We hive recently demonstrated that TRB3, a novel endoplasmic reticulum (ER) stress-inducible protein, is induced by CHOP and ATF4 to regulate their function and ER stress-induced cell death, however, the regulation of TRB3 function has not been well characterized Here we demonstrate that TRB3 is an unstable protein regulated by the ubiquitin-proteasome system The carboxyl-terminal domain of TRB3 is necessary for protein degradation. and in this region, we found the typical D-box motif, which is a critical sequence for the anaphase-promoting complex/cyclosome (APC/C) dependent proteolysis. TRB3 proteins were stabilized by deletion of its D-box motif and interacted with APC/C coactivator proteins, Cdc20 and Cdh1. The expression level of TRB3 protein is down-regulated by over-expression of Cdh1 but not by that of Cdc20 In addition, knockdown of Cdh1 enhanced the endogenous TRB3 expression level and suppressed Its ubiquitination level These results suggest that APC/C-Cdh1 is involved in ubiquitination and down-regulating the stability of TRB3 protein (C) 2010 Elsevier Inc. All rights reserved