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 controlling 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 effector domain, the GTP form interacts with specific downstream effector proteins. The GTP form exhibits a weak intrinsic GTPase activity for GTP hydrolysis, requiring a GTPase-activating protein (GAP) for efficient 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.