title: SM-20 is a novel mitochondrial protein that causes caspase-dependent cell death in nerve growth factor-dependent neurons.
abstract: Sympathetic neurons undergo protein synthesis-dependent apoptosis when deprived of nerve growth factor (NGF). Expression of SM-20 is up-regulated in NGF-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 mitochondrial protein that promotes cell death through a caspase-dependent mechanism. SM-20 immunofluorescence was present in the cytoplasm in a punctate pattern that colocalized with cytochrome oxidase I and with mitochondria-selective dyes. Analysis of SM-20/dihydrofolate reductase fusion proteins revealed that the first 25 amino acids of SM-20 contain a functional mitochondrial targeting sequence. An amino-terminal truncated form of SM-20 was not restricted to mitochondria but instead localized throughout the cytosol and nucleus. Nevertheless, the truncated SM-20 retained the ability to induce neuronal death, similar to the wild type protein. SM-20-induced death was accompanied by caspase-3 activation and was blocked by a general caspase inhibitor. Additionally, overexpression of SM-20, under conditions where cell death is blocked by a general caspase inhibitor, did not result in widespread release of cytochrome c from mitochondria. These results indicate that SM-20 is a novel mitochondrial protein that may be an important mediator of neurotrophin-withdrawal-mediated cell death.
authors.affiliation: Department of Environmental Medicine, University of Rochester School of Medicine and Dentistry, Rochester, NY 14642, USA.
authors.forename: E A
authors.initials: EA
authors.lastname: Lipscomb
authors.forename: P D
authors.initials: PD
authors.lastname: Sarmiere
authors.forename: R S
authors.initials: RS
authors.lastname: Freeman
mesh_terms: D016475:3T3 Cells; D000818:Animals; D017209:Apoptosis; D001224:Aspartic Acid / Q000031:analogs & derivatives* / Q000494:pharmacology; D053148:Caspase 3; D061945:Caspase Inhibitors; D020169:Caspases / Q000378:metabolism*; D016923:Cell Death; D002460:Cell Line; D002478:Cells, Cultured; D003574:Cytochrome c Group / Q000378:metabolism; D004268:DNA-Binding Proteins; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005728:Ganglia, Sympathetic / Q000166:cytology / Q000378:metabolism; D064799:Hypoxia-Inducible Factor-Proline Dioxygenases; D017874:Immediate-Early Proteins / Q000235:genetics / Q000502:physiology*; D051379:Mice; D008928:Mitochondria / Q000378:metabolism*; D020932:Nerve Growth Factor / Q000502:physiology*; D009474:Neurons / Q000166:cytology* / Q000378:metabolism; D011393:Procollagen-Proline Dioxygenase; D021381:Protein Transport; D017384:Sequence Deletion
chemical_list: D061945:Caspase Inhibitors; D003574:Cytochrome c Group; D004268:DNA-Binding Proteins; D004791:Enzyme Inhibitors; D017874:Immediate-Early Proteins; C413643:benzyloxycarbonyl-aspartyl(O-methyl)fluoromethane; D001224:Aspartic Acid; D020932:Nerve Growth Factor; D011393:Procollagen-Proline Dioxygenase; C490189:Egln1 protein, mouse; D064799:Hypoxia-Inducible Factor-Proline Dioxygenases; C505409:Casp3 protein, mouse; D053148:Caspase 3; D020169:Caspases