Model for TR3-mediated conversion from protector to killer. Bcl-2 can assume 2 conformational states, including an antiapoptotic conformation in which its BH3 domain is buried and a proapoptotic state in which its BH3 domain is exposed. The proapoptotic form can either activate proapoptotic proteins Bax and Bak or inactivate antiapoptotic proteins such as Bcl-X_L.