BH3 peptides (Degterev et al., 2001), or using docking simulations for Bcl-2 protein
structures (modeled on the BH3 peptide: Bcl-xL complex) (Wang, J.L. et al., 2000;
Enyedy et al., 2001), led to the identification of inhibitors of BH3 peptide: protein
association, and in the case of the BH3Is, intracellular heterodimerization with Bax.
The relative cytotoxicities of a panel of BH3Is are consistent with the above
mechanism, and, as predicted, cell killing with these compounds is inhibited by
increased expression of the Bcl-xL target protein (Degterev et al., 2001). In contrast,
the activity of 2-methoxy antimycin A on Bcl-XL was revealed by a cell screen for
selective cytotoxicity based on Bcl-xL expression, demonstrating a direct relationship
between AA cytotoxicity and Bcl-xL expression (Tzung et al., 2001). Although this
compound is shown to bind to Bcl-xL alone, with displacement by BH3 peptide, it
appears to have little or no ability to displace the BH3 peptide from a peptide-Bcl-xL
complex (D.H., unpublished results). Differences between the BH3Is and 2-methoxy
antimycin A also appear in their effects on Bcl-xL pore formation, as 2-methoxy AA
inhibits pore formation and the BH3Is are ineffective in these assays.
In summary, studies with these inhibitors suggest that occupancy of the Bcl-xL
hydrophobic groove can inhibit structural rearrangement to form a membrane pore,
as well as dimerization with proapoptotic partners. Detailed mapping of the ligand-
binding sites may lead to new insights into the membrane topology and function of
these proteins as well as refinement of strategies to exploit these proteins as drug
targets.
2.10
Bcl-2 and Bax: identical twins?It is surprising that proapoptotic and antiapoptotic BH proteins have highly similar
three-dimensional structures. For the limited number of structures available (Bcl-xL,
Bcl-2, Bax, and Bid), it is not possible to predict function from the structure alone
(Muchmore et al., 1996; Chou et al., 1999; McDonnell et al., 1999; Suzuki et al.,
2000; Petros et al., 2001). In one attempt to identify distinguishing features of pro-
and antiapoptotic halves of this family, Korsmeyer proposed that packing of the
amphipathic second α-helical domain from the NH 2 -terminus constituted a critical
determinant (McDonnell et al., 1999). This domain contains the BH3 sequence that
is conserved among all BH proteins. In this model, proapoptotic proteins have an
Table 5. Properties of Bcl-xL inhibitors62 GENETICS OF APOPTOSIS