G-Proteins as Molecular Switches in Signal Transduction - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 168

GEF

PAMP

GDP

GTP

Effector

Inactive state Active state

GAP

Fig. 3.2 Small GTPases-mediated elementary signal transduction pathway

(Yalovsky et al. 2010 ; Wu et al. 2011 ). These GTPases act as a simple binary switch
(the 'off' GDP-bound and the 'on' GTP-bound states). Shuttling between GDP-
bound and GTP-bound states is controlled by two major regulators, guanine nucleo-
tide exchange factors (GEFs) and GTPase activating proteins (GAPs). GEFs catalyze
GDP release, which is exchanged with GTP, while GAPs enhance GTP hydrolysis,
thereby accelerating RAC/ROP inactivation (Mucha et al. 2011 ; Wu et al. 2011 ).
Upon stimulation by an upstream PAMP signal, GEF, which is also known as a
G-protein-coupling receptor (GPCR) (Pandey et al. 2010 ), converts the GDP-bound
inactive form of the small GTPase into the GTP-bound active form through GDP/
GTP replacement (Yang 2002 ). Subsequent to signal propagation, the intrinsic
GTPase activity results in hydrolysis of bound GTP to GDP. The GTP form exhibits
a weak intrinsic GTPase activity for GTP hydrolysis, requiring specifi c GTPases
activating proteins (GAPs) with unique domain composition for effi cient deactiva-
tion (Fig. 3.2 ; Yang 2002 ; Mucha et al. 2011 ).
Most small G-proteins cycle between membrane-bound and cytosolic forms.
Only membrane-associated GTPases can be activated by GEF and their removal by
a cytosolic factor called guanine nucleotide dissociation inhibitor (GDI) negatively
regulates these GTPases (Yang 2002 ). Activated RAC/ROPs are capable of receiv-
ing a wide variety of inputs and accordingly generating a multitude of specifi c
inputs (Yang and Fu 2007 ; Liu et al. 2010 ). They interact with immediate cellular
effectors that interact with cellular components, relaying the signal to the ultimate
target systems to effect the corresponding signal-induced responses (Wu et al.
2011 ). Rho is known to orchestrate a great number of signaling networks through a
large number of interacting partners in plants (Yang and Fu 2007 ).

3.4 Heterotrimeric G-Protein G May Act Upstream

of Small G-Protein in Immune Signaling

α

Both the heterotrimeric G-protein and small G-protein may function together and
trigger immune responses in plants (Suharsono et al. 2002 ). In rice cells it was shown
that the PAMP signal was recognized by unknown receptor in the plasma membrane
and transmitted to the heterotrimeric G-protein G α subunit. G α mRNA accumulation

was induced by the signals from the receptor. The activated G triggered accumula-

tion of the small G-protein OsRac1 mRNA, which in turn strongly induced transcrip-
tion of the defense-related PR10 protein gene PBZ1 (Fig. 3.3 ; Suharsono et al. 2002 ).

α