Protein Phosphorylation and Dephosphorylation in Plant Immune Signaling Systems - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 387

BAK1 (Schulze et al. 2010 ). The de novo phosphorylation of both FLS2 and BAK1
has been detected within 15 s of stimulation with fl g22. It is suggested that sev-
eral LRR-RKs form tight complexes with BAK1 almost instantaneously after ligand
binding and the subsequent phosphorylation events are key initial steps in signal
transduction (Schulze et al. 2010 ). Collectively, these studies suggest that autophos-
phorylation of receptor-like kinases is an important event in PAMP-activated
defense response signal transduction system.

9.5 PAMP/Elicitor Induces Phosphorylation

of Calcium-Dependent Protein Kinases

Phosphorylation of calcium-dependent protein kinases plays an important role in
plant immune responses. Romeis et al. ( 2000 ) identifi ed a membrane-bound
calcium-dependent protein kinase (CDPK) that showed a shift in electrophoretic
mobility from 68 to 70 kDa within 5 min after an elicitor was added in tobacco cell
cultures. The interconversion of the corresponding CDPK forms could be induced
in vitro in both directions by treatments with either phosphatase or ATP. CDPK
activity of the phosphorylated 70-kDa CDPK form was greater than that of nonelic-
ited 68-kD form (Romeis et al. 2000 ). The conversion of the nonelicited CDPK into
active form was not due to autophosphorylation (Romeis et al. 2000 ). The results
suggest that phosphorylation of the CDPK results in activation of the kinase to trig-
ger the downstream events in the signal transduction system (Romeis et al. 2001 ).
Autophosphorylation of CDPK due to elicitor action has also been reported.
CDPK is autoinhibited by an interaction of a pseudosubstrate site within its junction
domain that blocks the active site of the kinase domain. Binding of Ca 2+ to the
calmodulin-like domain of the CDPK causes a conformational change that extends
to the adjacent junction domain and fi nally disengages the autoinhibitor of the active
site (Huang et al. 1996 ).

9.6 PAMP/Elicitor Triggers Phosphorylation

of MAP Kinases

Mitogen-activated protein kinase cascade involves sequence of phosphorylation
events. MAPKKK (MEKK) phosphorylates and activates a particular MAPKK
(MKK) by the phosphorylation of serine/threonine residues in the SXXXS/T motif.
As a dual-specifi city kinase, MAPKK then activates MAPK through the phosphory-
lation of threonine and tyrosine residues in the TXY motif located between kinase
subdomains VII and VIII (Ligterink and Hirt 2000 ; Liu et al. 2000 ). The MAP
kinase itself may be autophosphorylated (Mayrose et al. 2004 ). The tomato MAP
kinase LeMPK3 is specifi cally induced at the mRNA level upon treatment with a