Lysine-specific demethylase 1 (LSD1) has been reported to repress and activate transcription by mediating histone H3K4me1/2 and H3K9me1/2 demethylation, respectively. The molecular mechanism that underlies this dual substrate specificity has remained unknown. Here we report that an isoform of LSD1, LSD1+8a, does not have the intrinsic capability to demethylate H3K4me2. Instead, LSD1+8a mediates H3K9me2 demethylation in collaboration with supervillin (SVIL), a new LSD1+8a interacting protein. LSD1+8a knockdown increases H3K9me2, but not H3K4me2, levels at its target promoters and compromises neuronal differentiation. Importantly, SVIL co-localizes to LSD1+8a-bound promoters, and its knockdown mimics the impact of LSD1+8a loss, supporting SVIL as a cofactor for LSD1+8a in neuronal cells. These findings provide insight into mechanisms by which LSD1 mediates H3K9me demethylation and highlight alternative splicing as a means by which LSD1 acquires selective substrate specificities (H3K9 versus H3K4) to differentially control specific gene expression programs in neurons.

Death domain (DD) superfamily members play a central role in apoptotic and inflammatory signaling through formation of oligomeric molecular scaffolds. These scaffolds promote the activation of proinflammatory and apoptotic initiator caspases, as well as Ser/Thr kinases. Interactions between DDs are facilitated by a conserved set of interaction surfaces, type I, type II, and type III. Recently structural information on a ternary complex containing the DDs of MyD88, IRAK4, and IRAK2 and a binary complex containing Fas and FADD DDs has become available. This review will focus on how the three DD interaction surfaces cooperate to facilitate the assembly of these oligomeric signaling complexes.

SUMMARY Trans -autophosphorylation is among the most prevalent means of protein kinase activation, yet its molecular basis is poorly defined. In Toll-like receptor and interleukin-1 receptor signaling pathways, the kinase IRAK4 is recruited to the membrane proximal adapter MyD88 through death domain (DD) interactions, forming the oligomeric Myddosome and mediating NF-κB activation. Here we show that unphosphorylated IRAK4 dimerizes in solution with a Kd of 2.5 μM and that Myddosome assembly greatly enhances IRAK4 kinase domain (KD) autophosphorylation at sub-Kd concentrations. The crystal structure of the unphosphorylated IRAK4KD dimer captures a conformation that appears to represent the actual trans-autophosphorylation reaction, with the activation loop phosphosite of one IRAK4 monomer precisely positioned for phosphotransfer by its partner. We show dimerization is crucial for IRAK4 autophosphorylation in vitro and ligand-dependent signaling in cells. These studies identify a mechanism for oligomerization-driven allosteric autoactivation of IRAK4 that may be general to other kinases activated by autophosphorylation.