in Bcl-2). Additionally, these residues form hydrogen bonds at the NH 2 -terminal end
of α5, from the loop joining α4 and α5 (Arg^136 : Asp^133 , Gly134, Asp^136 in Bcl-xL)
or intrahelical bonds (Gly^138 : Ala^142 in Bcl-xL; Gly^145 :A la^149 in Bcl-2). Thus, an
alternative explanation for the loss of survival function with these mutations may be
destabilizing effects on the BH tertiary structure, especially in hydrophobic lipid
environments. Of possible relevance to this interpretation, Asoh et al. observed a gain
of function phenotype for three Bcl-xL mutations (Tyr^22 Phe, Gln^26 Asn, and
Arg^165 Lys) that were predicted to weaken hydrogen bonding to the distal portion of
the α5-α6 hairpin (Asoh et al., 2000).
A recent development in this field is the identification of small molecule inhibitors
of Bcl-xL/Bcl-2 that act by binding to the hydrophobic cleft. There are interesting
differences in the activities of these compounds, at the cellular level as well as with
purified protein (Table 5). These properties correspond to the screening strategy used
to identify each compound. Assays based on competitive binding with proapoptotic
Table 4. Mutational studies of Bcl-xL
Abbrev. wLOF weak Loss of Function.
MAKING SENSE OF THE BCL-2 FAMILY OF APOPTOSIS REGULATORS 61