TI - Activation of apoptosis signal-regulating kinase 1 by the stress-induced activating phosphorylation of pre-formed oligomer . AB - Apoptosis signal-regulating kinase 1 ( ASK1 ) is a MAPKKK family member which activates c-Jun N-terminal kinase ( JNK ) and p38 . In non-stressed cells , ASK1 exists as an inactive complex with the reduced form of thioredoxin . Oxidative stress such as hydrogen peroxide (H2O2) disrupts the ASK1-thioredoxin complex by oxidization of thioredoxin and thereby activates ASK1 . The precise mechanism by which ASK1 is activated after its release from thioredoxin is unknown . Here we show that phosphorylation of Thr845 at the activation loop is essential for ASK1 to be activated by H2O2 . ASK1 appears to form a silent homo-oligomer through its C-terminal coiled-coil region in non-stressed cells . Following H2O2 treatment , pre-existing ASK1 oligomer undergoes conformational change and creates a new interface within an oligomer , which ultimately leads to trans-autophosphorylation of Thr845 . Thus , direct interaction via the coiled-coil region is required for self-scaffolding but not sufficient for activation of ASK1 . Importantly , Thr845 of ASK1 can also be trans-phosphorylated by an unidentified Thr845 kinase in response to H2O2 treatment . We propose that this potential Thr845 kinase may be an ignition kinase that triggers Thr845 phosphorylation in oligomerized and activation-competent forms of ASK1 .