Ion Channel Activity of the BH3 Only Bcl-2 Family Member, BID

Abstract

BID is a member of the BH3-only subgroup of Bcl-2 family proteins that displays pro-apoptotic activity. The NH 2 -terminal region of BID contains a caspase-8 (Casp-8) cleavage site and the cleaved form of BID translocates to mitochondrial membranes where it is a potent inducer of cytochrome c release. Secondary structure and fold predictions suggest that BID has a high degree of ␣-helical content and structural similarity to Bcl-X L , which itself is highly similar to bacterial pore-forming toxins. Moreover, circular dichroism analysis confirmed a high ␣-helical content of BID. Amino-terminal truncated BID⌬1-55, mimicking the Casp-8-cleaved molecule, formed channels in planar bilayers at neutral pH and in liposomes at acidic pH. In contrast, full-length BID displayed channel activity only at nonphysiological pH 4.0 (but not at neutral pH) in planar bilayers and failed to form channels in liposomes even under acidic conditions. On a single channel level, BID⌬1-55 channels were voltage-gated and exhibited multiconductance behavior at neutral pH. When full-length BID was cleaved by Casp-8, it too demonstrated channel activity similar to that seen with BID⌬1-55. Thus, BID appears to share structural and functional similarity with other Bcl-2 family proteins known to have channel-forming activity, but its activity exhibits a novel form of activation: proteolytic cleavage.

Is MAC the knife that cuts cytochrome c from mitochondria during apoptosis?

“…Recombinant Bcl-2 can form channels in planar bilayers. 45,48 In contrast, no new channel activities are detected when Bcl-2 is overexpressed in FL5.12 or MDA-231 cells, suggesting that this protein does not form channels in native mitochondrial membranes. 20,51 However, channels whose conductance is between 0.75 and 1 nS are detected in isolated mitochondria after addition of caspase-cleaved recombinant Bcl-x L (DN-Bcl-x L ).…”

“…50 Unlike Bax and Bak, antiapoptotic Bcl-2 family proteins and t-Bid only form large-conductance channels when they are assayed or pre-inserted at low pH (Table 2). 45,48 Importantly, no channel activity has been detected that can be attributed to Bcl-2 in mitochondria of cells overexpressing this protein. 20,51 These observations raised questions concerning the physiological relevance of channels formed by the antiapoptotic and 'BH3 domain-only' members of the Bcl-2 family.…”

“…The principal results of these investigations are presented in Table 2. [45][46][47][48][49] Surprisingly, both antiapoptotic and proapoptotic proteins have channel-forming activity in artificial lipid membranes. Typically, the channels formed are slightly cation-selective and voltage independent.…”

Mitochondria as therapeutic targets for cancer chemotherapy

“…Nevertheless, it has also been suggested that t-Bid forms membrane-inserted homotrimers that do not interact with other mitochondrial proteins (Grinberg et al, 2002). Moreover, t-Bid can form channels in planar lipid bilayers (Schendel et al, 1999) and destabilize them (Kudla et al, 2000). The question of whether t-Bid's effects on mitochondria depend on interactions with sessile mitochondrial proteins, including a CsA target, has been controversial .…”

Single-point mutations of a lysine residue change function of Bax and Bcl-xL expressed in Bax- and Bak-less mouse embryonic fibroblasts: novel insights into the molecular mechanisms of Bax-induced apoptosis

“…[9][10][11][12] . The ability of Bax to form ion-conducting pores in synthetic and mitochondrial membranes suggests that such pores are involved in the release of cytochrome c. [13][14][15] Bax however might work as a twin-arginine translocation (Tat)-like protein rather than as an always-open ion-conducting pore in the OMM.…”