Cytochrome c: Can't Live with It—Can't Live without It

quence of PT pore opening. Every student of biochemistry is taught the importance A second model predicts a specific channel located of cytochrome c for oxidative phosphorylation in mitoin the outer membrane that allows release of cytochondria, where it assists with production of lifechrome c. Moreover, a good candidate was identified sustaining ATP by participating in electron transport. in Bax, a pro-apoptotic member of the Bcl-2 family. Apoptosis researchers, however, are poised to rewrite The three-dimensional structure of Bcl-XL suggests that the textbooks, painting a more nefarious picture of cytomany Bcl-2 family proteins share similarity with the porechrome c. The first indication that cytochrome c could forming domains of certain bacterial toxins, including be bad for cells came when Xiaodong Wang and his diphtheria toxin (DT). The purported function of the porecolleagues set out to purify factors from the cytosol that forming domain of DT is to allow the ADP-ribosylating induced proteolytic processing and activation of the cell subunit of the toxin to transit from the interior of lysodeath protease, caspase-3. One of the required factors somes into the cytosol. By analogy, it is conceivable surprisingly proved to be cytochrome c (Liu et al., 1996). that some Bcl-2 family proteins could form channels Subsequent investigations using both subcellular fraclarge enough to transport even proteins, despite their tionation methods and confocal immunofluorescence a-helical character, which is generally more indicative microscopy demonstrated that following exposure of of smaller ion channels. Addition of recombinant Bax cells to apoptotic stimuli, cytochrome c is rapidly reprotein to synthetic membranes results in channels that, leased from mitochondria into the cytosol (Kluck et al., while generally having small conductances (z30 pS to 1997; Yang et al., 1997). Moreover, the anti-apoptotic 280 pS), may under some circumstances become quite proteins Bcl-2 and Bcl-XL, which reside in the outer large (z2 nS) (Antonsson et al., 1997; Schlesinger et al., mitochondrial and some other intracellular membranes, 1997). Moreover, when expressed in yeast, Bax induces prevent release of cytochrome c and activation of cascytochrome c release from mitochondria and cell death pase-3, while protecting cells from death. However, two (Manon et al., 1997), supporting the idea of a direct role questions remained: (1) How does cytochrome c get for this proapoptotic protein in controlling cytochrome out of mitochondria?; and (2) How does cytochrome c transport. c activate caspase-3 once released? Recent reports, Reports that cytochrome c release from mitochondria including one in this issue of Cell by Vander Heiden et can precede dissipation of the voltage gradient (DC) al. (1997) and another in the previous issue of Cell by across the inner membrane argue in favor of the specific Li et al. (1997), make strides toward answering these channel hypothesis, suggesting that escape of cytoquestions. chrome c from mitochondria occurs prior to PT pore Cytochrome c is encoded in the nucleus as apo-cytoopening (Kluck et al., 1997; Yang et al., 1997). Moreover, chrome c, and undergoes transport across the outer the finding that many caspases directly induce mitomembrane of mitochondria, where it combines with chondrial PT (Susin et al., 1997) suggests that the seheme to become the mature protein. Cytochrome c requence of events might entail release of cytochrome c, sides in the space between the outer and inner memfollowed by activation of caspases, and then caspasebranes of mitochondria, where it snuggles up to the induced proteolytic events that trigger PT. cytochrome c oxidase complex located in the inner In this issue, Vander Heiden et al. (1997) offer a third membrane. How apo-cytochrome c crosses the outer hypothesis, suggesting that mitochondrial swelling and membrane of mitochondria is enigmatic, thus providing rupture of the outer membrane explain how cytochrome no clues as to how holo-cytochrome c might escape. c is released, but without the dissipation of DC usually At least two competing theories have been advanced. associated with PT pore opening. The evidence proFirst, a strong correlation has been established between vided includes electron micrographs that areparticularly the phenomenon of mitochondrial permeability transiimpressive but raises the question of why no one has tion (PT) and apoptosis (Petit et al., 1996). Mitochondrial seen this before. In the classical descriptions of apoPT is caused by the opening of a large conductance, ptosis by Kerr, Wyllie, and Currie, the ultrastructure of cyclosporin-inhibited channel (z1.5 nS) in the inner mitochondria reportedly remained largely intact. Does membrane of mitochondria, causing dissipation of the this imply that not all apoptotic cell deaths involve mitoH gradient and osmotic swelling due to the high solute chondrial swelling and outer membrane rupture? Equally concentration of the mitochondrial matrix. The PT pore puzzling is the question of whydoes the organelle swell? remains poorly characterized but appears to consist Swelling of mitochondria implies a dysregulation of osof several proteins located in both the inner and outer motic homeostasis caused by inner membrane permemitochondrial membranes that collaborate with each ability alternations, since the channel formed by the other at the contact sites where these two membranes outer membrane protein porin (a.k.a. VDAC) has an esti-