PAMP Signaling in Plant Innate Immunity - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 75

as co-chaperones in the ER. All Hsp 40s contain a J domain, named for a conserved
about 70 amino acid motif in DnaJ, and are often referred to as J domain containing
proteins or J proteins (Buck et al. 2007 ). DnaJ is a type I Hsp40, and contains an
N-terminal J domain, a glycine/phenylalanine-rich domain and a cysteine-rich
domain. DnaJ proteins specifi cally interact with the ATP-bound form of Hsp70s
(Buck et al. 2007 ).
The ER-localised co-chaperone Hsp40 protein ERdj (ER DnaJ like protein) fi rst
directly binds to the misfolded substrate. ERdj then recruits BiP and activates BiP
ATPase activity present in its N-terminus, leading to interaction of the C-terminal
region of BiP with the substrate and the release of ERdj (Jin et al. 2008 , 2009 ). The
BiP retention system acts independently of, or subsequent to, the CNX/CRT cycle
(Buck et al. 2007 ).
Another protein SDF2 (for Stromal Derived Factor 2) is also required for PRR
biogenesis (Nekrasov et al. 2009 ). SDF2 resides in ER protein complex with the
Hsp40 ERdj and the Hsp70 BiP, which are components of the ER-QC. Loss of SDF2
results in ER retention and degradation of PRR, suggesting a role for the BiP/ERdj/
SDF2 in ER-QC system (Nekrasov et al. 2009 ). ER protein complex comprising
stromal-derived factor-2 (SDF2), Erdj3B and BiP is required for the proper biogen-
esis of the PRR EFR (Nekrasov et al. 2009 ). AtSDF2 is a single copy gene in
Arabidopsis and orthologs exist in all eukaryotes. AtSDF2 is a small protein of 218
amino acids (24 kDa) consisting of a 23 amino-acid (aa) predicted N-terminal signal
peptide and three repeats of the MIR domain. MIR domain is named after three of the
proteins in which it occurs: protein mannosyltransferase, inositol 1,4,5-trisphosphate
receptor (IP3R) and the ryanodine receptor (RyR). Although many eukaryotic pro-
teins contain MIR domains, SDF2 is the only MIR domain-containing protein in
plants (Nekrasov et al. 2009 ). SDF2 seems to be required for the function of EFR
(the PRR for the PAMP elf18) and FLS2 (the PRR for the PAMP fl g22). However,
the requirement of SDF2 for the function of FLS2 is only to a lesser extent and SDF2
doesn't seem to be required for the function of CERK1, the PRR for the fungal
PAMP chitin (Nekrasov et al. 2009 ). The oxidative burst induced by elf18 was
strongly diminished in sdf2 mutant Arabidopsis plants and it was less reduced after
fl g22 treatment. In contrast, the oxidative burst triggered by the fungal PAMP chitin
was not impaired at all in sdf2 mutant plants (Nekrasov et al. 2009 ).

2.17.4 Function of ERD2 in ER-QC

Another protein involved in ER-QC is ERD2 (for ER Retention Defective2), the ER
lumen protein-retaining receptor (Semenza et al. 1990 ). ERD2 binds the ER-escaped
proteins and retrieves them back to the ER (Li et al. 2009b ). The ERD2b protein is
highly homologous to the yeast HDEL receptor. Five ERD2 paralogs (ERD2-
likeproteins or ERPs) have been detected in Arabidopsis . ERD2b is essential for
the calreticulin CRT3 accumulation. The retention of soluble ER protein relies
mainly of the recognition of its C-terminal sorting signal (i.e. HDEL;