Accumulation of unfolded proteins in the endoplasmic reticulum (ER) of eukaryotic cells triggers the transcriptional induction of ER-resident molecular chaperones to maintain cellular homeostasis, termed the ER stress response. Previously we isolated AtbZIP60, a membrane-bound transcription factor involved in the Arabidopsis ER stress response whose activity is controlled by proteolytic cleavage. In this study we characterized the active form of AtbZIP60 localized in the nucleus during the ER stress response. Transient assay using Arabidopsis protoplasts revealed that activation of BiP promoters by AtbZIP60 is dependent on the cis-elements plant-unfolded protein response element (P-UPRE) and ER stress response element (ERSE). Transcriptional activation activity of AtbZIP60 was mainly located in the region for amino acids 41-80 of AtbZIP60. Size exclusion chromatography analysis showed that the nuclear form of AtbZIP60 exists as a protein complex of approximately 260 kDa. On the basis of the present study combined with observations described in the literature, possible mechanisms of AtbZIP60's action in the nucleus are discussed.