PAMP Signaling in Plant Innate Immunity - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 74

2.17.2 Calnexin (CNX)/Calreticulin (CRT)/UGGT System

ER-QC depends on two main different pathways involving different chaperones.
The fi rst pathway is specifi c to glycoproteins and is dependent on the calnexin
(CNX)/calreticulin (CRT) cycle that relies on specifi c interaction between CNX/CRT,
two ER-resident lectin-like chaperones, and Asn-linked monoglucosylated glycans.
The CNX/CRT-glycan interaction depends on the availability of the terminal glucose
residue, which is generated through sequential removal of two glucose residues
of the core glycans on nascent proteins by glucosidases I and II. Eliminating
the remaining glucose residue by glucosidase II releases the glycoproteins from
the ER lectins. The released glycoprotein that has successfully acquired its native
conformation can exit the ER to continue its secretory journey. By contrast, a deglu-
cosylated glycoprotein with an incompletely/improperly folded conformation is
recognized by the luminal enzyme UDP-glucose:glycoprotein glucosyltransferase
(UGGT), which specifi cally functions as a folding sensor. UGGT transfers a glucose
residue from UDP-glucose to glycans (Jin et al. 2007 ). This UGGT-catalyzed
reglucosylation promotes its reassociation with CNX/CRT lectins to initiate another
round of CNX/CRT-mediated folding (Williams 2006 ; Jin et al. 2007 ). CNX and CRT
need assistance of

-glucosidase II as well as UGGT to release and re-bind substrate
glycoprotein, respectively (Kleizen and Braakman 2004 ). The alternate action of
glucosidase II and UGGT drives cycles of glycoprotein release from and binding to
CNX/CRT until the glycoprotein is correctly folded. Terminally misfolded proteins are
retrotranslocated into the cytosol for proteasome-mediated ER-associated degradation
(ERAD) in the cytosol (Jin et al. 2007 ).

α

2.17.3 BiP/ERdj/SDF2 System

The second pathway in ERQC system involves B inding P rotein (BiP), also called
G lucose- R elated P rotein 78 (GRP78), which is a member of the heat shock pro-
tein70 (Hsp70) family of chaperones. It activates an adaptive signaling pathway
termed the “unfolded protein response” (Kleizen and Braakman 2004 ). BiP consists
of approximately 45 kDa domain at the N-terminus that is predicted to carry ade-
nosine triphosphatase (ATPase) activity and a domain of 25 kDa at the C-terminus
having a predicted substrate-binding domain (Mayer et al. 2003 ). BiP is localized to
the ER (Park et al. 2010a , b ). BiP interacts with the growing nascent chain of
substrates containing N-linked glycans, facilitating their translocation into the ER
(Molinary and Helenius 2000). In addition, it is involved in the ER-QC system by
which misfolded or unassembled proteins are selectively retained in the ER (Kleizen
and Braakman 2004 ; Park et al. 2010a ). BiP targets permanently misfolded pro-
teins for ER-associated degradation (ERAD) (Kleizen and Braakman 2004 ).
BiP ATPase cycle is controlled by a number of cofactors that regulate either
ATP hydrolysis or nucleotide exchange. These include Hsp40 proteins, which act