PAMP Signaling in Plant Innate Immunity - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 52

extracellular domain shows homology to lectin proteins known to bind carbohy-
drates (van Damme et al. 1998 ). NbLRK1 belongs to the class B lectin-like receptor
kinases (Kanzaki et al. 2008 ).
The 31 amino acids fragment of NbLRK1 kinase domain within VIb subdo-
main has been shown to interact with INF1 in vitro . The VIb subdomain of Ser/
Thr kinase is known to contain the catalytic loop with an invariant Asp serving as
the catalytic base necessary for the kinase function. This site is close to the VII
and VIII domains where the activation loop is located, which is necessary for
autophosphorylation of kinases (Dardick and Ronald 2006 ; Kanzaki et al. 2008 ).
It is suggested that INF1 binding to the VIb subdomain of NbLRK1 alters its
kinase activity presumably by autophosphorylation (Kanzaki et al. 2008 ).
NbLRK1 contains a conserved arginine (R) at immediately preceding the invari-
ant aspartate (D) in subdomain VIb. It suggests that NbLRK1 is a typical RD
kinase (Kanzaki et al. 2008 ). It does not belong to the non-RD kinases known to
harbor many kinases involved in pathogen recognition receptors signaling
(Dardick and Ronald 2006 ).
INF1 and NbLRK1 proteins also interact in vitro . INF1 treatment induced
autophosphorylation of NbLRK1 in vivo . Virus-induced gene silencing of
NbLRK1 delayed INF1-mediated defense responses in N . benthamiana (Kanzaki
et al. 2008 ). These results suggest that NbLRK1 recognizes INF1 elicitor and
transduces the defense signals.
NbLRK1 has been found to be localized at plasma membrane in tobacco (Kanzaki
et al. 2008 ). INF1 is known to be secreted by P . infestans through its N-terminal
signal peptide and was suggested to localize at the extracellular space of plant tissue
(Kamoun et al. 1997a ). Tyler ( 2002 ) suggested that plant recognition of elicitins
takes place inside the plant cells and elicitins would have been transported inside
plant cells by receptor-mediated endocytosis. The elicitin quercinin of Phytophthora
quercina was reported to be localized inside the cells of host oak plants (Brummer
et al. 2002 ). Collectively these studies suggest that INF1 protein initially localizes
in the apoplast but then traffi cks inside plant cells by endocytosis, where it interacts
with kinase domain of NbLRK1 (Kanzaki et al. 2008 ).

2.11.8 NgRLK1, the PRR for the PAMP Elicitin Capsicein

A PRR for the elicitin capsicein has been identifi ed in tobacco ( Nicotiana glutinosa )
and it was designated NgRLK1 (Kim et al. 2010 ). NgRLK1 has a domain structure
similar to that of all plant RLKs. The extracellular domain of NgRLK1 contains
both lectin-like and S-locus glycoprotein domains, in addition to a PAN AP domain,
which is known to mediate protein-protein or protein-carbohydrate interactions.
Extracellular NgRLK1 was found to interact with the elicitin capsicein. Capsicein
was found to bind to the intracellular kinase domain of NgRLK1 (Kim et al. 2010 ).
NgRLK1 was more closely related to the protein kinase homologous to PR5 K from
Arabidopsis thaliana than to the lectin-like receptor kinases. It has been suggested