PAMP Signaling in Plant Innate Immunity - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 76

His-Asp-Glu-Leu tetrapeptide) by ERD2 (Semenza et al. 1990 ). Arabidopsis erd2b
mutants show insensitivity to EFR, suggesting the importance of ERD2 in EFR
function (Li et al. 2009b ). The erd2b mutation specifi cally affects CRT3, which
carries the C-terminal HDEL signal and CRT3 may be a likely substrate for ERD2b
(Li et al. 2009b ).

2.17.5 ER Quality Control Components Required

for Biogenesis of the Pattern Recognition Receptor EFR

EFR is the PRR detected in Arabidopsis for binding the PAMP EF-Tu (Zipfel et al.
2006 ; Albert et al. 2010 ). It is a transmembrane glycoprotein and it needs to transit
through the secretory pathway to mature and reach their fi nal destination at the
plasma membrane. The protein is secreted into the ER and undergoes quality con-
trol during folding and maturation in the ER. Several chaperones and enzymes
resident in the ER take part in the ER-QC process. Arabidopsis genes encoding
glucosidase II, UGGT, CRT3, ERdj3b, and ERD2b have been shown to be required
for the EFR function and accumulation. SDF2 and STT3A, a subunit of the oligo-
saccharyltransferase complex are also necessary for EFR biogenesis (Li et al. 2009b ;
Nekrasov et al. 2009 ; Dodds and Rathjen 2010 ).
In the ER, the PRR proteins may be modifi ed at glycosylable Asn residues by
an oligosaccharyltransferase complex (OST). This function specifi cally depends
on STT3A ( Staurosporin and temperature sensitive-3A ) coding for a component
of the OST complex involved in N-glycosylation of nascent proteins (Nekrasov
et al. 2009 ). Loss of STT3A-containing OST complex markedly decreases
accumulation and signaling activity of the PRR EFR (Saijo et al. 2009 ; Häweker
et al. 2010 ), suggesting the importance of the OST complex in ER-QC. The OST
complex is involved in N-glycosylation of nascent proteins (Nekrasov et al.
2009 ). It covalently attaches a complex polysaccharide containing three terminal
glucose residues. The glucose moieties may be subsequently trimmed by gluco-
sidases I and II (Dodds and Rathjen 2010 ). A single glucose residue is added
back by UGGT near regions of protein disorder. Monoglucosylated proteins
interact with the lectin calreticulin (CRT) to retain misfolded substrates in the
ER. In this way, UGGT acts as a folding sensor, and glycosylation is intimately
related to protein maturation. Terminally misfolded proteins are degraded (Dodds
and Rathjen 2010 ). UGGT mutant alleles that compromise EFR signaling have
been identifi ed (Li et al. 2009b ). The allele uggt-3 and a null insertion line
( uggt-4 ) showed the PAMP elf18 insensitivity confi rming that UGGT is required
for EFR function (Li et al. 2009b ).
Arabidopsis carries three CRT genes, including CRT1 , CRT2 , and CRT3 . The
crt3-1 mutant was completely insensitive to elf18, as measured by oxidative burst,
callose deposition, ethylene production, mitogen-activated protein kinase activation,
and defense gene activation. Loss of CRT1 together with CRT2 compromises EFR