Signal peptide peptidase-like proteases OsSPPL1 and OsSPPL2 facilitate ER-associated protein degradation in rice.

Signal peptide peptidases (SPPs) play a critical role in intramembrane proteolysis of signal peptides in mammals. However, their function in plants remains poorly understood. Here, we uncover the critical role of two rice SPP-like proteins, OsSPPL1/ 2, in ER-associated degradation (ERAD). Their expression is directly upregulated by OsbZIP50 under ER stress conditions. Mutations in OsSPPL1/2 result in increased ER stress sensitivity, whereas their overexpression enhance ER stress tolerance. We further demonstrate that OsSPPL1/2 localize in ER, and physically interact with the ERAD components OsDER1/2, indicating their involvement in ERAD. Using a GFP protein fused with a segment of maize floury-2 protein defective in signal peptide cleavage (ZmFL2m-GFP), we show that OsSPPL1/2 interact with ZmFL2m-GFP in ER and facilitate its degradation in tobacco leaves and rice plants. Additionally, OsSPPL1/2 double mutants exhibit exaggerated thermal sensitivity, while OsSPPL1/2-overexpressing plants display improved thermotolerance. Together, our findings identify OsSPPL1/2 as components of ERAD and highlight the importance of ERAD in plant thermotolerance.