Heterotrimeric GTP-binding proteins (G-proteins) play a critical role in switching on or off a signaling cascade to create a cellular response to external stimuli.
What are Heterotrimeric G-proteins?
Heterotrimeric G-protein complexes participate in cell proliferation, differentiation, chemoreception and neurotransmission in animals and in plants, they are involved in a range of mech- anisms from growth control, cell proliferation, pathogen defense, stomata movements, channel regulation, sugar sensing and some hormonal responses.
The heterotrimeric complex consists of the α, β and γ subunits and the signaling mechanism further in- volves putative heptahelical G-protein-coupled-receptors (GPCRs), whose presences are still questioned in plants and regulator of G-protein signaling proteins (RGS). The structural differences between α, β and γ subunits in different plants are given in Table 1. Recently, a fifth Nobel Prize concerning G-protein signaling was awarded to B. Kobilka and RJ. Lefkowitz for studying these pathways, and mechanisms in ani- mals are well understood. However,
G-proteins and their signaling mechanisms stand out as not so clear in plants and questions like “Can plants tell us something new about G-protein signaling?” have come up. In the past years, studies on G-protein signaling have revealed significant differences between plants and animals, so findings in animal studies are probably limited to one small corner of the eukaryotic system in the big picture.
G-protein signaling in animals
In animals, fungi and also in some amoebae, as shown schematically in Figure 1 the heterotrimeric complex is found to be attached to the seven-transmembrane (7TM) recep- tors in the cytoplasmic part of the membrane. In the resting state, the α subunit contains a GDP and is bound to βγ dimer. Binding of a signal molecule to the GPCR causes the exchange of the GDP with GTP and results in a conformational change in the heterotrimer with dissociation α subunit from the βγ dimer.
The βγ dimer is attached to the membrane by a prenyl group while α subunit is tethered to the membrane by myristyl group. After dissociation by GDP-GTP exchange, βγ dimer and α subunits can interact separately with their different target proteins called “effectors”. Effectors are pro- teins, usually enzymes, which change their activity when bound to G-protein subunits. In animals, adenylyl cyclase and phospholipase C are among the identified effectors in the G-protein signaling mechanism. Regulators of G-protein signaling proteins (RGSs) are other players in this process.
They are also known as “GTPase-accelerating proteins”, and they speed termination of signaling by increasing the GTP hydrolysis rate on the α subunit. In humans, there are at least 37 RGS proteins with domains that permit membrane localization, such as lipids or those that facilitate G-protein coupled receptor (GPCR) binding . RGS can participate in many protein-protein interactions via amino and carboxy extensions from the RGS domain. Zhong et al. presented a concept “kinetic scaffolding” and its contribution to the spatial focusing of G-protein signals.
According to this study, the ability of RGS to narrow the spatial range of signal output from GPCRs to the 10-100 nm scale could permit a similar fine localization of signaling via G-proteins system. This process may occur around a single receptor but could also play a major role in localizing signals around small clusters of receptors in dendrites or synaptic areas of neuronal cell bodies. This mechanism is not found in plant G-protein signaling yet so it can be counted as a difference among plants and animals.
Functional studies on plant G-proteins
In plants, heterotrimeric G-proteins have roles in defense mechanism against to bacte- rial and fungal attacks. However, recent studies has shown that G-protein signaling is also involved in resistance against to viral pathogens by activating the cell death. In this pathway, signaling mediated by Gβ subunit initiates the defense mechanism by cell-death associated mechanisms in Arabidopsis.
Choudhury and Pandey has been modeled a soybean nodulation through G-protein signaling activation. Based on the model, nodule factor receptor (NFR) kinases by collaborating with heterotrimeric G-proteins components.