Extra Large G Protein 2 (XLG2) in Plant Immunity: What Is Its Molecular Mode of Action?

Humans use a large number of heterotrimeric G proteins for transducing various extracellular signals perceived by G protein-coupled receptors into appropriate intracellular physiological responses. Plants such as Arabidopsis only have a few genes encoding the G protein subunits, but G proteins have been implicated in many plant biological processes. However, very few downstream effectors of G proteins have been identified and the molecular mechanisms of G protein-mediated processes remain largely unknown in plants.

In addition to a single prototypical G protein (GPA1), there are three unique G-like proteins in Arabidopsis, known as XLG1, XLG2, and XLG3. Their C-terminal regions are similar to prototypical G, but they have a 400 amino-acid N-terminal region that shares significant sequence similarity only to plant XLG proteins. We previously identified XLG2 as a positive regulator in plant disease resistance. However, its molecular mode of action remains elusive.

Unlike other G proteins that are generally localized in the cytoplasm, XLG2 was found to be localized in nuclei. Sequence analysis suggests that the N-terminal region of XLG proteins contains a PHD/FYVE-like zinc finger domain followed by a bundle of alpha helices, and this region shares some sequence similarity to the PHD-Bromo domain of human TRIM24 which functions in transcriptional regulation by recognizing histone modifications. XLG2 was recently reported by another group to interact with RTV1, a DNA-binding protein with a sequence similarity to VRN1 which is involved in chromatin remodeling. We also found that ectopic expression of XLG2 led to accumulation of transcript isoforms of defense-related genes which might be generated by alternative promoter usage. The preliminary studies from ours and others raise an intriguing possibility that XLG2 might function in transcriptional regulation including the defense-associated differential use of alternative promoters through recognizing specific histone modifications. In this study, we will test such a working hypothesis and unravel the molecular function of XLG2 by a combination of molecular, functional genomics, and genetics approaches. The study will provide novel insights into the molecular mechanism of the XLG- mediated pathways and expand our knowledge of G protein functions.