Akt signaling plays a central role in many biological functions, such as cell proliferation and apoptosis. Since Akt resides primarily in the cytosol, it is not known how these signaling molecules are recruited to the plasma membrane and subsequently activated by growth factor stimuli. Here, we found that the protein kinase Akt undergoes lysine 63 chain ubiquitination, which is important for Akt membrane localization and phosphorylation. TRAF6 was found to be a direct E3 ligase for Akt and was essential for Akt ubiquitination, membrane recruitment, and phosphorylation upon growth-factor stimulation. The human cancer-associated Akt mutant (E17K) displayed an increase in Akt ubiquitination, in turn contributing to the enhancement of Akt membrane localization and phosphorylation. Thus, Akt ubiquitination is an important step for oncogenic Akt activation.

TRAF6, a crucial adaptor molecule in innate and adaptive immunity, contains three distinct functional domains. The C-terminal TRAF domain facilitates oligomerization and sequence-specific interaction with receptors or other adaptor proteins. In conjunction with the dimeric E2 enzyme Ubc13-Uev1A, the N-terminal RING domain of TRAF6 functions as an E3 ubiquitin (Ub) ligase that facilitates its own site-specific ubiquitination through the generation of a Lys-63-linked poly-Ub chain. This modification does not cause its proteasomal degradation but rather serves as a scaffold to activate both the IKK and stress kinase pathways. Connecting the N-and C-terminal regions, the four internal zinc finger (ZF) motifs have yet to be functionally defined. In this study, we examined the role of the ZF domains in interleukin-1, lipopolysaccharide, and RANKL signaling by reconstitution of TRAF6-deficient cells with point mutations or deletions of these ZF motifs. Although ZF domains 2-4 are dispensable for activating IKK, p38, and JNK by interleukin-1 and lipopolysaccharide, the first ZF domain together with an intact RING domain of TRAF6 is essential for activating these pathways. Furthermore, TRAF6 autoubiquitination and its interaction with Ubc13 are dependent on ZF1 and an intact RING domain. Additionally, expression of TRAF6 lacking ZF2-4 in TRAF6-deficient monocytes rescues RANKL-mediated osteoclast differentiation and LPS-stimulated interleukin-6 production. These data provide evidence for the critical role of the Ub ligase activity of TRAF6, which is coordinated via the RING domain and ZF1 to supply the necessary elements in signaling by cytokines dependent upon TRAF6.TRAF6, a member of the tumor necrosis factor receptorassociated factor (TRAF) 2 family, is a crucial docking molecule that mediates signaling events initiated by cytokines of the tumor necrosis factor superfamily, interleukin-1 (IL-1) family, and pathogen-associated microbial patterns that are recognized by the Toll-like receptor family (1, 2). Following binding to their respective receptors, these ligands induce a cascade of signaling events leading to the activation of transcription factors, such as the nuclear factor-B (NF-B) and AP1 (activator protein-1) family, through activation of upstream kinases, including inhibitor of -B kinase (IKK) and the mitogen-activated protein kinase (MAPK) family (i.e. p38, JNK, and extracellular signal-regulated kinase) (1, 2). As a result, these ligands induce numerous genes involved in the innate immune and inflammatory responses. Generation of TRAF6-deficient mice revealed that in addition to playing a critical role in the innate and adaptive immunity, TRAF6 has a crucial role in a wide range of biological functions, such as lymph node organogenesis, formation of skin appendices, nervous system development, and bone metabolism (3-9). TRAF6 contains three major domains: 1) the C-terminal domain, which facilitates oligomerization and interaction with receptors and other adaptor proteins in a sequence-specific manner; 2) t...

Previous studies of the conditional ablation of TGF-β activated kinase 1 (TAK1) in mice indicate that TAK1 has an obligatory role in the survival and/or development of hematopoietic stem cells, B cells, T cells, hepatocytes, intestinal epithelial cells, keratinocytes, and various tissues, primarily because of these cells’ increased apoptotic sensitivity, and have implicated TAK1 as a critical regulator of the NF-κB and stress kinase pathways and thus a key intermediary in cellular survival. Contrary to this understanding of TAK1’s role, we report a mouse model in which TAK1 deletion in the myeloid compartment that evoked a clonal myelomonocytic cell expansion, splenomegaly, multi-organ infiltration, genomic instability, and aggressive, fatal myelomonocytic leukemia. Unlike in previous reports, simultaneous deletion of TNF receptor 1 (TNFR1) failed to rescue this severe phenotype. We found that the features of the disease in our mouse model resemble those of human chronic myelomonocytic leukemia (CMML) in its transformation to acute myeloid leukemia (AML). Consequently, we found TAK1 deletion in 13 of 30 AML patients (43%), thus providing direct genetic evidence of TAK1’s role in leukemogenesis.