THE E1B 19K PROTEIN BLOCKS APOPTOSIS BY INTERACTING WITH AND INHIBITING THE P53-INDUCIBLE AND DEATH-PROMOTING BAX PROTEIN

The E1B 19K protein is a potent apoptosis inhibitor and the putative a denovirus Bcl-2 homolog. To investigate the mechanism of apoptosis reg ulation, 19K-interacting cellular proteins were identified using the y east two-hybrid system, and Bar was one of seven 19K-interacting clone s. Residues 50-78 of Bax containing a conserved region designated Bcl- 2 homology region 3 (BH3) were sufficient for specific binding to both the E1B 19K and Bcl-2 proteins. The Bax-E1B 19K interaction was detec table in vitro and in lysates from mammalian cells, and Bar expression antagonized E1B 19K protein function. box mRNA and protein levels wer e p53-inducible with kinetics identical to that of p21/Waf-1/Cip-1, an d E1B 19K and Bcl-2 expression did not affect Bax or p21/Waf-1/Cip-1 a ccumulation. In cells where p53 was mutant, Bar expression induced apo ptosis, suggesting that Bar was sufficient for apoptosis, and acted do wnstream of p53. p53 may simultaneously activate the transcription of genes required for both growth arrest (p21/Waf-1/Cip-1) and death (box ), and E1B 19K and Bcl-2 may act distally and function through interac tion with and antagonism of Bar to prevent apoptosis. With the death p athway disabled, induction of growth arrest by p53 can then be manifes ted.