| Neuronal apoptotic paradigms such as trophic factor deprivation (TFD) require de novo transcription in order for death to occur. One critical transcriptional pathway whose activity is substantially elevated during TFD and other apoptotic stresses is the c-Jun N-terminal kinase (JNK) pathway. This activation occurs via PJAC--the POSH-JIP Apoptotic Complex. This complex includes two scaffolds, POSH and JIP, that between them directly bind activated Rac1, MLKs, MKKs and JNKs. POSH (plenty of SH3 domains ) was used as a bait in a yeast two-hybrid screen to identify additional modulators of JNK signaling that might play a role in cell death. The main aim of this thesis is to describe both a novel activator (Siah1) and a novel brake of JNK signaling (Cbl) discovered in that screen. Siah1 (seven in absentia homologue) plays a role in TFD and DNA damage models of neuronal death by driving JNK signaling. This activation occurs via a mutual stabilization mechanism of SIAH1 and PJAC components, is enhanced by tyrosine phosphorylation of SIAH1 on tyr100/126, and requires SIAH1's E3 ligase activity. On the other hand, Cbls (casitas B-lineage lymphoma) negatively regulate JNK signaling in healthy cells by a mechanism that includes c-Cbl interfering with the ability of MLKs to signal to downstream components. This inhibition is released in a variety of neuronal apoptotic paradigms by reduction of Cbl proteins, which at least in part is mediated by protease cleavage. These findings thus reveal how apoptotic JNK signaling is positively and negatively modulated by two POSH-associated proteins of the RING finger family. |
