Analysis of rice ER-resident J-proteins reveals diversity and functional differentiation of the ER-resident Hsp70 system in plants

Abstract

The heat shock protein 70 (Hsp70) chaperone system participates in protein folding and quality control of unfolded proteins. To examine the roles of co-chaperones in the rice Hsp70 chaperone system in the endoplasmic reticulum (ER), the functions of six ER-resident J-proteins (OsP58A, OsP58B, OsERdj2, OsERdj3A, OsERdj3B, and OsERdj7) in rice were investigated. The expression of OsP58B, OsERdj3A, and OsERdj3B was predominantly up-regulated in roots subjected to ER stress. This response was mediated by signalling through ATF6 orthologues such as OsbZIP39 and OsbZIP60, but not through the IRE1/OsbZIP50 pathway. A co-immunoprecipitation assay demonstrated that OsP58A, OsP58B, and OsERdj3B preferentially interact with the major OsBiP, OsBiP1, while OsERdj3A interacts preferentially with OsBiP5, suggesting that there are different affinities between OsBiPs and J-proteins. In the endosperm tissue, OsP58A, OsP58B, and OsERdj2 were mainly localized in the ER, whereas OsERdj2 was localized around the outer surfaces of ER-derived protein bodies (PB-Is). Furthermore, OsERdj3A was not expressed in wild-type seeds but was up-regulated in transgenic seeds accumulating human interleukin-7 (hIL-7). Since ERdj3A–green fluorescent protein (GFP) was also detected in vacuoles of callus cells under ER stress conditions, OsERdj3A is a bona fide vacuole-localized protein. OsP58A, OsP58B and OsERdj3A were differentially accumulated in transgenic plants expressing various recombinant proteins. These results reveal the functional diversity of the rice ER-resident Hsp70 system.

Emerging features of ER resident J-proteins in plants

“…1); and AtP58 IPK , AtERdj2A, AtERdj2B, AtERdj3A, AtERdj3B and AtERdj7 (At1g61770), respectively. 5,6 Humans have seven ERdjs (ERdj1/Mtj1, ERdj2/Sec63, ERdj3/HEDJ, ERdj4/MDG-1, ERdj5/JPDI, ERdj6/P58 IPK and ERdj7), and yeast has four ERdjs (Sec63p, Scj1p, Jem1p and Erj5p) (Fig. 1).…”

“…1). Since Arabidopsis and rice ERdjs were designated according to their homology to mammalian ERdjs, 5,6 plants have orthologs of mammalian ERdjs corresponding to ERdj2, ERdj3, ERdj6/ P58 IPK and ERdj7. While yeast nomenclature for ERdjs is not consistent with plant ERdj names, relationships are apparent between ERdjs from the different kingdoms.…”

“…Expression of the yeast Erj5 is induced by ER stress, 10 whereas expression of mammalian and plant ERdj7 are not affected by ER stress. 5,6,11 Thus, mammalian and plant ERdj7 and yeast Erj5 may be distinct, and yeast Erj5 is likely to be a yeast-specific ERdj.…”

“…3,4 Six of the plant ERdjs are localized in the ER. 5,6 The ER resident J-proteins are often termed ERj in mammalian systems; in this plant review, the ER J-proteins are denoted ERdj. The functions of most ERdjs in plants remain unclear; however, recent studies have revealed physiological roles for Arabidopsis ERdjs such as AtERdj2A, AtERdj3A and AtERdj3B.…”

“…5,7,8 In addition, rice OsERdj3A is localized in vacuoles under ER stress conditions, indicating that plant ERdjs display distinct characteristics from those in mammals. 6 This mini-review summarizes the current knowledge surrounding ERdjs in plants, as well as in mammals and yeast. In addition, plant ERdjs are compared with mammalian and yeast ERdjs to allow functional inference for plant ERdjs that are not yet fully characterized.…”