Nix and Nip3 form a subfamily of pro-apoptotic mitochondrial proteins

We have identified Nix, a homolog of the E1B 19K/Bcl-2 binding and pro-apop totic protein Nip3, Human and murine Nix have a 56 and 53% amino acid ident ity to human and murine Nip3, respectively. The carboxyl terminus of Nix, i ncluding a transmembrane domain, is highly homologous to Nip3 but it bears a longer and distinct asparagine/proline-rich N terminus. Human Nip3 maps t o chromosome 14q11.2-q12, whereas Nix/BNip3L was found on 8q21, Nix encodes a 23.8-kDa protein but it is expressed as a 48-kDa protein, suggesting tha t it homodimerizes similarly to Nip3, Following transfection, Nix protein u ndergoes progressive proteolysis to an 11-kDa C-terminal fragment, which is blocked by the proteasome inhibitor lactacystin, Nix colocalizes with the mitochondrial matrix protein HSP60, and removal of the putative transmembra ne domain (TM) results in general cytoplasmic and nuclear expression. When transiently expressed, Nix and Nip3 but not TM deletion mutants rapidly act ivate apoptosis, Nix can overcome the suppressers Bcl-2 and Bcl-X-L, althou gh high levels of Bcl-X-L expression will inhibit apoptosis, We propose tha t Nix and Nip3 form a new subfamily of pro-apoptotic mitochondrial proteins .