title: Extra-Large G Proteins Expand the Repertoire of Subunits in Arabidopsis Heterotrimeric G Protein Signaling.
abstract: Heterotrimeric G proteins, consisting of Gα, Gβ, and Gγ subunits, are a conserved signal transduction mechanism in eukaryotes. However, G protein subunit numbers in diploid plant genomes are greatly reduced as compared with animals and do not correlate with the diversity of functions and phenotypes in which heterotrimeric G proteins have been implicated. In addition to GPA1, the sole canonical Arabidopsis (Arabidopsis thaliana) Gα subunit, Arabidopsis has three related proteins: the extra-large GTP-binding proteins XLG1, XLG2, and XLG3. We demonstrate that the XLGs can bind Gβγ dimers (AGB1 plus a Gγ subunit: AGG1, AGG2, or AGG3) with differing specificity in yeast (Saccharomyces cerevisiae) three-hybrid assays. Our in silico structural analysis shows that XLG3 aligns closely to the crystal structure of GPA1, and XLG3 also competes with GPA1 for Gβγ binding in yeast. We observed interaction of the XLGs with all three Gβγ dimers at the plasma membrane in planta by bimolecular fluorescence complementation. Bioinformatic and localization studies identified and confirmed nuclear localization signals in XLG2 and XLG3 and a nuclear export signal in XLG3, which may facilitate intracellular shuttling. We found that tunicamycin, salt, and glucose hypersensitivity and increased stomatal density are agb1-specific phenotypes that are not observed in gpa1 mutants but are recapitulated in xlg mutants. Thus, XLG-Gβγ heterotrimers provide additional signaling modalities for tuning plant G protein responses and increase the repertoire of G protein heterotrimer combinations from three to 12. The potential for signal partitioning and competition between the XLGs and GPA1 is a new paradigm for plant-specific cell signaling.
authors.affiliation: Biology Department, Pennsylvania State University, University Park, Pennsylvania 16802.
authors.forename: David
authors.identifier: http://orcid.org/0000-0002-6591-4853
authors.initials: D
authors.lastname: Chakravorty
authors.affiliation: Biology Department, Pennsylvania State University, University Park, Pennsylvania 16802.
authors.forename: Timothy E
authors.identifier: http://orcid.org/0000-0002-0994-0790
authors.initials: TE
authors.lastname: Gookin
authors.affiliation: Biology Department, Pennsylvania State University, University Park, Pennsylvania 16802.
authors.forename: Matthew J
authors.identifier: http://orcid.org/0000-0001-9184-7423
authors.initials: MJ
authors.lastname: Milner
authors.affiliation: Biology Department, Pennsylvania State University, University Park, Pennsylvania 16802.
authors.forename: Yunqing
authors.initials: Y
authors.lastname: Yu
authors.affiliation: Biology Department, Pennsylvania State University, University Park, Pennsylvania 16802 sma3@psu.edu.
authors.forename: Sarah M
authors.identifier: http://orcid.org/0000-0003-4541-1594
authors.initials: SM
authors.lastname: Assmann
mesh_terms: D000595:Amino Acid Sequence; D017360:Arabidopsis / Q000187:drug effects / Q000378:metabolism*; D029681:Arabidopsis Proteins / Q000737:chemistry / Q000378:metabolism*; D002462:Cell Membrane / Q000378:metabolism; D003198:Computer Simulation; D019204:GTP-Binding Proteins / Q000378:metabolism*; D020962:Heterotrimeric GTP-Binding Proteins / Q000737:chemistry / Q000378:metabolism*; D008969:Molecular Sequence Data; D009154:Mutation / Q000235:genetics; D019913:Nuclear Localization Signals / Q000737:chemistry; D010641:Phenotype; D011485:Protein Binding / Q000187:drug effects; D021122:Protein Subunits / Q000378:metabolism*; D015398:Signal Transduction / Q000187:drug effects; D013347:Subcellular Fractions / Q000378:metabolism; D014026:Nicotiana / Q000378:metabolism; D014415:Tunicamycin / Q000494:pharmacology; D020798:Two-Hybrid System Techniques
chemical_list: D029681:Arabidopsis Proteins; D019913:Nuclear Localization Signals; D021122:Protein Subunits; C120055:XLG1 protein, Arabidopsis; D014415:Tunicamycin; D019204:GTP-Binding Proteins; D020962:Heterotrimeric GTP-Binding Proteins; C527829:XLG2 protein, Arabidopsis; C527830:XLG3 protein, Arabidopsis