Agriculture Reference
In-Depth Information
known to be specifically targeted by autophagy (Weidberg et al.
2011
). Each
specific cargo was used as a base of providing the names for different autophagy
types: mitophagy, pexophagy, ribophagy, xenophagy, aggrephagy etc. (see also
above). In most cases the molecular machinery involved in specificity of cargo
recognition is not characterised. However, it is evident that the various cargos in
animal cells are specifically recognised and targeted for lysosomal degradation by
the action of specific cargo-recognising receptors, called selective autophagy
receptors (Lynch-Day and Klionsky
2010
; Johansen and Lamark
2011
; Behrends
and Fulda
2012
; Isakson et al.
2013
). The main feature of these proteins is their
ability to interact directly with both the ATG8 proteins through the LIR (LC3-
interating region) motif and the cargo. The selective cargo receptors are degraded
along with the cargo in the autophagosomes (Komatsu and Ichimura
2010
). Rec-
ognition of the cargo designed for degradation from the one needed in the cell
requires Ub as a specific ligand. The ubiquitinated cargo is specifically recognised
by cargo receptors, such as p62, NBR1, NDP52, possess the Ub-binding domain
(Johansen and Lamark
2011
). Yet, some cargoes can be targeted for lysosomal
degradation in a Ub-independent mechanism and some selective autophagy recep-
tors, which have a Ub-binding domain, directly recognise cargo in a
Ub-independent mechanism (Komatsu et al.
2010
). In addition to cargo receptors
able to bind to both the cargo and ATG8 proteins, another category of proteins,
autophagy adaptors (defined as scaffold proteins capable of binding both the cargo-
receptor complex and the core components of autophagic machinery) have been
characterised. The best described examples of such proteins are the mammalian
ALFY (autophagy-linked FYVE protein) and ATG11 from yeast (Lynch-Day and
Klionsky
2010
; Isakson et al.
2013
).
Figure
7.5
illustrates selective autophagy types reported for plants. Some have
already been discussed above. There is no indication that in every case a specific
autophagy receptor exists. For example, although importance of lipophagy in
animals is well documented (Christian et al.
2013
), few data for plants are available
and the presence of this process in plants can only be inferred by accumulation of
lipid peroxides in
atg
mutants (Xiong et al.
2007
). The involvement of mammalian
p62 and NBR1 proteins in degradation of protein aggregates is well known and
therefore it seems reasonable that the plant NBR1-like proteins work as selective
receptors for protein aggregates and possibly also for single Ub-tagged proteins.
The involvement of autophagy in response to biotic stresses has been well studied
in plants but the authors of this review are not aware of any report about direct
targeting of viral or bacterial pathogens by selective cargo receptors. Xenophagy
involving specific cargo receptors is reported for animal pathogens only (Mostowy
et al.
2011
). An interesting example are anthocyanins, plant pigments of various
functions maintained in the vacuole: a significant change in anthocyanin profiles
(overall reduction) was observed in
atg
mutants (Pourcel et al.
2010
). Although no
direct evidence exists for the specific authophagy-dependent vacuolar transport of
anthocyanins, it is tempting to speculate that it might be an example of autophagy
involvement in a plant anabolic process (compare to CVT in yeast as an example of
such a process).
