Are GTGs a new class of plant anion channels - Williams - BBSRC

Membrane proteins play important physiological roles in all organisms with fundamental functions including transport, signalling, and bioenergetics. This project will focus on a unique membrane protein class which is highly conserved in eukaryotes: the G protein coupled receptor type-G proteins/Golgi pH regulator (GTG/GPHR) family. Contrasting theories exist for the roles of these proteins and the physiological function of this family remains enigmatic. One model suggests the Arabidopsis GTG/GPHRs are plasma membrane receptors for the plant hormone, abscisic acid (ABA; Pandey et al, 2009, Cell 136, 136-148). In contrast, a second group used patch clamp technology, which can be used to measure current carried across membranes by proteins called ion channels, and found that the mammalian GTG/GPHR possesses voltage-dependent anion-channel activity and is critical in regulating Golgi acidification (Maeda et al 2008 Nat Cell Biol, 10, 1135-1145). They made these finding using a Chinese hamster ovary cell line which had a mutated GTG/GPHR, and showed defects in Golgi function. The Golgi is part of the endomembrane system and is a critical organelle in eukaryotes required for packaging and sorting of molecules to be delivered to other parts of the cell and for secretion. This vital process in eukaryotic cell biology is thought to be dependent on a pH gradient along the endomembrane pathway. The proposal builds on our recent breakthroughs using the model plant, Arabidopsis, and model animal, C.elegans, demonstrating that plant and animal GTGs are critical for growth and fertility. We have shown in Arabidopsis that GTG proteins are required for Golgi function, cell wall synthesis and light-regulated growth (Jaffé et al, 2012, Plant Cell 24, 3649-68). This was carried out using mutants that we have isolated independently and in which we observe normal responses to ABA treatments. This and the fact that we find them localised to the Golgi questions their role as plasma membrane ABA receptors. In addition, we have produced the first whole animal model (C.elegans) where both GTGs are mutated and this mutant also shows defects in fertility and growth. Transformation of C. elegans GTG1 into plant gtg1gtg2 mutants shows that its expression restores normal root and hypocotyl (seedling stem) growth. As the animal protein restores these defects in plants we propose a common function for plant and animal GTGs. This project will define the function of this novel membrane protein class, further investigating conservation of function and specifically testing the hypothesis that they function as anion channels regulating Golgi pH in plants. To demonstrate whether there is conservation of function across kingdoms, we will determine if the Arabidopsis GTG1 gene can restore the defects in two animal mutant systems which lack GTG function. The first will be the mammalian Chinese hamster ovary GTG/GPHR-mutant cell line which shows defects in protein secretion due to poor Golgi acidification; the second will be the C. elegans gtg1gtg2 mutant. To determine directly whether plant GTGs have channel activity we will use patch clamp technology to demonstrate anion transport activity following purification and insertion of AtGTG1 into giant unilamellar vesicles. This system will be used to determine biophysical and pharmacological properties of these putative channels and structure/function analysis. We will develop systems for assessing Golgi/ER pH in Arabidopsis using a range of pH probes and test whether plant GTGs can function as pH regulators allowing acidification of these endomembrane compartments. Finally, a regulatory protein called Galpha has been shown to interact with AtGTG1 in yeast. To address the importance of this interaction in the function of GTGs we will determine whether a Galpha-GTG interaction can be observed in planta and the extent to which the phenotype of the gtg1 gtg2 double mutant is dependent on Galpha.