| ATG4B Protein — Autophagy Related 4B | |
|---|---|
| Protein Name | Autophagy-related protein 4B |
| Gene | [ATG4B](/genes/atg4b) |
| UniProt | Q9Y4P1 |
| Molecular Weight | 52 kDa |
| Structure | Cysteine protease fold, N-terminal regulatory domain |
| Subcellular Localization | Cytoplasm, Autophagosome membrane |
| Protein Family | ATG4 family (cysteine proteases) |
ATG4B (Autophagy-related protein 4B) is a cysteine protease encoded by the ATG4B gene that plays a critical role in the autophagy pathway. As the master regulator of LC3/GABARAP processing, ATG4B is essential for autophagosome biogenesis and the degradation of damaged proteins and organelles. This function is particularly important in post-mitotic neurons, which cannot dilute toxic protein aggregates through cell division and rely heavily on autophagy for proteostasis. UniProt ID: Q9Y4P1.
ATG4B is a 393-amino acid cysteine protease that catalyzes the proteolytic processing of LC3 (Microtubule-associated protein 1A/1B-light chain 3) and GABARAP (GABA receptor-associated protein) family members. Unlike other ATG4 isoforms (ATG4A, ATG4C, ATG4D), ATG4B exhibits broad substrate specificity and can process all known ATG8 homologs, making it the predominant functional protease in basal autophagy 1.
ATG4B possesses a bipartite domain structure essential for its function:
N-terminal regulatory domain (residues 1-150): Contains the LC3-interacting region (LIR) docking site and substrate recognition motifs. This domain regulates access to the catalytic site and determines substrate specificity.
C-terminal protease domain (residues 151-393): Features the conserved cysteine protease fold with the catalytic triad:
The protease activity follows a classic cysteine protease mechanism:
ATG4B catalyzes three essential reactions in the autophagy cycle:
Pro-LC3 → LC3-I + Glycine
ATG4B cleaves the C-terminal arginine of pro-LC3, exposing a conserved glycine residue. This is the essential first step for LC3 lipidation.
LC3-I + Phosphatidylethanolamine → LC3-II
Though ATG4B doesn't catalyze this reaction (performed by ATG3/ATG7), the primed LC3-I can be conjugated to phosphatidylethanolamine (PE) in the autophagosome membrane.
LC3-II (membrane-bound) → LC3-I (free)
ATG4B can reverse the lipidation, cleaving LC3-II from the autophagosome membrane to recycle LC3 for new autophagosome formation. This function is crucial for maintaining the ATG8 pool during sustained autophagy 3.
ATG4B processes the entire ATG8 family:
The broad substrate specificity makes ATG4B essential for both canonical autophagy and specialized forms like mitophagy (mitochondrial autophagy) and aggrephagy (aggregate autophagy).
ATG4B dysfunction contributes to Alzheimer's disease pathogenesis through multiple mechanisms:
ATG4B plays a critical role in clearing α-synuclein aggregates:
Motor neurons exhibit high baseline autophagy activity due to:
Several strategies are being explored to enhance ATG4B activity:
ATG4B is the processing enzyme for all LC3 family members (2009). Autophagy.
Phosphorylation of ATG4B regulates its activity (2013). Journal of Biological Chemistry.
ATG4B mediates LC3 delipidation for autophagosome recycling (2011). Nature Cell Biology.
Autophagy dysfunction in Alzheimer's disease (2010). Nature Reviews Neuroscience.
Tau clearance by autophagy (2014). Acta Neuropathologica.
Autophagy and α-synuclein in Parkinson's disease (2012). Journal of Parkinson's Disease.
Mitophagy in dopaminergic neurons (2015). Cell Death & Differentiation.
Autophagy in ALS pathogenesis (2014). Molecular Neurodegeneration.
ATG4B and huntingtin clearance (2011). Nature Cell Biology.
Small molecule autophagy activators (2015). Trends in Pharmacological Sciences.
AAV-mediated gene therapy for neurodegeneration (2018). Molecular Therapy.
The study of Atg4B Protein — Autophagy Related 4B has evolved significantly over the past decades. Research in this area has revealed important insights into the underlying mechanisms of neurodegeneration and continues to drive therapeutic development.
Historical context and key discoveries in this field have shaped our current understanding and will continue to guide future research directions.