Alternative splicing variants of Arabidopsis G protein β subunit AGB1 function in plant development and endoplasmic reticulum stress response

The Arabidopsis heterotrimeric G protein β subunit, GTP BINDING PROTEIN BETA 1 (AGB1), has multiple functions in plant development and response to various environmental stimuli including endoplasmic reticulum (ER) stress. However, how the single gene produces the pleiotropic effect remains elusive. Here, we show that AGB1 has 4 alternative splice isoforms with isoform-specific features. AGB1.2 failed to rescue the agb1-3 mutant defects and thus was considered a non-functional isoform. Although AGB1.1 and AGB1.4 were both localized at the plasma membrane and the ER, AGB1.1 fully rescued the defects of agb1-3, and AGB1.4 only partially rescued the defects even though its transcript level was higher than that of AGB1.1. Intriguingly, AGB1.3 was localized at the nucleus and further enhanced the leaf-shape phenotype of agb1-3 . The protein structure of AGB1.3 is unique because of the termination of translation in the 7th-WD40 motif by alternative splicing, which produced an incomplete propeller structure. AGB1.1 and AGB1.4 but not AGB1.3 interacted with the Gγ subunits, AGG1, AGG2, and AGG3, possibly because of the lack of the 7th-WD40 motif in AGB1.3. AGB1 may produce its multifaceted functions in plant development and ER stress tolerance via its alternative splice isoforms with distinct structural features and subcellular localization.