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 (G beta-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 (G alpha-null mutation) mutants. Leaves of older G beta mutant plants show much less cell death when infiltrated with Tm than leaves of wild-type plants. The transcriptional response of G beta 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 G beta protein is associated with the endoplasmic reticulum (ER) and cofractionates with membrane-associated ER luminal BiP. Consistent with its ER localization, Gig protein is degraded during the UPR, whereas G alpha protein is not. Taken together, these observations imply that the G beta protein, which forms a stable heterodimer with the G gamma subunit, is involved in the signaling events that trigger UPR-associated cell death. The different Tm sensitivities of G alpha and G beta mutants, the ER localization of G beta, and the differential stabilities of G alpha and G beta proteins during the UPR suggest that the G beta gamma complex serves a signaling function in the ER independent of its function in the Ga beta gamma heterotrimer.