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Endoplasmic Reticulum-Associated Degradation (ERAD) is one of the major processes in maintaining protein homeostasis. Class I α-mannosidases MNS4 and MNS5 are involved in the degradation of misfolded variants of the heavily glycosylated proteins, playing an important role for glycan-dependent ERAD  in planta . MNS4 and MNS5 reportedly have functional redundancy, meaning that only the loss of both MNS4 and MNS5 shows phenotypes. However, MNS4 is a membrane-associated protein while MNS5 is a soluble protein, and both can localize to the endoplasmic reticulum (ER). Furthermore, MNS4 and MNS5 differentially demannosylate the glycoprotein substrates. Importantly, we found that their gene expression patterns are complemented rather than overlapped. This raises the question of whether they indeed work redundantly, warranting a further investigation. Here, we conducted an exhaustive genetic screen for a suppressor of the  bri1-5 , a brassinosteroid (BR) receptor mutant with its receptor downregulated by ERAD, and isolated  sbi3 , a suppressor of  bri1-5  mutant named after  sbi1  (suppressor of  bri1 ). After genetic mapping together with whole-genome re-sequencing, we identified a point mutation G343E in AT1G27520 (MNS5) in  sbi3 . Genetic complementation experiments confirmed that  sbi3  was a loss-of-function allele of  MNS5 . In addition,  sbi3  suppressed the dwarf phenotype of  bri1-235  in the proteasome-independent ERAD pathway and  bri1-9  in the proteasome-dependent ERAD pathway. Importantly,  sbi3  could only affect BRI1/bri1 with kinase activities such that it restored BR-sensitivities of  bri1-5 ,  bri1-9 , and  bri1-235  but not null  bri1 . Furthermore,  sbi3  was less tolerant to tunicamycin and salt than the wild-type plants. Thus, our study uncovers a non-redundant function of MNS5 in the regulation of ERAD as well as plant growth and ER stress response, highlighting a need of the traditional forward genetic approach to complement the T-DNA or CRISPR-Cas9 systems on gene functional study.

frontiers in plant science

A Non-redundant Function of MNS5: A Class I α-1, 2 Mannosidase, in the Regulation of Endoplasmic Reticulum-Associated Degradation of Misfolded Glycoproteins