Agriculture Reference
In-Depth Information
and both bind ATG8 (Honig et al.
2012a
,
b
). Although in the case of these proteins
the cargo is unknown, and it was speculated that ATI1 and ATI2 are involved either
in removal of germination inhibiting compounds and their deposition into the
vacuole or in transport of germination-promoting compounds to the vacuole.
They probably transport membrane proteins or non-protein compounds into the
vacuole, especially after C starvation.
Additionally, the tomato AGC protein kinase Adi3, known to function as a
suppressor of programmed cell death, was shown to bind to ATG8 (Devarenne
2011
). Although the interaction was not further investigated, the phenotypes of the
double
atg
(
atg3
,
atg6
and
atg7
) and
adi3
mutants suggest that Adi3 might work in
coordination with autophagy to control cell death. However, it is unclear if it can be
considered a selective autophagy cargo receptor.
The evolutionary conserved NBR1-like proteins, which were described also in
plants as selective autophagy cargo receptors, will be discussed below.
An interesting example of the specific target of autophagic degradation in plants
is ARGONAUTE1 (AGO1), a key component of RNA-induced silencing complex
(RISC) (Derrien et al.
2012
). Autophagy is involved in the turnover of this protein.
Thus, it is appealing to hypothesize that autophagy is involved in degradation of
proteins from this family during cellular stress, namely when the fast
reprogramming of the RISC is needed following rapid changes of the population
of miRNA and siRNA.
Knowledge of the selective autophagy cargo receptors in plants is very limited
and extensive work is needed to fill the multiple gaps in this field.
NBR1-Like Proteins in Plants
The plant NBR1-like selective cargo receptors, AtNBR1 and Joka2 were identified
independently by two research groups in
Arabidopsis
(Svenning et al.
2011
) and
tobacco (Zientara-Rytter et al.
2011
), respectively. Plant NBR1-like proteins seem
to be good autophagy markers (Svenning et al.
2011
; Zientara-Rytter et al.
2011
;
Minina et al.
2013
). Moreover, it has been shown that AtNBR1, similarly to the
mammalian p62 and NBR1, is degraded by the autophagy pathway (Minina
et al.
2013
). With respect to the phenotype, the enhanced sensitivity of
nbr1
mutants
to heat, oxidative, drought and salt stress was reported (Zhou et al.
2013
).
NBR1-like proteins identified in plants are functional hybrids of the mammalian
NBR1 (next to breast cancer 1 gene 1) and p62/SQSTM1 (Sequestosome 1) and
have a modular composition (Fig.
7.6
). They have four main domains: PB1 (Phox
and Bem1), ZZ (ZZ-type Zinc finger domain), NBR1/FW and double UBA
(ubiquitin-associated domain), UBA1 and UBA2. The roles of individual domains
and the protein partners specifically binding to these domains are characterised in
mammalian cargo receptors (Kirkin et al.
2009
; Mardakheh et al.
2010
; Salminen
et al.
2012
).
