G-Proteins as Molecular Switches in Signal Transduction - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 162

Chapter 3
G-Proteins as Molecular Switches
in Signal Transduction

Abstract Guanosine triphosphate (GTP)-binding proteins (G-proteins) are the
regulatory GTPases that have the ability to bind GTP and hydrolyze it to guanosine
diphosphate (GDP). GDP locks G proteins into their inactive state, while GTP locks
G-proteins into their activated state. Active or inactive states of G-proteins depend
on the binding of GTP or GDP, respectively. G-proteins have been found to be key
players in plant innate immunity. The GTPases act as molecular switches control-
ling the transmission of extracellular signals like pathogen-associated molecular
patterns (PAMPs) to intracellular signaling pathways. The PAMPs have been shown
to activate GTP binding to G-protein. The GTPase is normally inactive. The PAMP
stimulates exchange of GTP for GDP and thus converts the G-proteins from their
inactive state to their active state. Upon stimulation by an upstream PAMP signal, a
guanine nucleotide exchange factor (GEF) converts the GDP-bound inactive form
into the GTP-bound active form through GDP/GTP replacement. Through its effec-
tor domain, the GTP form interacts with specifi c downstream effector proteins. The
GTP form exhibits a weak intrinsic GTPase activity for GTP hydrolysis, requiring
a GTPase-activating protein (GAP) for effi cient deactivation. Most small GTPases
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.
G-proteins include two major subfamilies: heterotrimeric G-proteins and small
G-proteins (also called small GTPases). The heterotrimeric G-proteins contain G α -,

G - subunits. The small G-proteins are monomeric G-proteins and they

appear to be similar to

β -, and G γ

-subunits. Both
classes of G-proteins use the GTP/GDP cycle as a molecular switch for signal trans-
duction. Both heteromeric and monomeric small G-proteins trigger immune
responses by activating several immune signaling systems. These include Ca 2+ chan-
nel activation, K + channel regulation, generation of reactive oxygen species through
activation of NADPH oxidase, regulation of redox signaling, activation of nitric
oxide (NO) signaling system, activation of mitogen-activated protein kinase (MAPK)
signaling cascade, activation of phospholipases, effl ux of vacuolar H + , biosynthesis

α -subunits, operating without the β -, and γ