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  • Review Article
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Ubiquitin and proteasomes

Molecular dissection of autophagy: two ubiquitin-like systems

Nature Reviews Molecular Cell Biology volume 2, pages 211–216 (2001)Cite this article

Key Points

  • This review discusses recent analyses of the genes required for autophagy ? intracellular bulk protein degradation ? in yeast, where two ubiquitin-like systems have now been revealed.

  • Light microscopy studies show that starved yeast cells take up their own cytoplasm into vacuoles through autophagic bodies. Closer analysis using electron microscopy show that these bodies form a double-membraned structure called the autophagosome, which subsequently fuses with the vacuole/lysosome. The whole process is topologically the same in mammals.

  • Screens in yeast defective in autophagy morphologically and biochemically revealed two sets of genes, the APG and AUT genes, respectively. A specific vacuolar enzyme biosynthetic pathway requires the cytosol-to-vacuole-targeting (CVT) genes. There is extensive overlap between the CVT genes and the APG/AUT genes.All apg mutants have defects at or before formation of the autophagosome.

  • Two ubiquitin-like systems have been discovered. The first ? the Apg12 conjugation system ? is unique in that Apg12 is synthesized as a mature form; it seems to have just one target, Apg5, and at steady state almost all Apg12 molecules are conjugated with Apg5.

  • The second ? Apg8/Aut7 ? is processed at its carboxy-terminal region by Aut2/Apg4. Apg8 exists in two forms, one of which is membrane bound through a phospholipid. This lipidation is mediated by ubiqutin-like system; Apg8 is activated by Apg7 and transferred to Apg3 and finally forms a conjugate with phosphatidylethaolamine (PE). Apg4 cleaves Apg8?PE, releasing Apg8 from membrane.

  • Morphological studies show that Apg8 localizes on the membrane of intermediate structures of the autophagosome; this transient association seems to be essential for formation of the autophagosome.

  • Both Apg12 and Apg8 are highly conserved, with apparent homologues in the worm, mammals and plants. In higher eukaryotes, Apg8 consists of a multigene family.

Abstract

Recent analyses of the genes required for autophagy ? intracellular bulk protein degradation ? in yeast have revealed two ubiquitin-like systems, both of which are involved in the membrane dynamics of the process. Molecular dissection of these systems is now revealing some surprises.

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Figure 1: Electron microscopic images of autophagy in yeast.
Figure 2: Two ubiquitin-like systems are required for autophagy.

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Notes

  1. *Macroautophagy is generally a non-selective sequestration.But in some physiological situations, excess or unnecessary organelles areselectively degraded in a lysosome/vacuole. The best-studied case is peroxisomedegradation, called pexophagy by Daniel Klionsky. In the yeast Pichia pastoris, peroxisomes are sequestered through either macroautophagy or microautophagy,depending on the nutrient conditions40.

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Acknowledgements

Thanks to T. Noda and N. Mizushima for critical discussion and reading of this manuscript.

Author information

Authors and Affiliations

  1. Department of Cell Biology, National Institute for Basic Biology, Myodaiji, 444-8585 , Okazaki, Japan

    Yoshinori Ohsumi

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  1. Yoshinori Ohsumi
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DATABASE LINKS

APG16

APG17

Apg12

Apg1

apg6

vps30

Apg7/Cvt2

Apg10

Aut7/Apg8/Cvt5

Aut2/Apg4

Aut1

Apg3

Apg5

MAP1-LC3

GABARAP

ENCYCLOPEDIA OF LIFE SCIENCES

Lysosomal degradation of proteins

Glossary

E1 ENZYME

An enzyme that activates the carboxy-terminal glycine of the small protein ubiquitin, or ubiquitin-like proteins, allowing them to form a high-energy bond to a specific cysteine residue of the E1.

E2 ENZYME

An enzyme that accepts ubiquitin or a ubiquitin-like protein from an E1 and transfers it to the substrate, mostly using an E3 enzyme.

NSF

N-ethylmaleimide-sensitive factor, an AAA-type ATPase essential for membrane fusion during vesicle transport.

V-SNARE

A family of proteins on secretory vesicles. They form a complex with t-SNAREs on the target membrane during vesicle fusion.

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Ohsumi, Y. Molecular dissection of autophagy: two ubiquitin-like systems. Nat Rev Mol Cell Biol 2, 211–216 (2001). https://doi.org/10.1038/35056522

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