Signaling Specificity in Plant G Protein Networks

Heterotrimeric G-protein complexes couple extracellular signals perceived by a plasma membrane receptor to cytoplasmic target proteins in order to regulate shoot and root development and to mediate various stress and hormone responses in plants. The Arabidopsis genome encodes one canonical Gα protein that binds the obligate Gβγ dimer and three non-canonical Extra-Large Gα proteins (XLG1 through 3), the latter, of which the function remains unknown. Specifically, it is not known if the XLG proteins have a Gα subunit-like signaling property. The physiological function of XLG proteins in the G protein signaling network was elucidated genetically using Arabidopsis loss-of-function mutants of xlg1/xlg2/xlg3 triple (hereafter, “triple”) and gpa1/xlg1/xlg2/xlg3 quadruple (hereafter “quad”) mutants. Five independent physiological traits were analyzed: etiolated hypocotyl length, root length, lateral root formation, endoplasmic reticulum (ER) stress response, and CLAVATA3 hormonal peptide response. The triple and quad mutations decreased hypocotyl length relative to wildtype plants, although to a lesser degree than the Gβ mutant. The quad mutant phenocopied the Gβ mutant root, indicating that an XLG/Gβγ signal suppresses root lengths in parallel with the canonical Gαβγ heterotrimer. XLG mutants also exhibited decreased survival against ER-induced stress, along with Gβ mutants, suggesting that the XLG/Gβγ complex transmits an ER stress tolerance signal. It is concluded that the non-canonical XLG proteins biologically function with Gβγ dimer and confers signaling specificity in heterotrimeric G protein signaling network in plants. Thus, despite the unusual structure of XLG proteins, they may biochemically partner with the Gβγ dimer.Bachelor of Scienc