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J. Exp. Med.,
Volume 186, Number 12, December 15, 1997 1975-1983
By

From the * Manitoba Institute of Cell Biology and the § Department of Human Genetics, University of
Manitoba, Winnipeg, Manitoba, Canada; and the Nip3 (nineteen kD interacting protein-3) is an E1B 19K and Bcl-2 binding protein of unknown function. Nip3 is detected as both a 60- and 30-kD protein in vivo and in vitro and exhibits strong homologous interaction in a yeast two-hybrid system indicating that it can homodimerize. Nip3 is expressed in mitochondria and a mutant (Nip3163) lacking the putative
transmembrane domain and COOH terminus does not dimerize or localize to mitochondria.
Transient transfection of epitope-tagged Nip3 in Rat-1 fibroblasts and MCF-7 breast carcinoma induces apoptosis within 12 h while cells transfected with the Nip3163 mutant have a
normal phenotype, suggesting that mitochondrial localization is necessary for induction of cell
death. Nip3 overexpression increases the sensitivity to apoptosis induced by granzyme B and topoisomerase I and II inhibitors. After transfection, both Nip3 and Nip3163 protein levels decrease steadily over 48 h indicating that the protein is rapidly degraded and this occurs in the
absence of cell death. Bcl-2 overexpression initially delays the onset of apoptosis induced by
Nip3 but the resistance is completely overcome in longer periods of incubation. Nip3 protein
levels are much higher and persist longer in Bcl-2 expressing cells. In conclusion, Nip3 is an
apoptosis-inducing dimeric mitochondrial protein that can overcome Bcl-2 suppression.
Department of Biochemistry, University of
Alberta, Edmonton, Alberta, Canada
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