A kinase-independent function of Ask1 in caspase-independent cell death

Ask1 ((a) under bar poptosis (s) under bar ignal-regulating (k) under bar i nase (1) under bar) is activated as a consequence of cell exposure to a var iety of stresses and can then initiate apoptosis. A known pathway of apopto sis downstream of Ask1 involves the activation of the stress-activated prot ein kinases, the release of cytochrome c from mitochondria, the activation of caspases, and the fragmentation of nuclei. Here, we characterized a nove l mechanism of Ask1-mediated cell killing that is triggered by the interact ion with Daxx. Co-transfection of Ask1 and Daxx induced a caspase-independe nt cell-death process characterized at the morphological level by distincti ve crumpled nuclei easily distinguishable from the condensed and fragmented nuclei seen during classical caspase-dependent apoptosis. The kinase activ ity of Ask1 was not involved in this process, because mutants lacking kinas e activity were as efficient as wild type Ask1 in mediating Daxx-induced ce ll death. Ask1N, a deletant that lacks the C-terminal half including the ki nase domain of Ask1, was constitutively active in producing crumpled nuclei . In contrast, Ask1 DeltaN, the reciprocal deletant that possesses constitu tive kinase activity, produced fragmented nuclei typical of caspase-depende nt death processes. We conclude that in addition to a caspase-dependent pro apoptotic function that depends on its kinase activity, Ask1 possesses a ca spase-independent killing function that is independent on its kinase activi ty and is activable by interaction with Daxx. In the physiological situatio n, such an activity is induced as a consequence of the translocation of Dax x from the nucleus to the cytoplasm, a condition that occurs following acti vation of the death receptor Fas.