Genetics of Apoptosis

8.4

Bax-ANT channels

Interactions between Bax and the adenine nucleotide translocator (ANT), a protein
of the inner mitochondrial membrane and part of the PTP complex, have also been
demonstrated. Bax coimmunoprecipitates with ANT, and the two proteins were
shown to interact in a yeast two-hybrid system. Furthermore, Bax was shown to
induce cell death when expressed in wild-type yeast, but not in an ANT-deficient
yeast strain (Marzo et al., 1998a). Apart from its translocator activity, ANT can form
channels in lipid membranes (Brustovetsky and Klingenberg, 1994; Ruck et al.,
1998). Bax has been shown to enhance the ANT channel activity in artificial
membranes. The channel activity was stabilized in the presence of the ANT inhibitor
atractyloside and was completely inhibited by the natural physiologic ANT ligand,
ATP, or by Bcl-2 (Brenner et al., 2000). The physiologic importance of the
stimulating effect by Bax on the ANT channel remains unclear, since a non-
physiologic inhibitor appears to be required, and the activity is completely inhibited
at physiologic ATP concentrations. However, it is possible that Bax could interact
with ANT in the inner membrane at the contact sites, and these interactions could
modulate the activity of a Bax channel in the outer membrane.

8.5

The permeability transition pore

Proteins, including cytochrome c, can be released from the intermembrane space after
opening of the PTP (Marzo et al., 1998b). PTP opening results in matrix swelling,
leading to rupture of the outer mitochondrial membrane. The main arguments for
the involvement of the PTP in Bax-induced apoptosis are early observations of a
decrease in the transmembrane potential, and the finding that cytochrome c release
can be inhibited by cyclosporin A, a PTP inhibitor (Halestrap et al., 1997; Narita et
al., 1998). However, it now appears clear that, at least in most forms of apoptosis,
the mitochondrial structures remain intact. These would suggest that activation of
the PTP is not involved. However, the intracellular ATP concentration has been
shown to influence whether a cell dies by apoptosis or necrosis (Nicotera et al., 1998).

MITOCHONDRIA IN APOPTOSIS INDUCTION 137

It is therefore possible that cells starting to die by the apoptotic pathway could switch

to necrotic death, thus activating the PTP.

It appears that the multidomain proteins Bax and Bak form channels in the outer

mitochondrial membrane independently of the PTP or its components. The

multidomain proteins permeabilize the outer membrane, but the mitochondrial

structural integrity remains intact. However, it cannot be excluded that under certain

conditions the proteins might work in synergy with PTP or its components.

9.

Activities of the mitochondrial proteins released

Cytochrome c is a component of the respiratory electron transfer chain, normally