Agriculture Reference
In-Depth Information
the vacuole for breakdown (Suttangkakul et al.
2011
). In addition, it has been
reported that induction of autophagy upon nutrient starvation and salt stress
depends on the activity of a plasma membrane NADPH oxidase, while induction
of autophagy upon osmotic stress (by mannitol) is NADPH oxidase-independent
(Liu et al.
2009
).
At the cellular level, the phagophore assembly site (PAS) initiates the
pre-autophagosomal nucleation and formation of the autophagy unit, the
autophagosome. The PAS is molecularly defined by having ATG1-ATG13-
ATG17-ATG101 and class III phosphatidylinositol 3-kinase (PIK3) complexes
bound by lipidated ATG8 (ATG8-PE). In yeast and animals the PIK3 complex is
composed of ATG6, ATG14 and kinase subunits, VPS15 and VPS34 (Rabinowitz
and White
2010
; Johansen and Lamark
2011
). In yeast, ATG14 targets the kinase
complex to the probable site of autophagosome formation, while the scaffold
protein, ATG17, organises the PAS and creates a recruiting point for further ATG
proteins, which contribute to the elongation step (Suzuki et al.
2007
). A membrane
delivery protein, ATG9, contributes to the phagophore expansion (Li and Vierstra
2012a
; Orsi et al.
2012
) and works in conjunction with integral membrane protein,
ATG27, and with proteins ATG2 and ATG18. ATG18 attaches phosphatidy-
linositol 3-phosphate (PI3P) and phosphatidylinositol 3,5-bisphosphate (PI(3,5)
P2) and together with ATG2 is involved in retrograde transport of ATG9 (Klionsky
et al.
2011
). ATG2 and ATG18 are functionally related, independently localise to
the PAS by means of ATG1, and can interact and form a complex but do not recruit
other proteins to the PAS; ATG1 is drawn by ATG13 to the PAS. The plant
orthologues of ATG14, ATG17, ATG22 and ATG27 remain unidentified (see
also Table
7.1
).
The steps described above, namely autophagy initiation and nucleation
(phagophore assembly) are followed by a step called vesicle elongation, where
two pathways utilising UPS-like E1-E3 enzymatic cascades are involved in activa-
tion of two ubiquitin-like proteins, ATG8 and ATG12. In the first cascade the
Cys-protease, ATG4, exposes the C-terminal Gly of ATG8. It allows ATG7 (E1) to
bind to the C-terminus of ATG8 that, in turn, permits ATG3 (E2) to lipidate the
C-terminus of ATG8 with phosphatidylethanolamine (PE). The PE tag enables
ATG8 anchoring into the autophagosomal membrane. In the second cascade,
ATG7 (E1) also participates by activating the ubiquitin like protein ATG12,
enabling its conjugation to ATG10 (E2) which in turn, conjugates ATG12 to
ATG5 (Noda et al.
2013
). The ATG12-ATG5 platform oligomerizes with ATG16
and forms the E3-like ATG12-ATG5-ATG16 ligase complex. This complex further
promotes ATG8 lipidation and phagophore growth (Fig.
7.4
; Maiuri et al.
2007
;
Johansen and Lamark
2011
). On the other hand, ATG7 activates also the C-terminal
residue of ATG3 (E2) enabling its conjugation to ATG12. In addition, ATG4
protease acts as deubiquitinating (DUB) enzyme and recycles ATG8 through PE
deconjugation from the outer autophagosomal membrane in a reactive oxygen
species (ROS) regulated way (Rabinowitz and White
2010
; Johansen and Lamark
2011
).
