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N‐glycosylation of ATF6β is essential for its proteolytic cleavage and transcriptional repressor function to ATF6α

Activating transcription factor 6 (ATF6), a member of the ATF/CREB family of transcription factors, has two isoforms of 90‐kDa (p90ATF6α) and 110‐kDa (p110 ATF6β) as endoplasmic reticulum (ER) transmembrane glycoprotein. ATF6β contains five evolutionarily conserved N‐linked glycosylation sites and is a key transcriptional repressor of ATF6α, which contribute to regulating the strength and duration of ATF6‐dependent ER stress response (ERSR) gene induction. Although it is well established that p110ATF6β can be cleaved and generate a nuclear form of 60‐kDa (p60ATF6β) that inhibits ATF6α‐mediated ERSR genes activation, the functional significance of p110 ATF6β N‐linked glycosylation is unknown. Herein, we found that the fully unglycosylated ATF6β cannot be proteolytic cleaved, be detectable in nucleus after dithiothreitol treatment, and repress the transcriptional activity of ATF6α. These results provide the first evidence that unglycosylated ATF6β may directly facilitate the expression of ERSR genes by losing its repressor function to ATF6α. J. Cell. Biochem. 108: 825–831, 2009. © 2009 Wiley‐Liss, Inc.