The TRAF6 Ubiquitin Ligase and TAK1 Kinase Mediate IKK Activation by BCL10 and MALT1 in T Lymphocytes

Abstract

The CARD domain protein BCL10 and paracaspase MALT1 are essential for the activation of IkappaB kinase (IKK) and NF-kappaB in response to T cell receptor (TCR) stimulation. Here we present evidence that TRAF6 ubiquitin ligase and TAK1 protein kinase mediate IKK activation by BCL10 and MALT1. RNAi-mediated silencing of MALT1, TAK1, TRAF6, and TRAF2 suppressed TCR-dependent IKK activation and interleukin-2 production in T cells. Furthermore, we have reconstituted the pathway from BCL10 to IKK activation in vitro with purified proteins of MALT1, TRAF6, TAK1, and ubiquitination enzymes including Ubc13/Uev1A. We find that a small fraction of BCL10 and MALT1 proteins form high molecular weight oligomers. Strikingly, only these oligomeric forms of BCL10 and MALT1 can activate IKK in vitro. The MALT1 oligomers bind to TRAF6, induce TRAF6 oligomerization, and activate the ligase activity of TRAF6 to polyubiquitinate NEMO. These results reveal an oligomerization --> ubiquitination --> phosphorylation cascade that culminates in NF-kappaB activation in T lymphocytes.

Signals from within: the DNA-damage-induced NF-κB response

“…It has been proposed that the different modifications target the molecule to different subcellular compartments, that is, sumoylation would lead to NEMO nuclear accumulation and thus recognition by ATM, while ubiquitination most likely releases the molecule from the ATM scaffold and allows its cytoplasmic translocation. 54,55 As has been shown before for other signalling cascades, [56][57][58][59] ubiquitination of NEMO is a prerequisite for IKK activation. As such, NEMO could act as the factor mediating the link between DNA damage and ATM activation in the nucleus and activation of the IKK complex in the cytosol.…”

A dual role for the API2 moiety in API2-MALT1-dependent NF-κB activation: heterotypic oligomerization and TRAF2 recruitment

“…It is thought that upon antigenic stimulation, Bcl10 binds Multiple roles for the API2 moiety of API2-MALT1 PC Lucas et al to MALT1 and induces its oligomerization and that enforced oligomerization of the MALT1 C-terminus triggers NF-kB activation Sun et al, 2004). Because API2-MALT1, unlike wild-type MALT1, potently stimulates NF-kB in the absence of Bcl10 (Uren et al, 2000;Lucas et al, 2001;Ruland et al, 2003), we wondered whether this oncoprotein might promote lymphocyte proliferation by acting as a 'gain-of-function' mutant of MALT1, which could induce constitutive NF-kB activation via API2 moietymediated oligomerization.…”

“…Zhou et al (2004Zhou et al ( , 2005 suggest that MALT1 possesses E3 ubiquitin ligase activity and that, together with the Ubc13/MMS2 E2 enzyme complex, MALT1 directly catalyses ubiquitination of NF-kB essential modulator (NEMO), the regulatory subunit of the IKK complex, upon oligomerization. Other groups suggest that MALT1 oligomerization triggers TRAF6-dependent ubiquitination of NEMO (Sun et al, 2004;Dong et al, 2006). Further studies are required to resolve this controversy.…”

NF-κB and the immune response

“…BCL10 oligomerization has been implicated in IKK activation through a process that involves ubiquitination of NEMO (Zhou et al, 2004). This ubiquitination event appears to be mediated by MALT1, and perhaps TRAF6, although no TCR signaling deficits have been reported in TRAF6-deficient mice (Lomaga et al, 1999;Sun et al, 2004). If this is true however, it is possible that the effect is mediated through the kinase TAK1, which functions in IKK activation downstream of TRAF6 in other pathways.…”