The VDAC1 N-terminus is essential both for apoptosis and the protective effect of anti-apoptotic proteins.

The release of mitochondrial-intermembrane-space pro-apoptotic proteins, such as cytochrome c, is a key step in initiating apoptosis. Our study addresses two major questions in apoptosis: how are mitochondrial pro-apoptotic proteins released and how is this process regulated? Accumulating evidence indicates that the voltage-dependent anion channel (VDAC) plays a central role in mitochondria-mediated apoptosis. Here, we demonstrate that the N-terminal domain of VDAC1 controls the release of cytochrome c, apoptosis and the regulation of apoptosis by anti-apoptotic proteins such as hexokinase and Bcl2. Cells expressing N-terminal truncated VDAC1 do not release cytochrome c and are resistant to apoptosis, induced by various stimuli. Employing a variety of experimental approaches, we show that hexokinase and Bcl2 confer protection against apoptosis through interaction with the VDAC1 N-terminal region. We also demonstrate that apoptosis induction is associated with VDAC oligomerization. These results show VDAC1 to be a component of the apoptosis machinery and offer new insight into the mechanism of cytochrome c release and how anti-apoptotic proteins regulate apoptosis and promote tumor cell survival.

Voltage-dependent anion channel 1-based peptides interact with hexokinase to prevent its anti-apoptotic activity.

In brain and tumor cells, the hexokinase isoforms, HK-I and HK-II, bind to the voltage-dependent anion channel (VDAC) in the outer mitochondrial membrane. The VDAC domains interacting with these anti-apoptotic proteins were recently defined using site-directed mutagenesis. Now, we demonstrate that synthetic peptides corresponding to the VDAC1 N-terminal region and selected sequences bound specifically, in a concentration- and time-dependent manner, to immobilized HK-I, as revealed by real time surface plasmon resonance technology. The same VDAC1-based peptides also detached HK bound to brain or tumor-derived mitochondria. Moreover, expression of the VDAC1-based peptides in cells overexpressing HK-I or HK-II prevented HK-mediated protection against staurosporine-induced release of cytochrome c and subsequent cell death. One loop-shaped VDAC1-based peptide corresponding to a selected sequence and fused to a cell-penetrating peptide entered the cell and prevented the anti-apoptotic effects of HK-I and HK-II. This peptide detached mitochondrial-bound HK better than did the same peptide in its linear form. Both cell-expressed and exogenously added cell-penetrating peptide detached mitochondrial-bound HK-I-GFP. These results point to HK-I and HK-II as promoting tumor cell survival through binding to VDAC1, thereby inhibiting cytochrome c release and apoptotic cell death. Moreover, VDAC1-based peptides interfering with HK-mediated anti-apoptotic activity may potentiate the efficacy of conventional chemotherapeutic agents.

A photoactivable probe for calcium binding proteins.

Ca2+ as a signaling molecule carries information pivotal to cell life and death via its reversible interaction with a specific site in a protein. Although numerous Ca2+-dependent activities are known, the proteins responsible for some of these activities remain unidentified. We synthesized and characterized a photoreactive reagent, azido ruthenium (AzRu), which interacts specifically with Ca2+ binding proteins and strongly inhibits their Ca2+-dependent activities, regardless of their catalytic mechanisms or functional state as purified proteins, embedded in the membrane or in intact cells. As expected from a Ca2+ binding protein-specific reagent, AzRu had no effect on Ca2+-independent and Mg2+-dependent activities. Az103Ru covalently bound, and specifically labeled, known Ca2+ binding proteins. AzRu is a photoreactive reagent that provides an approach for identification of Ca2+ binding proteins, characterization of their binding sites, and exploration of new Ca2+-dependent processes.