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Fig. 7.4 Recruitments of ATG8 and ATG12 proteins to the autophagosome. The ubiquitin-like
proteins, ATG12 and ATG8, are processed in a UPS-like E1-E3 manner through two separate
conjugation cascades with some overlapping elements. In the first, ATG12 covalently linked to
ATG5 (by subsequent action of E1-like ATG7 and E2-like ATG10) promotes formation of
ATG12-ATG5-ATG16 complex needed for creation of pre-autophagosomal structure. In the
second system, cleavage of ATG8 by ATG4 enables attachment of phosphatidylethanolamine
(
PE
) to the C-terminus of ATG8 by subsequent action of E1-like ATG7 and E2-like ATG3.
Activated ATG8 (ATG8-PE) associates with the expanding autophagosome membranes and
remains through autophagosome maturation until fusion with the vacuole. This property is
commonly exploited in many labs for reliable autophagy monitoring (Figure based on Suzuki
et al. (
2007
) and Li and Vierstra (
2012a
))
such as mitochondria (mitophagy), peroxisomes (pexophagy), ribosomes
(ribophagy), ER (reticulophagy or ERphagy) lipid droplets (lipophagy), pathogens
(xenophagy), protein aggregates (aggrephagy) and choroplast material
(chlorophagy). In addition, in yeast a mechanism also exists called CVT (Cyto-
plasm to Vacuole Targeting; (Bassham et al.
2006
), which is only hypothesised in
plants (Nakatogawa et al.
2009
; Li and Vierstra
2012a
), leaving open the question
of how functional vacuolar proteins reach the plant vacuole.
Work on mitophagy in yeast indicated that the ATG32 mitochondrial outer
membrane protein is the main factor involved in selective degradation in fungal
mitochondria. Following the induction of mitophagy, Atg32 binds Atg11, an
adaptor protein for selective types of autophagy, then it is recruited to mitochondria
and imported into the vacuole along with mitochondria. Therefore, in yeast Atg32
confers selectivity for mitochondrial sequestration as a cargo and is necessary
for recruitment of this organelle by the autophagy machinery for mitophagy
