Stress conditions affecting the functions of the endoplasmic reticulum (ER) cause the accumulation of unfolded proteins. ER stress is counteracted by the unfolded-protein response (UPR). However, under prolonged stress the UPR initiates a proapoptotic response. Mounting evidence indicate that the ER chaperone calnexin is involved in apoptosis caused by ER stress. Here, we report that overexpression of calnexin in Schizosaccharomyces pombe induces cell death with apoptosis markers. Cell death was partially dependent on the Ire1p ER-stress transducer. Apoptotic death caused by calnexin overexpression required its transmembrane domain (TM), and involved sequences on either side of the ER membrane. Apoptotic death caused by tunicamycin was dramatically reduced in a strain expressing endogenous levels of calnexin lacking its TM and cytosolic tail. This demonstrates the involvement of calnexin in apoptosis triggered by ER stress. A genetic screen identified the S. pombe homologue of the human antiapoptotic protein HMGB1 as a suppressor of apoptotic death due to calnexin overexpression. Remarkably, overexpression of human calnexin in S. pombe also provoked apoptotic death. Our results argue for the conservation of the role of calnexin in apoptosis triggered by ER stress, and validate S. pombe as a model to elucidate the mechanisms of calnexin-mediated cell death. INTRODUCTIONThe endoplasmic reticulum (ER) is a specialized organelle playing essential and central roles in the biology of the cell. The ER is the site of synthesis and folding of secreted, membrane-bound and some organelle-targeted proteins (Bukau et al., 2000;Fewell et al., 2001). Protein folding in the ER is assisted by a battery of molecular chaperones and foldases (Bukau et al., 2000;Fewell et al., 2001;Trombetta and Parodi, 2003;Helenius and Aebi, 2004). In addition, the ER contains several factors required for optimum protein folding, including, ATP, Ca 2ϩ , and an oxidizing environment to allow disulphide-bond formation. Proper ER function is critical for numerous aspects of cell physiology, including vesicle trafficking and lipid and membrane biogenesis, as well as protein targeting and secretion (Lai et al., 2007).The ER is highly sensitive to stresses perturbing the cellular energy levels and ER lipid or glycolipid imbalances or changes in the redox state or Ca 2ϩ concentration (Breckenridge et al., 2003;Boyce and Yuan, 2006;Szegezdi et al., 2006). Such stresses reduce the protein folding capacity of the ER, which results in the accumulation and aggregation of unfolded proteins, a condition referred to as ER stress. When the capacity of the ER to fold proteins properly is compromised or overwhelmed, a highly conserved unfolded-protein response (UPR) signal-transduction pathway is activated. The ER response to stress is basically conserved from yeast to mammalian cells (Patil and Walter, 2001;Ron and Walter, 2007). To counter ER stress, the UPR halts general protein synthesis and up-regulates the transcription of genes encoding ER resident chaperone...

No abstract