Interplay between BCL10, MALT1 and IkappaBalpha during T-cell-receptor-mediated NFkappaB activation.

T-cell-receptor (TCR) signalling to NFkappaB requires the assembly of a large multiprotein complex containing the serine/threonine kinase CK1alpha, the scaffold protein CARMA1, the heterodimer BCL10-MALT1 (the CBM complex) and the IkappaB kinase complex (IKK). Although the mechanisms regulating recruitment and activation of IKK within the CBM microenvironment have been extensively studied, there is little understanding of how IKK subsequently binds and phosphorylates IkappaBalpha, the inhibitor of NFkappaB, to promote IkappaBalpha ubiquitylation and proteasomal degradation. Here, we show that BCL10, MALT1 and IKK inducibly associate with IkappaBalpha in a complex that is physically distinct from the early CK1alpha-CBM signalosome. This IkappaBalpha-containing complex probably maturates from the CBM, because siRNA-based knockdown of CARMA1, CK1alpha and BCL10 hampered its assembly, leading to a reduction in NFkappaB activation. By contrast, CK1alpha normally recruited both BCL10 and ubiquitylated species of MALT1 when IkappaBalpha levels were reduced. However, knockdown of IkappaBalpha led to an altered ubiquitylation profile of BCL10-MALT1 combined with a defect in MALT1 reorganisation within large cytoplasmic structures, suggesting that, following stimulation, IkappaBalpha might also participate in MALT1 recycling. Altogether, our data suggest a two-step mechanism to connect active IKK to IkappaBalpha, and further unveil a potential role for IkappaBalpha in resetting TCR-mediated signalling.