Heterotrimeric G protein signaling in the Arabidopsis unfolded protein response

We present evidence that heterotrimeric G protein signaling is involved in cell death associated with the unfolded protein response (UPR) in Arabidopsis . Seedlings of homozygous agb1-2 (Gβ-null mutation) mutant plants are markedly more resistant to growth inhibition by the protein glycosylation inhibitor tunicamycin (Tm) than either wild-type plants or gpa1-4 (Gα-null mutation) mutants. Leaves of older Gβ mutant plants show much less cell death when infiltrated with Tm than leaves of wild-type plants. The transcriptional response of Gβ mutant plants to Tm is less pronounced than that of wild-type plants, as is the accumulation of BiP chaperone proteins. A majority of the Arabidopsis Gβ protein is associated with the endoplasmic reticulum (ER) and cofractionates with membrane-associated ER luminal BiP. Consistent with its ER localization, Gβ protein is degraded during the UPR, whereas Gα protein is not. Taken together, these observations imply that the Gβ protein, which forms a stable heterodimer with the Gγ subunit, is involved in the signaling events that trigger UPR-associated cell death. The different Tm sensitivities of Gα and Gβ mutants, the ER localization of Gβ, and the differential stabilities of Gα and Gβ proteins during the UPR suggest that the Gβγ complex serves a signaling function in the ER independent of its function in the Gαβγ heterotrimer.