PAMP Signaling in Plant Innate Immunity - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 79

2.18 N -Glycosylation of PRRs

2.18.1 Glycosylation of PRRs Is Required

for Binding PAMP Ligands

N -glycans attached to ectodomains of plasma membrane PRRs constitute likely
initial contact sites between plant cells and PAMPs. Under-glycosylated EFR and
non-glycosylated FLS2 were not able to form functional ligand-binding sites. It
suggests that LRR glycosylation of these PRRs is required for stably binding their
elf18 and fl g22 peptide ligands, an essential function for plant immunity (Häweker
et al. 2010 ). Correct glycosylation also seems to be essential for the PRR EFR
function. EFR accumulation was signifi cantly reduced when synthesized without
N -glucans. EFR N143Q lacking a single conserved N -glycosylation site from the
EFR ectodomain accumulated to reduced levels, lost ability to bind to its ligand,
and that to mediate elf18-mediated oxidative burst. However, EFR N143Q in wild
type cells correctly targeted to the plasma membrane via the Golgi apparatus
(Häweker et al. 2010 ).
EFR requires at least an N residue (N143) for stable accumulation and ligand
binding (Häweker et al. 2010 ). This indicates that proper glycosylation on a particu-
lar site(s) is crucial for EFR function, despite extensive N-glycosylation of the
receptor. It appears that N143 is located on the convex surface in the middle of the
LRR domain. It is suggested that N143-glycosylation may mediate interactions
with ER folding machineries, durable LRR folding, ligand binding and/or combina-
tions there of (Saijo 2010 ). Ligand binding seems to occur in central LRRs 9-15 of
FLS2 (Dunning et al. 2007 ).

2.18.2 N-Glycosylation Is Required for Transport of PRRs

from Endoplasmic Reticulum to Plasma Membrane

N -glycans attached to PRRs seem to be critical for the export of proteins from the
endoplasmic reticulum (ER) to the plasma membrane. The glycosylation of aspara-
gines residues ( N -glycosylation) is an essential, highly conserved co-translational
modifi cation of secreted proteins occurring in plant cells. The oligosaccharyltrans-
ferase (OST) complex in the endoplasmic reticulum (ER) controls transfer of
N -glycans from dolicholpyrophosphate-linked lipid anchors to nascent polypeptide
chains. N -glycans attached to polypeptides within ER lumen monitor correct fold-
ing of proteins. Only successfully folded proteins are exported from the ER and
therefore, N -glycans are crucial for the transport of glycoproteins from ER to plasma
membrane (Häweker et al. 2010 ).
PRRs are subject to N -glycosylation (an enzymatic process that attaches glycans
to proteins), which in turn could be essential for the function of PRRs in triggering
plant immunity. PRRs require N -glycosylation to mediate plant immunity (Häweker