ATG5

ATG5 Full Name

autophagy related 5

ATG5 Introduction

ATG5 (autophagy related 5) is a core component of the autophagy machinery, a highly conserved cellular degradation pathway that delivers cytoplasmic contents, including damaged organelles and protein aggregates, to the lysosome for breakdown and recycling. The gene is located on chromosome 6q21 and encodes a protein of 275 amino acids with a molecular weight of approximately 32 kDa. ATG5 is ubiquitously expressed in human tissues and is essential for the formation of autophagosomes, the double-membrane vesicles that sequester cytoplasmic cargo during autophagy. The protein functions as part of a ubiquitin-like conjugation system together with ATG7 (E1-like enzyme), ATG10 (E2-like enzyme), and ATG12. In this system, ATG12 is activated by ATG7, transferred to ATG10, and then covalently conjugated to ATG5 at Lys130. The ATG12-ATG5 conjugate then forms a complex with ATG16L1, which localizes to the expanding phagophore membrane and functions as an E3-like enzyme that promotes the lipidation of LC3 (microtubule-associated protein 1 light chain 3). Lipidated LC3 (LC3-II) is incorporated into the autophagosome membrane and is essential for cargo recognition and autophagosome closure. Beyond its canonical role in autophagy, ATG5 participates in various non-autophagic functions including apoptosis, cell differentiation, innate immunity, and the regulation of the interferon response, highlighting its pleiotropic roles in cellular homeostasis and disease.

Figure 1. Revised model for ATG12–ATG5-ATG16L1 complex formation.(Wible D J, et al. 2019)

Biological Functions in Autophagy and Beyond
ATG5 is indispensable for the initiation and progression of autophagy, a process that is critical for cellular adaptation to starvation, removal of damaged organelles, clearance of protein aggregates, and defense against intracellular pathogens. During autophagy induction, the ATG12-ATG5-ATG16L1 complex is recruited to the isolation membrane (phagophore) where it facilitates the conjugation of phosphatidylethanolamine (PE) to LC3, converting LC3-I to LC3-II. This lipidation step is essential for autophagosome membrane expansion and closure. Beyond its canonical role in autophagosome formation, ATG5 has been implicated in several autophagy-independent functions. In apoptosis, a calpain-cleaved fragment of ATG5 (residues 1-191) translocates to the mitochondria where it interacts with BCL-XL and promotes cytochrome c release, thereby amplifying the apoptotic cascade. This pro-apoptotic function is independent of ATG7 and the canonical autophagy pathway. ATG5 also plays important roles in the regulation of the innate immune response. It interacts with the adaptor protein MAVS (mitochondrial antiviral signaling protein) downstream of the RIG-I-like receptor pathway, preventing excessive interferon production and autoinflammation. In addition, ATG5 is required for the clearance of intracellular bacteria through xenophagy, a selective form of autophagy that targets invading pathogens.

Clinical Significance: Neurodegenerative and Inflammatory Disorders
Given its essential role in cellular quality control, ATG5 dysfunction has been implicated in a wide range of human diseases, most notably neurodegenerative disorders and inflammatory conditions. In Alzheimer's disease (AD), ATG5 expression and autophagic flux are impaired, leading to the accumulation of amyloid-beta plaques and hyperphosphorylated tau tangles. Reduced ATG5 function contributes to defective clearance of these toxic protein aggregates, accelerating disease progression. In Parkinson's disease (PD), autophagy dysfunction mediated by ATG5 impairment leads to the accumulation of alpha-synuclein (SCNA)-containing Lewy bodies, a pathological hallmark of PD. In Huntington's disease (HD), mutant huntingtin protein interferes with ATG5 function, reducing autophagic clearance of protein aggregates and contributing to neuronal toxicity. In amyotrophic lateral sclerosis (ALS), mutations in autophagy-related genes including ATG5 have been identified, and impaired autophagy contributes to the accumulation of TDP-43 aggregates in motor neurons. Beyond neurodegeneration, ATG5 polymorphisms have been associated with susceptibility to systemic lupus erythematosus (SLE) and other autoimmune diseases.

Alternate Names for ATG5

ATG5; autophagy related 5; ASP; APG5; APG5L; hAPG5; APG5-LIKE; autophagy protein 5; apoptosis-specific protein; ATG5 autophagy related 5 homolog;

• 1.Wible D J, Chao H P, Tang D G, et al. ATG5 cancer mutations and alternative mRNA splicing reveal a conjugation switch that regulates ATG12–ATG5-ATG16L1 complex assembly and autophagy. Cell discovery, 2019, 5(1): 42.