¹Department of Biological Science and Technology, Science University of Tokyo, Noda, Japan

²Department of Molecular Immunology, Medical Institute of Bioregulation, Kyushu University, Fukuoka, Japan

Correspondence to: Takuma Nakajima, Department of Molecular Cellular Oncology and Microbiology, Faculty of Dentistry, Tokyo Medical Dental University 1-5-45 Yushima, Bunkyo-Ku, Tokyo 113-8549, Japan. tel: +81(0)3-5803-5458 fax: +81(0)3-5803-0189, e-mail: nakajima.mco@dent.tmd.ac.jp

^aCurrent address: Department of Molecular Cellular Oncology and Microbiology, Faculty of Dentistry, Tokyo Medical Dental University, Tokyo, Japan

Edited by Y. Kuchino

The adenovirus E1B19K protein inhibits apoptosis induced by E1A and other divergent signals. The cellular proteins that interact with E1B19K have been analyzed by isolating cDNA clones by the yeast two hybrid system. One of these clones encodes B5 which consists of 219 amino acid residues and contains the putative BH3 and transmembrane regions. B5 binds strongly to Nip3 and itself, weakly to E1B19K, but not to Bcl-2 and localizes in nuclear envelope, endoplasmic reticulum and mitochondria. B5 has sequence homology with Nip3 in the middle and C-terminal regions, but not in the N-terminal region. Unlike other E1B19K binding BH3 proteins so far characterized, B5 does not induce apoptosis, but inhibits apoptosis induced by Nip3. However the deletion mutant B51-31 lacking the N-terminus does induce apoptosis, although weaker than does Nip3, suggesting that the N-terminal region is masking the apoptosis-inducing capacity of B5.

apoptosis; E1A; E1B19K binding protein B5; Nip3