Protein Phosphorylation and Dephosphorylation in Plant Immune Signaling Systems - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 386

autophosphorylation by its own serine/threonine kinase (Gómez-Gómez et al. 2001 ;
Wang et al. 2001 ; Robatzek et al. 2006 ). Mutation of the threonine residue 867
hampers FLS2 response, suggesting that autophosphorylation of the general elicitor
occurs at threonine residue 867 (Robatzek et al. 2006 ).
Activation of other receptor-like kinases by induced autophosphorylation has
been reported (Nasrallah 2000 ; Gómez-Gómez et al. 2001 ; Wang et al. 2005b ).
Arabidopsis CERK1 is the receptor of the PAMP chitin. It is involved in the percep-
tion of the chitin oligosaccharide elicitor at the cell surface and the transduction of
the signal into the cytoplasm via its intracellular serine/threonine kinase activity
(Miya et al. 2007 ). It contains an intracellular serine/threonine kinase domain
(Eckardt 2008 ; Lohmann et al. 2010 ). The rice homolog of CERK1, OsCERK1
encoded a receptor-like kinase consisting of 624 amino acid residues, containing a
signal peptide, an extracellular domain, a transmembrane region and an intracellular
Ser/Thr kinase domain. The expression of OsCERK1 was up-regulated by elicitor
treatment (Shimizu et al. 2010 ). CERK1 is autophosphorylated in vitro (Iizasa et al.
2010 ). The autophosphorylated CERK1 has been shown to be essential for chitin
signaling in plants (Wan et al. 2008a , b ). The PRR for the PAMP elicitin INF1 of
Phytophthora infestans has been identifi ed as a lectin-like receptor kinase and it was
designated NbLRK1. NbLRK1 is a typical RD kinase (Kanzaki et al. 2008 ). The 31
amino acids fragment of NbLRK1 kinase domain within VIb subdomain 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 nec-
essary 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 autophos-
phorylation (Kanzaki et al. 2008 ). INF1 treatment induced autophosphorylation of
NbLRK1 in vivo (Kanzaki et al. 2008 ).
The importance of phosphorylation of receptor-like kinases in signal transduction
has been demonstrated by developing mutants with impaired kinase activity (Wang
et al. 2005a ). Mutation studies with BRI1, a receptor - like kinase in Arabidopsis
have revealed that mutations in the kinase domain activation loop nearly abolished
kinase activity, with respect to both autophosphorylation and protein substrate phos-
phorylation. The results suggest that autophosphorylation of the activation loop of
BRI1 is required for downstream signaling events (Wang et al. 2005a ).
In Arabidopsis , BAK1 belongs to the LRR-receptor-like kinase (RLK). BAK1 is
a positive regulator of PAMP-triggered plant immunity and it acts as an adaptor of
multiple LRR-RKs that act in defense signaling, including the PRRs FLS2, EFR,
PEPR1 and PEPR2 (Chinchilla et al. 2007b ; Ryan and Pearce 2003 ; Gao et al. 2009 ;
Postel et al. 2010 ; Schulze et al. 2010 ). It also acts as an adaptor of the receptor
kinases BIR1 and SOBIR1, which seem to act as part of a presumed PRR complex(es)
and/or at a downstream step in the signaling cascade (Saijo 2010 ). Flg22 perception
by the PRR FLS2 triggers an interaction between FLS2 and BAK1 (Chinchilla et al.
2007a , b ; Heese et al. 2007 ). The fl g22 induced FLS2-BAK1 association occurs
within seconds and is accompanied by increased phosphorylation on both FLS2 and