PAMP Signaling in Plant Innate Immunity - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 73

2.16.6 What Is the Role for Endocytosis in PAMP-PRR

Signaling?

PAMP-induced internalization of PRRs from the plasma membrane (PM) is closely
correlated with their immune function (Robatzek et al. 2006 ; Bar and Avni 2008 ).
It is still not known whether this endocytosis leads to signal activation or attenuation
of the PRRs (Saijo 2010 ). Endocytosis may help the exogenous ligands to provoke
plant responses that are rapid, but transient, to ensure proper defense while preventing
harm for the host cell by clearing the host cell of exogenous ligand (Geldner and
Robatzek 2008 ). Translocation of activated cell surface receptors is associated
with an attenuation of ligand-stimulated responses and also contributes to activate
downstream signaling cascades (von Zastrow and Sorkin 2007 ).

2.17 ER-QC (for E ndoplasmic R eticulum Q uality C ontrol)

Pathways in Biogenesis of PRRs

2.17.1 ERQC Mechanisms Monitor Protein Folding in ER

In plants, pattern recognition receptors (PRRs) are known to reside in cell surface
plasma membrane and no cytoplasmic PRRs have been reported so far (Zipfel
2009 ; Saijo 2010 ). In animals, extracellular PRRs are translated on the ER mem-
brane, enter the ER lumen, and then are transported to plasma membrane (Akashi-
Takamura and Miyake 2008 ). Similar mechanism of biogenesis and transport of
proteins may exist in plants (Li et al. 2009b ; Nekrasov et al. 2009 ; Park et al.
2010a , b ). The biogenesis of trans-membrane PRRs may occur through the endo-
plasmic reticulum (ER) with the aid of ER-resident chaperones (Dodds and
Rathjen 2010 ; Popescu 2012 ). After synthesis, proteins must rapidly fold to per-
form their biological activities (Anelli and Sitia 2008 ). ER is highly specialized
for folding proteins and it greatly enhances protein folding effi ciency (Kleizen
and Braakman 2004 ). In the ER lumen, chaperones and folding enzymes are abun-
dant and these folding factors in general prevent aggregation and thereby allow
more effi cient folding of a large variety of proteins (Kleizen and Braakman 2004 ).
Folding status of client proteins is monitored during their folding and maturation
by the process called ER-QC (for Endoplasmic R eticulum Q uality C ontrol)
(Anelli and Sitia 2008 ). Unfolded proteins are retained in the ER until they are
properly folded, or ultimately destroyed by ER-associated degradation (ERAD) in
the cytosol (Vembar and Brodsky 2008 ). Plant cells are equipped with several
ERQC mechanisms to monitor protein folding, allowing export of only correctly
folded proteins to their fi nal destinations but retaining misfolded proteins in the
ER (Jin et al. 2007 ).