Protein Phosphorylation and Dephosphorylation in Plant Immune Signaling Systems - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 388

fungal elicitor. The transcript accumulation was followed by an increase in LeMPK3
kinase activity. The LeMPK3 autophosphorylates in vitro mainly on tyrosine and
less so on threonine and serine, whereas it phosphorylates myelin basic protein on
serine and threonine (Mayrose et al. 2004 ). The autophosphorylation of LeMPK3
may not be suffi cient for its full activation, which requires an upstream MAPKK
(Mayrose et al. 2004 ). Arabidopsis AtMPK4 MAP kinase autophosphorylates in
vitro on tyrosine residues and is activated by the AtMEK1 MAPKK through phos-
phorylation of threonine residues (Huang et al. 2000 ). Phosphorylation of the MAP
kinase promotes its homodimerization and nuclear translocation (Khokhlatchev
et al. 1998 ). The activated MAPK phosphorylates transcription factors (Ligterink
and Hirt 2000 ).
The MAP kinases WIPK and SIPK have been shown to be involved in defense
signaling system (Liu et al. 2003 ). Reversible phosphorylation/dephosphorylation
events were involved in the activation of WIPK and SIPK in tobacco cells. The
protein kinase inhibitor staurosporine inhibited the activation of SIPK and WIPK
(Zhang et al. 2000 ; Liu et al. 2003 ) and the phosphatase inhibitors calyculin A and
okadaic acid activated SIPK and induced WIPK expression (Liu et al. 2003 ). These
results suggest the importance of transient phosphorylation/dephosphorylation
events in activation of the MAPK pathway.
A protein that interacts with SIPK has been identifi ed as a member of the MAP
kinase kinase family and named as SIPKK. SIPKK phosphorylates myelin basic
protein in vitro (Liu et al. 2000 ). SIPK is activated exclusively at the posttransla-
tional level by phosphorylation (Liu et al. 2000 ). Fungal elicitors transiently activated
a 47-kD putative MAPK via tyrosine phosphorylation in tobacco cells (Suzuki and
Shinshi 1995 ). Activation of this 47-kD kinase was inhibited by staurosporine, a
protein kinase inhibitor staurosporine and the Ca 2+ channel blocker Gd 2+ (gadolinium)
suggesting that upstream kinases and Ca 2+ might be involved in the activation of this
kinase (Suzuki and Shinshi 1995 ).

9.7 Role of 14-3-3 Proteins in Protein Phosphorylation

In some cases, besides the kinases, 14-3-3 proteins complete a multiplex signal-
induced change in the target protein (Roberts 2000 ; Yaffe 2002 ; Ferl 2004 ). 14-3-3
proteins physically interact with other protein families by binding with the phos-
phorylated proteins (Ferl 2004 ). 14-3-3 proteins can bind with a large number of
proteins, since 14-3-3 proteins require relatively simple amino acid sequence for
their binding (Yaffe et al. 1997 ). 14-3-3 proteins have been detected in different cel-
lular compartments. They are found in cytoplasm, inside chloroplasts and found
associated with mitochondria and some of them have been detected in plasma mem-
brane (Fuglsang et al. 1999 ; Roberts and Bowles 1999 ). It appears that phosphoryla-
tion cannot cause a change in protein activity, but phosphorylation is the only means
to connect the signal to the target protein (Ferl 2004 ). With the assistance of 14-3-3
proteins, phosphorylation signals can reach many additional targets (Ferl 2004 ).