ATG8

autophagy related protein 8
The crystal structure of microtubule-associated protein light chain 3, a mammalian homologue of Saccharomyces cerevisiae Atg8.
Identifiers
Organism	S. cerevisiae strain S288c (Baker's yeast)
Symbol	Atg8
Alt. symbols	Apg8, Aut7, Cvt5
Entrez	852200
RefSeq (mRNA)	NM_001178318
RefSeq (Prot)	NP_009475
UniProt	P38182
Other data
Chromosome	VII: 0.16 - 0.16 Mb

Autophagy-related protein 8 (Atg8) is a ubiquitin-like protein required for the formation of autophagosomal membranes. The transient conjugation of Atg8 to the autophagosomal membrane through a ubiquitin-like conjugation system is essential for autophagy in eukaryotes. Even though there are homologues in animals (see for example GABARAP, GABARAPL1, GABARAPL2, MAP1LC3A, MAP1LC3B, MAP1LC3B2, and MAP1LC3C), this article mainly focuses on its role in lower eukaryotes such as Saccharomyces cerevisiae.

Atg8 is a monomer of 117 aminoacids and a molecular weight of 13,6kDa. It consists of a 5-stranded β-sheet, which is enclosed by two α-helices at one side and one α-helix at the other side and exhibits a conserved GABARAP domain. Even though Atg8 does not show a clear sequence homology to ubiquitin, its crystal structure reveals a conserved ubiquitin-like fold.

Atg8 is one of the key molecular components involved in autophagy, the cellular process mediating the lysosome/vacuole-dependent turnover of macromolecules and organelles. Autophagy is induced upon nutrient depletion or rapamycin treatment and leads to the response of more than 30 autophagy-related (ATG) genes known so far, including ATG8. How exactly ATG proteins are regulated is still under investigation, but it is clear that all signals reporting on the availability of carbon and nitrogen sources converge on the TOR signalling pathway and that ATG proteins are downstream effectors of this pathway. In case nutrient supplies are sufficient, the TOR signaling pathway hyperphosphorylates certain Atg proteins, thereby inhibiting autophagosome formation. After starvation autophagy is induced through the activation of Atg proteins both on the protein modification and the transcriptional level.

Atg8 is especially important in macroautophagy which is one of three distinct types of autophagy characterized by the formation of double-membrane enclosed vesicles that sequester portions of the cytosol, the so-called autophagosomes. The outer membrane of these autophagosomes subsequently fuses with the lysosome/vacuole to release an inter single membrane (autophagic body) destined for degradation. During this process, Atg8 is particularly crucial for autophagosome maturation (lipidation).