OsERdj7 is an ER-resident J-protein involved in ER quality control in rice endosperm

OsERdj7 is one of six endoplasmic reticulum (ER)-resident J-domain-containing proteins (J-proteins) encoded by the rice genome that acts as a co-chaperone for Hsp70 and is characterized by the presence of two transmembrane domains. It is N-glycosylated and primarily exists in a dimeric form with a molecular mass of 64 kDa. When the microsomal fraction of maturing seeds was treated with alkaline, high salt or detergent compounds, OsERdj7 was solubilized, even in alkaline and high salt environments, indicating that it is not tightly integrated in the ER membrane. Next, to investigate its role during seed maturation, expression of OsERdj7 was specifically downregulated using RNA interference (RNAi) under the control of the endosperm-specific 16 kDa prolamin promoter in transgenic rice. As a result, the unfolded protein response (UPR) was induced in maturing seeds via activation of OsIRE1/OsbZIP50 and ATF6 orthologs, such as OsbZIP39 and OsbZIP60, leading to upregulation of several chaperones and folding enzymes. Furthermore, some prolamins (RM4 and RM9) were retained in the ER lumen in the form of a mesh-like structure without deposition to the inherent ER-derived protein bodies (PB-Is), although major storage protein glutelins were normally transported to protein storage vacuoles (PB-IIs). On the other hand, induction of ER associated degradation (ERAD) increased OsERdj7 expression in transgenic rice seeds in which ERAD related genes were highly expressed. Due to PDIL2-3 and OsHard3 co-immunoprecipitating with OsERdj7 in rice protoplasts, this result implicates OsERdj7 in the translocation of some seed proteins within the ER lumen and in the degradation of misfolded or unfolded proteins.