Autophagy-related protein 8 (Atg8) is a

ubiquitin-like protein Ubiquitin-like proteins (UBLs) are a family of small proteins involved in post-translational modification of other proteins in a cell, usually with a regulatory function. The UBL protein family derives its name from the first member of the class ...

required for the formation of autophagosomal membranes. The transient conjugation of Atg8 to the autophagosomal membrane through a

ubiquitin Ubiquitin is a small (8.6 kDa) regulatory protein found in most tissues of eukaryotic organisms, i.e., it is found ''ubiquitously''. It was discovered in 1975 by Gideon Goldstein and further characterized throughout the late 1970s and 1980s. Fo ...

-like conjugation system is essential for

autophagy Autophagy (or autophagocytosis; from the Ancient Greek , , meaning "self-devouring" and , , meaning "hollow") is the natural, conserved degradation of the cell that removes unnecessary or dysfunctional components through a lysosome-dependent re ...

eukaryotes Eukaryotes () are organisms whose cells have a nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the three domains of life. Bacter ...

. Even though there are homologues in animals (see for example

GABARAP Gamma-aminobutyric acid receptor-associated protein is a protein that in humans is encoded by the ''GABARAP'' gene. Function Gamma-aminobutyric acid A receptors ABA(A) receptorsare ligand-gated chloride channels that mediate inhibitory neurot ...

, GABARAPL1,

GABARAPL2 Gamma-aminobutyric acid receptor-associated protein-like 2 is a protein that in humans is encoded by the ''GABARAPL2'' gene. Interactions GABARAPL2 has been shown to interact with ULK1 ULK1 is an enzyme that in humans is encoded by the ''U ...

, MAP1LC3A,

MAP1LC3B Microtubule-associated proteins 1A/1B light chain 3B (hereafter referred to as LC3) is a protein that in humans is encoded by the ''MAP1LC3B'' gene. LC3 is a central protein in the autophagy pathway where it functions in autophagy substrate selec ...

MAP1LC3B2 Microtubule-associated proteins 1A/1B light chain 3B (hereafter referred to as LC3) is a protein that in humans is encoded by the ''MAP1LC3B'' gene. LC3 is a central protein in the autophagy pathway where it functions in autophagy substrate selec ...

, and MAP1LC3C), this article mainly focuses on its role in lower eukaryotes such as ''

Saccharomyces cerevisiae ''Saccharomyces cerevisiae'' () (brewer's yeast or baker's yeast) is a species of yeast (single-celled fungus microorganisms). The species has been instrumental in winemaking, baking, and brewing since ancient times. It is believed to have b ...

''.

Structure

Atg8 is a monomer of 117

aminoacids Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha ...

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

domain. Even though Atg8 does not show a clear sequence homology to

, its crystal structure reveals a conserved ubiquitin-like fold.

Function

In autophagy

Atg8 is one of the key molecular components involved in

, the cellular process mediating the

lysosome A lysosome () is a membrane-bound organelle found in many animal cells. They are spherical vesicles that contain hydrolytic enzymes that can break down many kinds of biomolecules. A lysosome has a specific composition, of both its membrane p ...

vacuole A vacuole () is a membrane-bound organelle which is present in plant and fungal cells and some protist, animal, and bacterial cells. Vacuoles are essentially enclosed compartments which are filled with water containing inorganic and organic m ...

-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 Autophagy (or autophagocytosis; from the Ancient Greek , , meaning "self-devouring" and , , meaning "hollow") is the natural, conserved degradation of the cell that removes unnecessary or dysfunctional components through a lysosome-dependent r ...

which is one of three distinct types of autophagy characterized by the formation of double-membrane enclosed

vesicles Vesicle may refer to: ; In cellular biology or chemistry * Vesicle (biology and chemistry), a supramolecular assembly of lipid molecules, like a cell membrane * Synaptic vesicle ; In human embryology * Vesicle (embryology), bulge-like features o ...

that sequester portions of the

cytosol The cytosol, also known as cytoplasmic matrix or groundplasm, is one of the liquids found inside cells ( intracellular fluid (ICF)). It is separated into compartments by membranes. For example, the mitochondrial matrix separates the mitochondri ...

, 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). Like most Atg proteins, Atg8 is localized in the cytoplasm and at the PAS under nutrient-rich conditions, but becomes membrane-associated in case of autophagy induction. It then localizes to the site of autophagosome nucleation, the phagophore-assembly site (PAS). Nucleation of the phagophore requires the accumulation of a set of Atg proteins and of class III

phosphoinositide 3-kinase Phosphoinositide 3-kinases (PI3Ks), also called phosphatidylinositol 3-kinases, are a family of enzymes involved in cellular functions such as cell growth, proliferation, differentiation, motility, survival and intracellular trafficking, which i ...

complexes on the PAS. The subsequent recruitment of Atg8 and other autophagy-related proteins is believed to trigger vesicle expansion in a concerted manner, presumably by providing the driving force for membrane curvature. The transient conjugation of Atg8 to the membrane lipid phosphatidylethanolamine is essential for phagophore expansion as its

mutation In biology, a mutation is an alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA. Viral genomes contain either DNA or RNA. Mutations result from errors during DNA or viral replication, m ...

leads to defects in autophagosome formation. It is distributed symmetrically on both sides of the autophagosome and it is assumed that there is a quantitative correlation between the amount of Atg8 and the vesicle size. After finishing vesicle expansion, the autophagosome is ready for fusion with the

and Atg8 can either be released from the membrane for recycling (see below) or gets degraded in the autolysosome if left uncleaved. ATG8 is also required for a different autophagy-related process called the Cytoplasm-to-vacuole targeting (Cvt) pathway. This yeast-specific process acts constitutively under nutrient-rich conditions and selectively transports hydrolases such as aminopeptidase I to the yeast vacuole. The Cvt pathway also requires Atg8 localised to the PAS for the formation of Cvt vesicles which then fuse with the vacuole to deliver hydrolases necessary for degradation.

Post-translational modification and regulatory cycle

Atg8 exists in a cytoplasmic and in a membrane-associated form. Membrane association is achieved by coupling Atg8 to phosphatidylethanolamine (PE) which is a lipid constituent of plasma membranes. This post-translational modification process, called lipidation, is performed by the Atg8 conjugation system comprising the cysteine protease ATG4 (belonging to the caspase family), as well as the proteins

ATG7 Autophagy related 7 is a protein in humans encoded by ''ATG7'' gene. Related to GSA7; APG7L; APG7-LIKE. ATG 7, present in both plant and animal genomes, acts as an essential protein for cell degradation and its recycling. The sequence associates ...

, ATG3 and the ATG5-

ATG12 Autophagy related 12 is a protein that in humans is encoded by the ''ATG12'' gene. Autophagy is a process of bulk protein degradation in which cytoplasmic components, including organelles, are enclosed in double-membrane structures called autoph ...

complex. The Atg8 conjugation system works in analogy to the

ubiquitination Ubiquitin is a small (8.6 kDa) regulatory protein found in most tissues of eukaryotic organisms, i.e., it is found ''ubiquitously''. It was discovered in 1975 by Gideon Goldstein and further characterized throughout the late 1970s and 1980s. Fo ...

system. However, it is Atg8 itself that represents the

(Ubl) being transferred to PE, while ATG7 functions like an E1 enzyme, ATG3 like an E2 enzyme and the ATG12-ATG5 complex like an E3 ligase. The lipidation process is initiated by an ATG4 dependent post-translational cleavage of the last C-terminal amino acid residue of Atg8. After the cleavage, Atg8 exposes a C-terminal glycine residue (Gly 116) to which PE can then be coupled during the following steps. In the first step, the Gly116 residue of Atg8 binds to a cysteine residue of

via a thioester bond in an ATP-dependent manner. During the second step, Atg8 is transferred to Atg3 assuming the same type of thioester bond. Finally, Atg8 is detached from Atg3 and coupled to the amine head group of PE via an amide bond. This final step was found to be facilitated and stimulated by the ATG5-

complex. Both proteins, Atg5 and Atg12 were originally identified as part of another Ubl conjugating system that promotes conjugation of

to ATG5 via

and Atg10. This implies, that the

and the Atg8 conjugation system are actually interdependent.

Mammalian homologues

In higher eukaryotes Atg8 is not encoded by a single gene as in yeast, but derived from a multigene family. Four of its homologues have already been identified in mammalian cells. One of them is LC3 ( MAP1LC3A), a light chain of the microtubule-associated protein 1 Like Atg8, LC3 needs to be proteolytically cleaved and lipidated to be turned into its active form which can localize to the autophagosomal membrane. Similar to the situation in yeast, the activation process of LC3 is triggered by nutrient depletion, as well as in response to hormones. Mammalian LC3 isoforms contain a conserved Ser/Thr12, which is phosphorylated by protein kinase A to suppress participation in autophagy/mitophagy. Other homologues are the transport factor GATE-16 (Golgi-associated ATPase enhancer of 16 kDa) which plays an important role in intra-golgi vesicular transport by stimulating NSF (N-ethylmaleimide-sensitive factor) ATPase activity and interacting with the Golgi v-SNARE GOS-28, and

(γ-aminobutyric acid type A receptor associated protein) which facilitates clustering of GABAA receptors in combination with microtubules. All three proteins are characterized by proteolytic activation processes upon which they get lipidated and localized to the plasma membrane. However, for GATE-16 and GABARAP membrane association seems to be possible even for the non-lipidated forms. Apart from LC3, GABARAP and GATE-16 the most recently but less well characterized mammalian homologue is ATGL8. Little is known about its actual activation process except for its interaction with one of the mammalian ATG4 homologues, hATG4A.

References

External links

www.yeastgenome.org
{{DEFAULTSORT:Atg8 Cell cycle Post-translational modification Saccharomyces cerevisiae genes