| Summary: | The demonstration that F<sub>1</sub>F<sub>O</sub> (F)-ATP synthase and adenine nucleotide translocase (ANT) can form Ca<sup>2+</sup>-activated, high-conductance channels in the inner membrane of mitochondria from a variety of eukaryotes led to renewed interest in the permeability transition (PT), a permeability increase mediated by the PT pore (PTP). The PT is a Ca<sup>2+</sup>-dependent permeability increase in the inner mitochondrial membrane whose function and underlying molecular mechanisms have challenged scientists for the last 70 years. Although most of our knowledge about the PTP comes from studies in mammals, recent data obtained in other species highlighted substantial differences that could be perhaps attributed to specific features of F-ATP synthase and/or ANT. Strikingly, the anoxia and salt-tolerant brine shrimp <i>Artemia franciscana</i> does not undergo a PT in spite of its ability to take up and store Ca<sup>2+</sup> in mitochondria, and the anoxia-resistant <i>Drosophila melanogaster</i> displays a low-conductance, selective Ca<sup>2+</sup>-induced Ca<sup>2+</sup> release channel rather than a PTP. In mammals, the PT provides a mechanism for the release of cytochrome <i>c</i> and other proapoptotic proteins and mediates various forms of cell death. In this review, we cover the features of the PT (or lack thereof) in mammals, yeast, <i>Drosophila melanogaster</i>, <i>Artemia franciscana</i> and <i>Caenorhabditis elegans</i>, and we discuss the presence of the intrinsic pathway of apoptosis and of other forms of cell death. We hope that this exercise may help elucidate the function(s) of the PT and its possible role in evolution and inspire further tests to define its molecular nature.
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