A plant-specific calreticulin is a key retention factor for a defective brassinosteroid receptor in the endoplasmic reticulum.

Mammalian calreticulin (CRT) is a multifunctional Ca(2+)-binding protein involved in more than 40 cellular processes in various subcellular compartments, such as Ca(2+) storage and protein folding in the endoplasmic reticulum (ER). CRT homologues were discovered in plants almost 15 years ago, and recent studies revealed that many plant species contain 2 or more CRTs that are members of 2 distinct families, the CRT1/2 family and the plant-specific CRT3 family. However, little is known about their physiological functions. Here we report ebs2 (EMS-mutagenized bri1 suppressor 2) as an allele-specific suppressor of bri1-9, a dwarf Arabidopsis mutant caused by retention of a defective brassinosteroid receptor in the ER. EBS2 encodes the Arabidopsis CRT3 that interacts with ER-localized bri1-9 in a glycan-dependent manner. Loss-of-function ebs2 mutations compromise ER retention of bri1-9 and suppress its dwarfism, whereas EBS2 over-expression enhances its dwarf phenotype. In contrast, mutations of 2 other CRTs or their membrane-localized homologues calnexins had little effect on bri1-9. A domain-swapping experiment revealed that the positively charged C-terminal tail of CRT3 is crucial for its "bri1-9-retainer" function. Our study revealed not only a functional role for a plant-specific CRT, but also functional diversity among the 3 Arabidopsis CRT paralogues.

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