Abstracts Biological Sciences

The anti-apoptotic function of adenovirus E1B 19kDa protein

by Gang Chen

E1B 19kDa protein (19K), a member of Bcl-2 family of protein, is an adenovirus early gene product. It functions to suppress apoptosis of the host cell caused by viral infection. The exact role of E1B 19kDa in regulating apoptosis in response to many different death stimuli has not been investigated. We found that hygromycin B, an aminoglycoside antibiotic that is used to establish stable mammalian cell lines, kill cells by apoptosis. Apoptosis induced by Hygromycin B is p53-independent and can be blocked both by E1B 19kDa and Bcl-2, showing that E1B 19kDa, like Bcl-2, may confer protection against apoptosis in a variety of circumstances. The sequence similarities between E1B 19kDa and Bcl-2 are largely restricted to the BH1 and BH3 domains. Our sequence alignment analysis shows that E1B 19kDa also contains a BH4 region overlapping its BH3 domain. We find that the BH1 domain of E1B 19kDa contributes to its anti-apoptotic function and its ability to interact with Bax. A conserved residue Gly 87 in the BH1 domain of E1B 19kDa that is critical for Bcl-2 and Bax interaction is also important for the E1B 19kDa and Bax interaction. We do not detect an interaction between E1B 19kDa and Bad, suggesting that E1B 19kDa is a structural and functional homolog of Bcl-2 but also has its unique features. Bap31, a 28kDa polytoptic integral protein of the endoplasmic reticulum, is part of an ER protein complex that also includes Bcl-2/Bcl-xL, procaspase-8, and a CED-4-like protein. E1B 19kDa does not interact with Bap31; nor does it interact with Bcl-2/Bcl-xL. We have made a set of chimeric E1B 19kDa proteins carrying different BH domains from Bcl-2 to test their protein-protein interaction properties. We show that the BH3 domain of Bcl-2, when substituted for the homologous region of E1B 19kDa, confers the properties of interaction with Bap31 and Bcl-xL onto 19K. The subcellular distribution and antiapoptotic function of the 19K/Bcl-2BH3 is very similar to that of wild type 19K. These results show that the BH3 domain of Bcl-2/Bcl-xL has an important role in mediating Bcl-2/Bcl-xL and Bap31 interaction.