The Toll-interleukin-1 receptor (TIR) 1 superfamily, a large family of proteins defined by the presence of an intracellular TIR domain, plays crucial roles in the immune response. This superfamily can be divided into two main subgroups, based on the extracellular domains: the immunoglobulin (Ig) domaincontaining receptors (1), and the leucine-rich repeat motifcontaining receptors (2). The Ig domain subgroup includes IL-1R1, IL-18 receptor, T1/ST2, and SIGIRR. IL-1 has been demonstrated to be a key player in the immune response and inflammatory response at both local and systemic levels by activating gene expression of such genes as MIP-2, KC, and C-reactive protein. IL-18 plays important roles in promoting Th1 cell differentiation and natural killer cell activation. T1/ ST2 (3) and SIGIRR (4), also known as TIR8 (5), have been shown to function as negative regulators for Toll-IL-1R-mediated signaling. The leucine-rich repeat motif subgroup consists of at least 11 Toll-like receptors (TLRs) (2, 6 -10). These receptors have received intense attention because different TLRs were found to be activated by specific pathogen products (8,(11)(12)(13)(14)(15)(16).Due to the similarity in their intracellular domain, the Toll-IL-1 receptors employ related yet distinct signaling components and downstream pathways. Genetic and biochemical studies revealed that IL-1R mediates a very complex pathway involving a cascade of kinases organized by multiple adapter molecules into signaling complexes, leading to activation of the transcription factors NF-B, ATF, and AP-1 (17-19). Based on published studies (20 -23), a model of the IL-1 pathway is postulated. Upon IL-1 stimulation, adapter molecule MyD88 (24) is first recruited to the IL-1 receptor, followed by the recruitment of two serine-threonine kinases, IRAK4 (25,26) and IRAK (27,28), and the adapter TRAF6 (29), resulting in the formation of the receptor complex (Complex I). During the formation of Complex I, IRAK and IRAK4 are activated, leading to the hyperphosphorylation of IRAK. Pellino 1⅐IRAK4⅐IRAK⅐TRAF6 complex is then formed, releasing these signaling molecules from the receptor (20). The released components interact with the membrane bound pre-associated TAK1⅐TAB1⅐TAB2⅐TAB3 (21, 23), resulting in the formation of Complex II (IRAK⅐TRAF6⅐TAK1⅐TAB1⅐TAB2⅐TAB3), followed by the translocation of TRAF6⅐TAK1⅐TAB1⅐TAB2⅐TAB3 (Complex III) from the membrane to the cytosol. The translocated Complex III interacts with additional factors in the cytosol, leading to TAK1 activation. It has been implicated that TRAF6 functions as part of a unique E3 complex, mediating TAK1 activation through nonclassical ubiquitination catalyzed by the ubiquitination proteins Ubc13 and Uev1A (30, 31). Once activated, TAK1 can directly phosphorylate IKK␤ and mitogenactivated protein kinase kinase 6, leading to the activation of both the JNK and NF-B signaling pathways (32-35). In addition to TAK1, mitogen-activated protein kinase/extracellular signal-regulated kinase kinase kinase 1 and 3 have also been i...

TNF receptor (TNFR) superfamily members, CD40, and BAFFR play critical roles in B cell survival and differentiation. Genetic deficiency in a novel adaptor molecule, Act1, for CD40 and BAFF results in a dramatic increase in peripheral B cells, which culminates in lymphadenopathy and splenomegaly, hypergammaglobulinemia, and autoantibodies. While the B cell-specific Act1 knockout mice displayed a similar phenotype with less severity, the pathology of the Act1-deficient mice was mostly blocked in CD40-Act1 and BAFF-Act1 double knockout mice. CD40- and BAFF-mediated survival is significantly increased in Act1-deficent B cells, with stronger IkappaB phosphorylation, processing of NF-kappaB2 (p100/p52), and activation of JNK, ERK, and p38 pathways, indicating that Act1 negatively regulates CD40- and BAFF-mediated signaling events. These findings demonstrate that Act1 plays an important role in the homeostasis of B cells by attenuating CD40 and BAFFR signaling.

The IL-1 2 /Toll receptors play essential roles in inflammation and innate immunity. The defining feature of members of the superfamily is a Toll/IL-1 receptor (TIR) domain on the cytoplasmic side of the receptors. The members of the IL-1 receptor subfamily contain three Ig domains in their extracellular regions (1). The other group in the superfamily is the recently identified pathogen-associated pattern recognition receptors, the Toll-like receptors (TLRs), 11 members of which contain two major domains characterized by extracellular leucine-rich repeats and an intracellular TIR domain (2-7).Much progress has been made in understanding the IL-1R-mediated signaling. Upon IL-1 stimulation, the TIR domaincontaining adaptor molecule MyD88 (8) is recruited to the TIR domain of the receptor complex, which then recruits serinethreonine kinases IRAK4 (IL-1 receptor associated kinase 4) (9, 10) and IRAK (11,12). Whereas IRAK4 is the kinase that functions upstream of and phosphorylates IRAK, the phosphorylated IRAK mediates the recruitment of TRAF6 to the receptor complex (13). IRAK-TRAF6 then leaves the receptor complex to interact with TAK1, a member of the mitogen-activated protein kinase kinase kinase family, and the proteins that bind to it, TAB1, TAB2, and TAB3 on the membrane (14, 15). TAK1 and TAB2 are phosphorylated on the membrane, followed by the formation and translocation of TRAF6-TAK1-TAB1-TAB2, from the membrane to the cytosol (15), where TAK1 is activated. Whereas genetic studies show that IRAK is required for the IL-1-induced activation of TAK1, in vitro biochemical analyses reveal that TRAF6-mediated ubiquitination may also play an important role in TAK1 activation (16). The activation of TAK1 eventually leads to the activation of IB kinase (IKK) by an unknown mechanism. Activated IKK phosphorylates IB proteins, which are degraded, releasing NF-B to activate transcription in the nucleus (17)(18)(19)(20). Activated TAK1 has also been implicated in the IL-1-induced activation of MKK6 and JNK (14). The definitive evidence for an essential role of TAK1 in IL-1 signaling is from studies with TAK1-deficient cells. Two groups (21,22) independently reported that TAK1 deficiency leads to a defect in IL-1 signaling. MEKK3 has also been implicated in IL-1-mediated IKK and JNK activation, possibly through its interaction with TRAF6 (23-25).

IL-1 receptor-associated kinase (IRAK) is phosphorylated, ubiquitinated, and degraded upon interleukin-1 (IL-1) stimulation. In this study, we showed that IRAK can be ubiquitinated through both Lys-48-and Lys-63-linked polyubiquitin chains upon IL-1 induction. Pellino 3b is the RING-like motif ubiquitin protein ligase that promotes the Lys-63-linked polyubiquitination on IRAK. Pellino 3b-mediated Lys-63-linked IRAK polyubiquitination competed with Lys-48-linked IRAK polyubiquitination for the same ubiquitination site, Lys-134 of IRAK, thereby blocking IL-1-induced IRAK degradation. Importantly, the negative impact of Pellino 3b on IL-1-induced IRAK degradation correlated with the inhibitory effect of Pellino 3b on the IL-1-induced TAK1-dependent pathway, suggesting that a positive role of IRAK degradation in IL-1 induced TAK1 activation. Taken together, our results suggest that Pellino 3b acts as a negative regulator for IL-1 signaling by regulating IRAK degradation through its ubiquitin protein ligase activity.Interleukin-1 (IL-1), 3 a major pro-inflammatory cytokine, has a wide range of biological activities in inflammation. Genetic and biochemical studies revealed that IL-1R mediates a very complex pathway, involving a cascade of kinases organized by multiple adapter molecules into signaling complexes, leading to activation of the transcription factor NFB. Based on studies by our group and others, we postulated a model for the IL-1 pathway (1-7). Upon IL-1 stimulation, the IL-1 receptor recruits adapter molecule MyD88 (8) and mediates the formation of complex I (IL-1R-MyD88-IRAK4-IRAK-TRAF6), where IRAK4 (IL-1 receptor-associated kinase 4 (9)) is activated, leading to hyperphosphorylation of IRAK (10), which creates an interface for its interaction with adapter Pellino 1 (11). The receptor proximal components are then released from the receptor to form an intermediate complex, followed by formation of complex II (IRAK-TRAF6-TAK1-TAB2-TAB3), leading to phosphorylation of TAK1 (transforming growth factor ␤-activated kinase, a MAP3K) and TAB2 (TAK1-binding protein 2) and TAB3 on the membrane (1, 2-7). Although the membraneassociated modified IRAK is ubiquitinated and degraded, complex III (TRAF6-TAK1-TAB2-TAB3) is then dissociated from complex II and translocated from the membrane to the cytosol, where TAK1 is activated, followed by the activation of IB kinase (IKK) and NFB (7).Chen and co-workers (14, 15) showed that protein ubiquitination plays an important role in TRAF6-mediated TAK1 and IKK activation. The ubiquitin pathway generally involves three types of enzymes, ubiquitin-activating enzyme (E1 or Uba), ubiquitin-conjugating enzyme (E2 or Ubc), and ubiquitin protein ligase (E3 or Ubr) (16). The E3 ubiquitin protein ligases play a key role in recognition and selection of proteins targeted for ubiquitination. Many RING finger proteins have been shown to act as E3s, either by themselves or as part of a multisubunit E3 protein complex (17). TRAF6, a RING domain protein, has been shown to function as a ubiquit...