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Several classes of signaling proteins contain autoinhibitory domains that prevent unwarranted signaling and coordinate the induction of activity in response to external cues. CARD11, a scaffold protein critical for antigen receptor signaling to NF-κB, undergoes autoregulation by a poorly understood inhibitory domain (ID), which keeps CARD11 inactive in the absence of receptor triggering through inhibitory intramolecular interactions. This autoinhibitory strategy makes CARD11 highly susceptible to gain-of-function mutations that are frequently observed in diffuse large B cell lymphoma (DLBCL) and that disrupt ID-mediated autoinhibition, leading to constitutive NF-κB activity, which can promote lymphoma proliferation. Although DLBCL-associated CARD11 mutations in the caspase recruitment domain (CARD), LATCH domain, and coiled coil have been shown to disrupt intramolecular ID binding, surprisingly, no gain-of-function mutations in the ID itself have been reported and validated. In this study, we solve this paradox and report that the CARD11 ID contains an unusual array of four repressive elements that function cooperatively with redundancy to prevent spontaneous NF-κB activation. Our quantitative analysis suggests that potent oncogenic CARD11 mutations must perturb autoinhibition by at least three repressive elements. Our results explain the lack of ID mutations in DLBCL and reveal an unusual autoinhibitory domain structure and strategy for preventing unwarranted scaffold signaling to NF-κB. [ABSTRACT FROM AUTHOR]

The activation of key signaling pathways downstream of antigen receptor engagement is critically required for normal lymphocyte activation during the adaptive immune response. CARD11 is a multidomain signaling scaffold protein required for antigen receptor signaling to NF-kB, c-Jun N-terminal kinase, and mTOR. Germline mutations in the CARD11 gene result in at least four types of primary immunodeficiency, and somatic CARD11 gain-of-function mutations drive constitutive NF- B activity in diffuse large B cell lymphoma and other lymphoid cancers. In response to antigen receptor triggering, CARD11 transitions from a closed, inactive state to an open, active scaffold that recruits multiple signaling partners into a complex to relay downstream signaling. However, how this signal- induced CARD11 conversion occurs remains poorly understood. Here we investigate the role of Inducible Element 1 (IE1), a short regulatory element in the CARD11 Inhibitory Domain, in the CARD11 signaling cycle. We find that IE1 controls the signal-dependent Opening Step that makes CARD11 accessible to the binding of cofactors, including Bcl10, MALT1, and the HOIP catalytic subunit of the linear ubiquitin chain assembly complex. Surprisingly, we find that IE1 is also required at an independent step for the maximal activation of HOIP and MALT1 enzymatic activity after cofactor recruitment to CARD11. This role of IE1 reveals that there is an Enzymatic Activation Step in the CARD11 signaling cycle that is distinct from the Cofactor Association Step. Our results indicate that CARD11 has evolved to actively coordinate scaffold opening and the induction of enzymatic activity among recruited cofactors during antigen receptor signaling. [ABSTRACT FROM AUTHOR]