Arabidopsis homologs of gp78 modulate the HRD1 complex component AtOS9 by ERAD tuning

Endoplasmic reticulum-associated degradation (ERAD) is an important mechanism for degrading misfolded proteins, and is mediated by different complexes containing several conserved ER-localized ubiquitin ligases, such as Hrd1, Doa10, and gp78. Recent studies have shown that the ERAD machinery is conserved in eukaryotes. However, it remains unknown whether plants have gp78 homologs. We report a functional study of Arabidopsis homologs of gp78 and their involvement in ERAD. T-DNA insertion mutations in Arabidopsis gp78 genes, AtGP78A and AtGP78B, increased degradation of mutated brassinosteroid (BR) receptors, bri1-5 and bri1-9, leading to lower activation of the signaling protein BES1, and thereby enhancing the dwarf phenotypes of bri1-5/9. This is different from the effects of knockout in known ERAD components, which suppress the dwarf phenotypes of bri1-5/9. AtGP78s interacted with and affected the stability of AtOS9, but not other components in the AtHRD1 complex. AtOS9 accumulated in atgp78a-1 atgp78b bri1-5/9, and knockout of AtOS9 rescued the enhanced-dwarf phenotypes of atgp78a-1 atgp78b bri1-5/9. We determined that AtGP78s were involved in plant ERAD by modulating the stability of AtOS9. Taken together, our results not only reveal AtGP78s as new ERAD components but also reveal a relationship between AtGP78s and the AtHRD1 complex in plants.