G-Proteins as Molecular Switches in Signal Transduction - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 167

The two pea G proteins PGG1 and PGG2 do not contain the highly conserved

DPLL/1 motif even though a possible prenylation motif is present at its C termini
(Misra et al. 2007 ).

γ

3.2.4 Activation of G-Protein Heterotrimer in Elementary

G-Protein Signaling

Heterotrimeric G-proteins are composed of subunits, which exist as associated

heterotrimers in their inactive state. The heterotrimeric G-protein signaling begins
with ligand (PAMP signal) binding, which results in a conformational change in a
G-protein-coupled receptor. Once activated by the GPCR, the G

α , β , γ

protein, which
possesses a GDP/GTP-nucleotide-binding site and GTP-hydrolase activity, changes
its form to a structure that allows exchange of GDP for GTP (Pandey et al. 2010 ).
The GPCR works as a guanine exchange factor (GEF) for G

α

α and facilitates the G α

subunit to exchange GDP for GTP and become active (Oki et al. 2009 ). GTP binding
is accompanied by structural rearrangements that disengage the G

interaction and
result in heterotrimer dissociation. The free subunits then relay signals by interacting
with downstream proteins called effectors. The GTP-bound G

βγ

α separates from the

associated G proteins can then interact with

downstream effector molecules, alone or in combination, to transduce the signal
(Pandey et al. 2010 ). G

βγ dimer and the freed G α and G βγ

independently interact with multiple downstream
effectors mediating specifi c signal transduction pathways. Subsequent to signal
propagation, the intrinsic GTPase activity of G

α and G βγ

α eventually results in hydrolysis of

bound GTP to GDP, which inactivates G α and allows its re-association with the G βγ

dimer to reform the inactive G-protein complex (Oldham and Hamm 2008 ; Oki et al.
2009 ; Liu et al. 2010 ; Pandey et al. 2010 ; Trusov et al. 2012 ).

3.3 Small G-Proteins Signaling

Small G-proteins (small GTPases) are monomeric guanine nucleotide binding pro-
teins related to the subunit of heterotrimeric G proteins (Yang 2002 ). All small

G-proteins belong to Ras superfamily. The small G-proteins constitute the sole
group of Rho family of small GTPases, called ROPs in plants (Berken 2006 ; Yang
and Fu 2007 ; Nakashima et al. 2008 ). Rops are also referred to as RAC (Gu et al.
2004 ; Kiirika et al. 2012 ). The common features of this super family include four
guanine nucleotide binding domains and an effector binding domain (Yang 2002 ).
Arabidopsis contains 11 Rac genes (Yang 2002 ), while rice contains seven Rac
genes (Miki et al. 2005 ). Six small G-proteins have been detected in barley
(Schultheiss et al. 2003 ).
The family of Rho GTPases termed either ROPs or RACs is known to act as the
major molecular switches in multitude of signal transduction pathways in plants

α