ABSTRACT
MALT1 channels proximal T-cell receptor (TCR) signalling to downstream signalling pathways. With MALT1A and MALT1B two conserved splice variants exist and we demonstrate here that MALT1 alternative splicing supports optimal T-cell activation. Inclusion of exon7 in MALT1A facilitates the recruitment of TRAF6, which augments MALT1 scaffolding function, but not protease activity. Naive CD4(+) T cells express almost exclusively MALT1B and MALT1A expression is induced by TCR stimulation. We identify hnRNP U as a suppressor of exon7 inclusion. Whereas selective depletion of MALT1A impairs T-cell signalling and activation, downregulation of hnRNP U enhances MALT1A expression and T-cell activation. Thus, TCR-induced alternative splicing augments MALT1 scaffolding to enhance downstream signalling and to promote optimal T-cell activation.
Subject(s)
Alternative Splicing/genetics , CD4-Positive T-Lymphocytes/immunology , Caspases/genetics , Lymphocyte Activation/immunology , Neoplasm Proteins/genetics , Signal Transduction , Animals , Caspases/metabolism , Down-Regulation , Enzyme Activation , Exons/genetics , HEK293 Cells , Heterogeneous-Nuclear Ribonucleoprotein U/metabolism , Humans , Interleukin-2/biosynthesis , JNK Mitogen-Activated Protein Kinases/metabolism , Jurkat Cells , Mice, Inbred C57BL , Mucosa-Associated Lymphoid Tissue Lymphoma Translocation 1 Protein , NF-kappa B/metabolism , Neoplasm Proteins/metabolism , Receptors, Antigen, T-Cell/metabolism , TNF Receptor-Associated Factor 6/metabolism , Th17 Cells/immunology , Up-RegulationABSTRACT
The IκB kinase (IKK) complex acts as a gatekeeper of canonical NF-κB signaling in response to upstream stimulation. IKK activation requires sensing of ubiquitin chains by the essential IKK regulatory subunit IKKγ/NEMO. However, it has remained enigmatic whether NEMO binding to Lys-63-linked or linear ubiquitin chains is critical for triggering IKK activation. We show here that the NEMO C terminus, comprising the ubiquitin binding region and a zinc finger, has a high preference for binding to linear ubiquitin chains. However, immobilization of NEMO, which may be reminiscent of cellular oligomerization, facilitates the interaction with Lys-63 ubiquitin chains. Moreover, selective mutations in NEMO that abolish association with linear ubiquitin but do not affect binding to Lys-63 ubiquitin are only partially compromising NF-κB signaling in response to TNFα stimulation in fibroblasts and T cells. In line with this, TNFα-triggered expression of NF-κB target genes and induction of apoptosis was partially compromised by NEMO mutations that selectively impair the binding to linear ubiquitin chains. Thus, in vivo NEMO interaction with linear and Lys-63 ubiquitin chains is required for optimal IKK activation, suggesting that both type of chains are cooperating in triggering canonical NF-κB signaling.
