Agriculture Reference
In-Depth Information
Ub moieties, unfolding and pore gating the substrate. The proteasome has the
ability to bind different lids or lid-like structures depending on the cellular needs.
A growing body of evidence indicates that monoubiquitinated proteins can be also
targeted to proteasomes indicating that monoubiquitination is sufficient for degra-
dation of small proteins (shorter than 150 aa) containing the unstructured region
(Shabek et al.
2012
). For more information about 26S proteasomes in plants, see
reviews and references within (Sullivan et al.
2003
; Vierstra
2003
,
2009
;
Sadanandom et al.
2012
).
What Is Autophagy?
Autophagy is a an important cellular process, and one of the two major degradation
pathways along with UPS, which are involved in maintaining homeostasis during
normal development and response to environmental stresses. Different kinds of
autophagy have been reported, including microautophagy, macroautophagy and
chaperone-mediated autophagy. Here (as in majority of other publications) the term
“autophagy” refers to the best-characterised type, macroautophagy. The general
scheme of the process is common in all eukaryotic organisms (Fig.
7.2
). The
process starts by the formation of a cup-like structure that engulfs selected cyto-
plasmic content (cargo) and progresses into a double-membrane autophagosome.
The outer membrane of the autophagosome then fuses with the tonoplast (in yeast
or plant cells) or lysosomal membrane (animal cells), while the inner membrane,
together with the cargo composes the autophagic body (or autolysosome in animal
cells). When such an organelle is present in the vacuolar lumen, its contents are
attacked by vacuolar proteases and then released through vacuolar permeases to the
cytosol for reuse (Klionsky
2007
).
Over 30 evolutionary conserved autophagy-related (ATG) proteins play a role in
the “core” autophagy pathway in yeast in mammals. Due to the high conservation of
autophagy among eukaryotes, mechanistic explanations for plants are often taken
from yeast or mammalian models (Thompson and Vierstra
2005
; Diaz-Troya
et al.
2008
; Nakatogawa et al.
2009
; Klionsky et al.
2011
). However, some ATG
proteins (e.g. ATG29 and ATG31) are not found in plants. The ATG11 protein was
also considered nonexistent in plants (Suzuki et al.
2007
) but a current TAIR search
shows its potential existence and hybrid similarity to ATG17; both proteins have
scaffolding properties in yeast.
Autophagy has an important role in regulating PCD. Both, excessive autophagy
as well as inefficient or defective autophagy may lead to cell death either by self-
eating (excessive autophagy) or by accumulation of damaged proteins and organ-
elles (defective autophagy) (Guiboileau et al.
2010
). For clarification of the nomen-
clature relating to different types of PCD in animals, the reader is referred to the
published guidance (Kroemer et al.
2009
) and for classification of plant PCD and
their merging with categories of PCD recognised in animals, to the review by van
Doorn (
2011
).
