Lethal mitochondrial membrane permeabilization has been depicted as the result of two fundamentally distinct processes, namely primary mitochondrial outer membrane permeabilization (MOMP) versus permeability transition (PT) ignited at the level of the mitochondrial inner membrane. MOMP and PT have been connected to apoptosis and necrosis, respectively. Moreover, it has been thought that MOMP was mediated by pro-apoptotic multidomain proteins of the Bcl-2 family (Bax and Bak), which would operate near-to-independently from the permeability transition pore complex (PTPC) composed by voltage-dependent anion channel (VDAC), adenine nucleotide translocase (ANT) and cyclophilin D. A recent paper in Molecular and Cellular Biology now reveals the obligate contribution of one particular ANT isoform to the execution of developmental and homeostatic cell death in Caenorhabditis elegans. The physical and functional interaction between CED-9, the sole multidomain Bcl-2 protein of C. elegans, and ANT emphasizes the existence of an intricate, phylogenetically conserved crosstalk between Bcl-2 family proteins and constituents of the PTPC. In this issue of Cell Death and Differentiation, Malorni et al. further corroborate this notion by showing that type 2 transglutaminase (TG2) is essential for the correct assembly/function of ANT1, and that, at least in some experimental settings, TG2 might be required to enable and/or stabilize the pro-apoptotic association of Bax with ANT1. The 'core' machinery of apoptosis was discovered by screening Caenorhabditis elegans mutations that suppressed developmental cell death, yielding a number of genes whose mammalian equivalents also have an important function in physiological and disease-associated apoptosis. Once it has been transcriptionally upregulated, EGL-1 (the only BH3-only protein encoded by C. elegans) disrupts a molecular complex composed by CED-9 (a Bcl-2 ortholog localized in mitochondrial membranes) and CED-4 (the nematode counterpart of Apaf-1, which is also normally present at mitochondria). As a consequence, CED-4 becomes free to translocate to the nuclear envelope and to activate CED-3 (the worm caspase), thereby inducing apoptosis. 1,2 Now, a new protein, WAN-1, the nematode ortholog of mammalian adenine nucleotide translocase (ANT) has been added to the 'core' machinery. 3
The 'Core' Machinery for Cell Death ExecutionThe scenario of a conserved cell death 'core' machinery composed by EGL-1, CED-9, CED-4 and CED-3 has been dominating the field of cell death research up to the point that some investigators suggested the existence of a ternary molecular complex made by Apaf-1, caspase-9 and the Bcl-2 homolog Bcl-X L also in mammalian cells, 4,5 which turned out to be a bold artifact. [6][7][8] Similarly, the idea that EGL-1, CED-9, CED-4 and CED-3 might have other functions than mediating cell death (EGL-1 as an inducer of autophagy, 9 CED-4 as part of the intra-S-phase DNA damage checkpoint, 2 CED-3 in the innate immune system 10 ) met incredulity and resistance. Prominent ...