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its displacement the ATPase gets activated (Palmgren and Harper
1999
). The
activation of H
+
-ATPase appears to be modulated by Ca
2+
infl ux - induced calcium
dependent protein kinase (CDPK) (Camoni et al.
1998b
; Schaller and Oecking
1999
). The CDPK phosphorylates H
+
-ATPase and the phosphorylation site is
located at the C-terminal domain of H
+
-ATPase (Camoni et al.
1998b
). The H
+
-
ATPase was found to be phosphorylated at serine and threonine residues (Schaller
and Sussman
1988
). Phosphorylation of H
+
-ATPase is stimulated by the addition
of Ca
2+
and by a decrease in pH, from 7.2 to 6.2, suggesting that changes in the
cytoplasmic Ca
2+
and pH are potentially important elements in modulating the
kinase-mediated phosphorylation (Schaller and Sussman
1988
). It is suggested
that the H
+
-ATPase might actually become activated following a dephosphoryla-
tion reaction preceded by phosphorylation reaction by CDPK (Palmgren and
Harper
1999
). The elicitor-induced stimulation of the plasma membrane H
+
-
ATPase was inhibited by okadaic acid, a phosphatase inhibitor, but not by strau-
rosporine, a protein kinase inhibitor in tomato, suggesting that protein
dephosphorylation was required for increased H
+
-ATPase activity (Vera-Estrella
et al.
1994
). G proteins may be involved in elicitor-receptor binding, which in turn
may stimulate the H
+
-ATPase by dephosphorylation (Vera-Estrella et al.
1994
;
Xing et al.
1997
).
Regulation of H
+
-ATPases appears to depend on the presence or absence of
14-3-3 proteins (Chung et al.
1999
; Fuglsang et al.
1999
; Kanczewska et al.
2005
;
Ottmann et al.
2007
; Duby and Boutry
2009
). An in vitro interaction between a
phosphorylated CDPK and 14-3-3 isoforms from
Arabidopsis
has been reported
(Camoni et al.
1998a
). There may be a functional link among phosphorylated
CDPK, H
+
-ATPase and 14-3-3 protein in defense signaling (Romeis et al.
2000
;
Duby et al.
2009
). Binding of 14-3-3 proteins to the plasma membrane H
+
-ATPase
involves the three C-terminal residues Tyr-Thr-Val and requires phosphorylation of
Thr (Fuglsang et al.
1999
; Duby and Boutry
2009
). The penultimate threonine
residue and accompanying mode III motif is widely conserved across H
+
- ATPases
throughout the plant kingdom, suggesting that this mechanism of activation is
highly conserved (Duby and Boutry
2009
). Additional phosphorylated residues
within the C-terminal domain have been reported to affect the enzyme activity
(Speth et al.
2010
). 14-3-3 proteins recognize phosphate-bearing amino acid and
regulate the H
+
-ATPase enzyme activity (Romeis et al.
2000
). Phosphorylation of
the penultimate Thr residue in the C-terminal, autoregulatory domain of the H
+
-
ATPase results in 14-3-3 protein-dependent activation of the pump (Fuglsang et al.
1999
,
2007
; Svennelid et al.
1999
; Maudoux et al.
2000
). Phosphorylation at a
second or additional unidentifi ed sites inhibits the H
+
-ATPase and this reaction
appears to be regulated by CDPK (Vera Estrella et al.
1994
; Xing et al.
1996
; De
Nishi et al.
1999
; Rutschmann et al.
2002
).
PAMPs have been shown to cause either H
+
-ATPase activation concomitant with
extracellular acidifi cation and membrane hyperpolarization, or H
+
-ATPase inactiva-
tion resulting in the depolarization of the plasma membrane (Wevelsiep et al.
1993
;
Vera-Estrella et al.
1994
; Hammond-Kosack et al.
1996
; Xing et al.
1996
).
Depolarization/hyperpolarization of cell membrane may modulate Ca
2+
infl ux in
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