Reconsidering phosphorylation in the control of inducible CARD11 scaffold activity during antigen receptor signaling

Phosphorylation of serine-893 in CARD11 suppresses the formation and activity of the CARD11-BCL10-MALT1 complex in T and B cells

CARD11 acts as a gatekeeper for adaptive immune responses after T cell or B cell antigen receptor (TCR/BCR) ligation on lymphocytes. PKCθ/β-catalyzed phosphorylation of CARD11 promotes the assembly of the CARD11-BCL10-MALT1 (CBM) complex and lymphocyte activation. Here, we demonstrated that PKCθ/β-dependent CARD11 phosphorylation also suppressed CARD11 functions in T or B cells. Through mass spectrometry-based proteomics analysis, we identified multiple constitutive and inducible CARD11 phosphorylation sites in T cells. We demonstrated that a single TCR- or BCR-inducible phosphorylation on Ser⁸⁹³ in the carboxyl terminus of CARD11 prevented the activation of the transcription factor NF-κB, the kinase JNK, and the protease MALT1. Moreover, CARD11 Ser⁸⁹³ phosphorylation sensitized BCR-addicted lymphoma cells to toxicity induced by Bruton's tyrosine kinase (BTK) inhibitors. Phosphorylation of Ser⁸⁹³ in CARD11 by PKCθ controlled the strength of CARD11 scaffolding by impairing the formation of the CBM complex. Thus, PKCθ simultaneously catalyzes both stimulatory and inhibitory CARD11 phosphorylation events, which shape the strength of CARD11 signaling in lymphocytes.

Mechanistic impact of oligomer poisoning by dominant-negative CARD11 variants

The CARD11 scaffold controls antigen receptor signaling to NF-κB, JNK, and mTOR. Three classes of germline mutations in CARD11 cause Primary Immunodeficiency, including homozygous loss-of-function (LOF) mutations in CARD11 deficiency, heterozygous gain-of-function (GOF) mutations in BENTA disease, and heterozygous dominant-negative LOF mutations in CADINS. Here, we characterize LOF CARD11 mutants with a range of dominant-negative activities to identify the mechanistic properties that cause these variants to exert dominant effects when heterozygous. We find that strong dominant negatives can poison signaling from mixed wild-type:mutant oligomers at two steps in the CARD11 signaling cycle, at the Opening Step and at the Cofactor Association Step. Our findings provide evidence that CARD11 oligomer subunits cooperate in at least two steps during antigen receptor signaling and reveal how different LOF mutations in the same oligomeric signaling hub may cause disease with different inheritance patterns.