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-GDP- G
complex and the receptor that can activate it are separately associated
with the membrane. On receptor activation, the receptor becomes highly affi ne for
the G-protein complex. On binding with the complex, GDP dissociates from the
complex and the free complex has a high affi nity for GTP. Upon GTP binding, both
G
βγ
α
-GTP and G
βγ
separate from both the receptor and from each other. Both G
α
-GTP and G
may activate separate effector molecules sending the signal further
down in the signal transduction chain. The GTPase activated by the intracellular
receptor domain activates other molecules of the signal transduction chain, either
via the
βγ
complex (Yang 2002 ; Agrawal et al. 2003 ).
G-proteins are located in the cytoplasmic face of the cell plasma membrane
(Casey 1995 ). Posttranslational lipid modifi cations of Arabidopsis G
α
unit or the
βγ
-subunits have
been shown to be required for plasma membrane targeting (Zeng et al. 2007 ). Many
G-protein coupled receptors have been identifi ed in plasma membrane (Kaziro et al.
1991 ). The PAMPs have been shown to activate GTP binding to G-protein (Gelli
et al. 1997 ; Zhao and Sakai 2003 ). Binding of the PAMP with a pattern-recognition
receptor (PRR) activates the G-protein (Kaziro et al. 1991 ; Tsukada et al. 2002 ). The
PAMP stimulates conformational changes in the G- proteins. It stimulates exchange
of GTP for GDP and thus converts the G-proteins from their inactive state to their
active state (Gelli et al. 1997 ; Pandey and Assmann 2004 ).
γ
3.10
PAMP-Activated G-Proteins Switch on Calcium
Ion-Mediated Immune Signaling System
3.10.1
G-Proteins Activate InsP3-Gated Channels
Perception of PAMP signals by pattern recognition receptors activates G-proteins.
One of the earliest events in the immune signaling system appears to be G-proteins-
triggered transient changes in permeability of the plasma membrane to Ca 2+ and
infl ux of extracellular Ca 2+ through the membrane (Garcia-Brugger et al. 2006 ;
Laohavisit et al. 2009 , 2010 ; Vadassery and Oelm
ller 2009 ). G-protein activates
Ca 2+ channels and enhances Ca 2+ infl ux through Ca 2+ -permeable channels (Wang
et al. 2001 ; Zhang et al. 2011 ). G-proteins have been shown to be involved in trig-
gering changes in cytosolic Ca 2+ concentrations (Blumwald et al. 1998 ; Schultheiss
et al. 2003 ). Massive infl ux of Ca 2+ was observed within 15-30 min after PAMP
treatment in tobacco-cultured cells (Lecourieux-Ouaked et al. 2000 ). The induced
calcium ([Ca 2+ ] cyt ) elevations predominantly result from a continuous Ca 2+ infl ux
through the plasma membrane (Hu et al. 2004 ; Vandelle et al. 2006 ).
RACK1 is an interactor with the G-protein Rac1 in rice. RACK1 homologs have
been isolated from several plant species (Shirasu and Schulze-Lefert 2003 ). RACK1
protein interacts with the GTP form of the G-protein Rac1 (Nakashima et al. 2008 ).
RACK1 binds to G
ū
units (Patterson
et al. 2004 ). RACK1 binds inositol 1,4,5-trisphosphate (InsP3) receptors and
β
and occurs in a complex with the G
α
and G
γ
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