SM-20 is a novel mitochondrial protein that causes caspase-dependent cell death in nerve growth factor-dependent neurons

Sympathetic neurons undergo protein synthesis-dependent apoptosis when depr ived of nerve growth factor (NGF). Expression of SM-20 is up-regulated in N GF-deprived sympathetic neurons, and ectopic SM-20 is sufficient to promote neuronal death in the presence of NGF. We now report that SM-20 is a mitoc hondrial protein that promotes cell death through a caspase-dependent mecha nism. SM-20 immunofluorescenee was present in the cytoplasm in a punctate p attern that colocalized with cytochrome oxidase I and with mitochondria-sel ective dyes. Analysis of SM-20/dihydrofolate reductase fusion proteins reve aled that the first 25 amino acids of SM-20 contain a functional mitochondr ial targeting sequence. An amino-terminal truncated form of SM-20 was not r estricted to mitochondria but instead localized throughout the cytosol and nucleus. Nevertheless, the truncated SM-20 retained the ability to induce n euronal death, similar to the wild type protein, SM-20-induced death was ac companied by caspase-3 activation and was blocked by a general caspase inhi bitor. Additionally, overexpression of SM-20, under conditions where cell d eath is blocked by a general caspase inhibitor, did not result in widesprea d release of cytochrome c from mitochondria, These results indicate that SM -20 is a novel mitochondrial protein that may be an important mediator of n eurotrophin-withdrawal-mediated cell death.