Genetics of Apoptosis

concentration. This is supported by the NMR structure study where a low octyl
glucoside concentration did not induce any significant changes in the structure.
However, at a concentration of 0.6 %, Bax structure changed dramatically, indicating
aggregation or oligomer formation (Suzuki et al., 2000). A study by Saito et al. (2000)
showed that Bax oligomers were able to release cytochrome c from liposomes. The
structure was estimated to be Bax tetramers. Channels formed by oligomeric Bax have
multiconductance levels, ranging from a few pS up to 3–4 nS. They are pH-sensitive,
slightly cation-selective, and Ca2+-insensitive (Antonsson et al., 1997; Schlesinger et
al., 1997). In a recent study, we tried to simulate Bax activation as it appears in Fas-
induced apoptosis in type 2 cells. In this pathway, caspase-8 cleaves Bid into a C-
terminal (tcBid) fragment and a N-terminal (tnBid) fragment. However, after
cleavage, the two fragments stay together in solution (cut Bid) (Kudla et al., 2000).
Monomeric Bax was incubated with cut Bid in the presence of liposomes (Roucou
et al., 2002). As expected, monomeric Bax alone was unable to trigger
carboxyfluorescein (CF) release from the liposomes; however, after co-addition of cut
Bid, Bax permeabilized the liposomes to CF. Surprisingly, the quaternary structure
of Bax activated by cut Bid was found to be monomeric; no oligomeric Bax was
detected, although the protein showed channel-forming activity. Further studies
revealed that, although oligomeric Bax also permeabilized the liposomes to
cytochrome c, the cut Bid-induced Bax channels were impermeable to cytochrome
c. Electrophysiologic comparison of the channel activities showed that the cut Bid-
induced Bax channel-forming activity was different from that of oligomeric Bax. Cut
Bid-induced Bax channels showed low conductance levels only and were highly cation
selectivity. Although the physiologic relevance of these new Bax channels remains
unclear, these results indicate that Bax and possibly other multidomain proteins might
form channels with different properties. It also shows that cut Bid alone in the
presence of lipid membranes is not sufficient to induce formation of Bax oligomers.
When monomeric Bax is incubated with cut Bid in the presence of mitochondria,
Bax oligomers are formed; thus, it appears that additional factors, presumably from
the mitochondria, are required to trigger the formation of large-conductance Bax
channels (Eskes et al., 2000). Furthermore, one study indicated that Bax can
destabilize lipid membranes through reducing the linear tension without forming ion
channels (Basanez et al., 1999). A membrane destabilizing activity was also detected
with the C-terminal fragment of the ‘BH3-domain-only’ protein Bid (tcBid) (Kudla
et al., 2000).

6.

Activation of the multidomain Bcl-2 proteins

Regulation of the multidomain protein activity appears to occur mainly on the post-
translational level, although Bax levels have been reported to change during apoptosis
(Krajewski et al., 1995b; Ekegren et al., 1999). The Bax promotor has been shown
to be transcriptionally activated by the tumor suppressor protein p53 (Miyashita and
Reed, 1995). Interestingly, a large number of human cancers have mutations in the

128 GENETICS OF APOPTOSIS