosolic form LC3-I. Next, E1-like (Atg7) and E2-like (Atg3) enzymes conjugate phosphatidylethanolamine to the newly exposed carboxyterminal glycine, generating LC3-II. Finally, the Atg12-Atg5-Atg16L1 complex participates in LC3 lipidation and autophagosome formation (10). LC3B-I to LC3B-II conversion correlates with autophagosome number and is considered the best marker to monitor autophagy.
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8. Wild, P., McEwan, D. G., & Dikic, I. (2014). The LC3 interactome at a glance. Journal of Cell Science. https://doi.org/10.1242/jcs.140426
9. Igloi, G. L. (2001). Cloning, expression patterns, and chromosome localization of three human and two mouse homologues of GABAA receptor-associated protein. Genomics. https://doi.org/10.1006/geno.2001.6555
10. Glick, D., Barth, S., & Macleod, K. F. (2010). Autophagy: Cellular and molecular mechanisms. Journal of Pathology. https://doi.org/10.1002/path.2697