Trim39 regulates neuronal apoptosis by acting as a SUMO-targeted E3 ubiquitin-ligase for the transcription factor NFATc3

NFATc3 is the predominant member of the NFAT family of transcription factors in neurons, where it plays a pro-apoptotic role. Mechanisms controlling NFAT protein stability are poorly understood. Here we identify Trim39 as an E3 ubiquitin-ligase of NFATc3. Indeed, Trim39 binds and ubiquitinates NFATc3 in vitro and in cells where it reduces NFATc3 protein level and transcriptional activity. In contrast, silencing of endogenous Trim39 decreases NFATc3 ubiquitination and increases its activity, thereby resulting in enhanced neuronal apoptosis. We also show that Trim17 inhibits Trim39-mediated ubiquitination of NFATc3 by reducing both the E3 ubiquitin-ligase activity of Trim39 and the NFATc3/Trim39 interaction. Moreover, we identify Trim39 as a new SUMO-targeted E3 ubiquitin-ligase (STUbL). Indeed, mutation of SUMOylation sites in NFATc3 or SUMO-interacting motifs in Trim39 reduces NFATc3/Trim39 interaction and Trim39-induced ubiquitination of NFATc3. In addition, Trim39 preferentially ubiquitinates SUMOylated forms of NFATc3 in vitro. As a consequence, a SUMOylation-deficient mutant of NFATc3 exhibits increased stability and pro-apoptotic activity in neurons. Taken together, these data indicate that Trim39 modulates neuronal apoptosis by acting as a STUbL for NFATc3. Trim39 acts as a SUMO-targeted E3 ubiquitin-ligase for the transcription factor NFATc3. Indeed, Trim39 preferentially binds and ubiquitinates the SUMOylated forms of NFATc3 by its SUMO interacting motifs (SIM), mostly its so-called SIM3. In the absence of Trim17 (black arrows), Trim39 ubiquitinates SUMOylated NFATc3, possibly on its SUMO chains or other sites. This induces the proteasomal degradation of NFATc3 and reduces the expression of its target genes, thereby promoting neuronal survival. In the presence of Trim17 (red arrows), Trim39-mediated ubiquitination and degradation of NFATc3 is inhibited, which stabilizes NFATc3 and increases the expression of its target genes, including Trim17 itself. This creates a positive feedback loop that favors neuronal apoptosis.