G-Proteins as Molecular Switches in Signal Transduction - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 166

3.2.2 G-Protein-Coupled Receptor

G-proteins physically couple the recognition of extracellular signals like
pathogen-associated molecular patterns (PAMPs) with specifi c cell-surface recep-
tors called G-protein coupled receptors (GPCRs) (Colucci et al. 2002 ; Apone et al.
2003 ; Chen et al. 2004 ; Gookin et al. 2008 ; Hu et al. 2010 ; Tesmer 2010 ). The
interaction of GPCRs with heterotrimeric G proteins is an important biological
process in activating various defense responses (Hu et al. 2010 ). The most promis-
ing GPCR, GCR1 , has been cloned from Arabidopsis thaliana (Josefsson and Rask
1997 ). Arabidopsis GCR1 encodes a protein with predicted seven-transmembrane-
spanning domain (Chen et al. 2004 ). It has been shown that GCR1 physically inter-
acts with GPA1, the G protein

α -subunit (Pandey and Assmann 2004 ).

3.2.3 G γ Protein Triggers Plasma Membrane Targeting

of G βγ to Trigger Immune Responses

The G γ subunit is an essential part of the heterotrimer, binding tightly to G β and

anchoring the G dimer to the plasma membrane (Anderson and Botella 2007 ;

Marrari et al. 2007 ). The G-protein

βγ

γ subunits are responsible for providing func-

tional selectivity in G protein of the

G-protein heterotrimer is crucial for its proper targeting at the plasma membrane
and correct functioning (Choudhury et al. 2011 ). Most of the G

βγ dimer (Trusov et al. 2007 , 2012 ). The G γ

subunits are small
proteins of about 8-11 kDa (Trusov et al. 2012 ) and contain a conserved prenylation
signal at their C-termini, which is a target for posttranslational prenylation (McIntyre
2009 ). The heterotrimer formation, together with isoprenylation, is required for
plasma membrane targeting of G

γ

βγ (Takida and Wedegaertner 2003 ).

proteins in their size and presence of prenylation signal
in the C-terminus have also been reported. The size of the Arabidopsis G

Variations among the G γ

subunit
AGG3 is approximately of 25 kDa, and the protein contains a large cysteine-rich
C-terminus (Chakravorty et al. 2011 ). The rice G

γ

γ subunit RGG2 does not contain

a C-terminal prenylation signal (Kato et al. 2004 ).
All known G

proteins contain a signature DPLL/l motif which together with
few additional conserved amino acids in the middle coiled-coil region is required
for interaction with the G

γ

proteins also contain a
CAXX motif at C termini which is isoprenylated, resulting in the targeting of the
proteins to the plasma membrane (Fukada 1995 ; Clapham and Neer 1997 ).
The Arabidopsis G

β proteins. Most of the known G γ

proteins AGG1 and AGG2 are involved in regulation of
defense responses of plants (Mason and Botella 2000 , 2001 ; Trusov et al. 2007 ).
The rice G

γ

2 proteins are highly
homologous to the AGG1 protein and contain all the conserved features and motifs
of G

γ protein RGG1 and soybean GmG γ 1 and GmG γ

proteins. The rice RGG2 protein has an extra 57 amino acid extension at its N
terminus (compared to RGG1) and does not contain the signature prenylation motif.

γ