During apoptosis, the pro-apoptotic protein Bax relocalizes from the cytosol to the mitochondrial outer membrane. This relocalization is associated to major conformational changes, namely at the N-and C-terminal ends of the protein. Substitution of residues located at critical positions within the protein potentially stimulates or inhibits this process. In the present study, we investigated the hypothesis that phosphorylation of serine residues might trigger these conformational changes, with a focus on Ser 163 and Ser 184 , which have been shown to be phosphorylatable by protein kinases GSK3␤ and Akt/PKB, respectively, and on Ser 60 , which is located in a consensus target sequence for PKA. Substitutions of these serine residues by alanine or aspartate were done in wild type or previously characterized Bax mutants, and the capacity of the resulting proteins to interact with mitochondria and to release cytochrome c was assayed in yeast, which provides a tool to study the function of Bax, independently of the rest of the apoptotic network. We conclude that sequential phosphorylation of these serine residues might participate in the triggering of the different conformational changes associated with Bax activation during apoptosis.The crucial function of Bcl-2 family proteins in the regulation of apoptosis is now well established (1). Among these proteins, membrane multidomain proteins Bax and Bak are essential for the completion of apoptosis because their double knock-out fully impairs this process (2). Their main function is the permeabilization of the outer mitochondrial membrane, leading to the relocalization of apoptogenic factors, such as cytochrome c, smac/diablo, omi/HtrA2, endonuclease G, and apoptosis-inducing factor (3-5).Bax function in mitochondria permeabilization relies on its relocalization from the cytosol to the outer mitochondrial membrane (6 -9). Membrane-associated Bax oligomerization (10) is further involved in the formation of the mitochondrial apoptosis-induced channel, a putative channel that allows relocating cytochrome c from the intermembrane space to the cytosol (11, 12). Bax may also participate in the opening of the permeability transition pore, leading to the rupture of the outer mitochondrial membrane and to the release of bigger proteins such as apoptosis-inducing factor (13), although the actual participation of this pore to the apoptotic process has been questioned (14). Bigger sized Bax channels might also directly participate in the unselective release of intermembrane space proteins (15).The cascade of events driving the conversion of inactive, cytosolic Bax to mitochondrial, active Bax requires further clarification. It has been suggested that the N-terminal end of Bax controls its localization (16) because removal of the first 19 N-terminal residues of Bax strongly stimulates Bax relocalization and membrane insertion on isolated mitochondria (16, 17) in mammalian cells (17) and after heterologous expression in yeast cells (18). Increasing the mobility of this N-terminal end...

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UTH1 is a yeast aging gene that has been identified on the basis of stress resistance and longer life span of mutants. It was also shown to participate in mitochondrial biogenesis. The absence of Uth1p was found to trigger resistance to autophagy induced by rapamycin. Uth1p is therefore the first mitochondrial protein proven to be required for the autophagic degradation of mitochondria. Since this protein is also involved in yeast cell death induced by heterologous expression of the pro-apoptotic protein Bax, the results are discussed in the light of evidence suggesting a co-regulation of apoptosis and autophagy in mammalian cells.