Autophagy, from Yeast to Human Diseases
Autophagy was first discovered in yeast as a form of cellular self-digestion for the purpose of providing nutrients to the cell in order to survive starvation. Research in autophagy has been expanding from its initial discovery in yeast to having implications in major drug discovery pipelines. Recent research and advances have shown an association of mammalian autophagy with diseases such as neurodegenerative disease, infection disease, cardiac disease as well as cancers.
A yeast autophagy-related gene product (Atg8) has
three mammalian homologues: LC3, GABAA receptorassociated
protein (GABARAP), and Golgi-associated
ATPase enhancer (GATE-16).
Among them, LC3 is most actively studied and
frequently used as a mammalian autophagy marker.
Shortly after translation (proLC3), LC3 is processed
at the C-terminus by Atg4B or Atg4A into LC3-I. Upon
induction of autophagy, LC3-I is conjugated to the
substrate phosphatidylethanolamine (PE) via E1 (Atg7)
and E2 (Atg3). The PE-LC3-I conjugate is referred to as LC3-II. Despite having a higher molecular weight than LC3-I, LC3-II is
more hydrophobic and shows a higher mobility in sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis (SDS-PAGE).
GABARAP and GATE-16 are also demonstrated to undergo the same process of conjugation with PE.
Anti-Atg5 and Anti-Atg12 antibodies
Unlike other ubiquitin-like proteins, the ubiquitin-like protein Atg12 has a
C-terminal glycine, which protects it from processing. Atg12 is conjugated to
the substrate Atg5 by Atg7 (an E1-like protein, and as seen in the conjugation
of Atg8 to PE) and Atg10 (an E2-like protein). The Atg12-Atg5 conjugate forms
a complex with Atg16. Self-oligomerization of Atg16 results in a multimer of
the Atg12-Atg5-Atg16 complex. Although its functions remain unknown, this
complex is shown to accumulate on the pre-autophagosomal structure (PAS)
in yeast and to play an essential role in autophagosome formation.