Noxa is a weak apoptosis activator consisting of a BH3 domain and a mitochondrial-targeting domain (MTD). BH3 binds Mcl-1 and Bcl2A1 and inactivates their anti-apoptotic activities, while MTD delivers BH3 to mitochondria. Previously we revealed that MTD may also function as an inducer of necrosis via conjugation with octa-arginine, which induces cytosolic Ca2+ influx from mitochondria. However, the mechanism(s) underlying this process has not been elucidated yet. Here, we show that calcium influx induced by an MTD peptide fused with octa-arginine residue (R8:MTD) originates not only from mitochondria but also from the extracellular space. However, calcium spikes were not sufficient for necrosis. R8:MTD induced mitochondrial permeability transition pore opening, fragmentation, and swelling. These mitochondrial events induced by MTD appeared to be necessary for necrosis induction, since DIDS, a VDAC inhibitor, inhibited the mitochondrial swelling and cell death induced by MTD. We show that R8:MTD disrupted endoplasmic reticulum (ER) structures but not peroxisomes or Golgi, indicating that R8:MTD causes necrosis by inducing ER events as well. (C) 2019 The Authors. Published by Elsevier Inc.