Site-1 protease cleavage site is important for the ER stress-induced activation of membrane-associated transcription factor bZIP28 in Arabidopsis.

Many sources of stress cause accumulation of unfolded or misfolded proteins in endoplasmic reticulum (ER), which elicits the unfolded protein response (UPR) to either promote cell survival or programmed cell death depending on different developmental context or stress severity. The Arabidopsis membrane-associated transcription factor, bZIP28, is the functional equivalent of mammalian ATF6, which relocates from the ER to the Golgi where it is proteolytically processed and released from the membrane to the nucleus to mediate the UPR. Although the canonical site-1 protease (S1P) cleavage site on the ER lumen-facing domain is well conserved between bZIP28 and ATF6, the importance of S1P cleavage on bZIP28 has not been experimentally demonstrated. Here we provide genetic evidence that the RRIL(573) site, but not the RVLM(373) site, on the lumen-facing domain of bZIP28 is critical for the biological function of bZIP28 under ER stress condition. Further biochemistry and cell biology studies demonstrated that the RRIL(573) site, but not the RVLM(373) site, is required for proteolytic processing and nuclear relocation of bZIP28 in response to ER stress. Our results reveal that S1P cleavage site plays a pivotal role in activation and function of bZIP28 during UPR in plants.