Dephosphorylation of Carma1 by PP2A negatively regulates T-cell activation

The Carma1-Bcl10-Malt1 (CBM) complex bridges T-cell receptor (TCR) signalling to the canonical I kappa B kinase (IKK)/NF-kappa B pathway. NF-kappa B activation is triggered by PKC theta-dependent phosphorylation of Carma1 after TCR/CD28 co-stimulation. PKC theta-phosphorylated Carma1 was suggested to function as a molecular scaffold that recruits preassembled Bcl10-Malt1 complexes to the membrane. We have identified the serine-threonine protein phosphatase PP2A regulatory subunit A alpha (PPP2R1A) as a novel interaction partner of Carma1. PPP2R1A is associated with Carma1 in resting as well as activated Tcells in the context of the active CBM complex. By siRNA-mediated knockdown and in vitro dephosphorylation, we demonstrate that PP2A removes PKC theta-dependent phosphorylation of Ser645 in Carma1, and show that maintenance of this phosphorylation is correlated with increased T-cell activation. As a result of PP2A inactivation, we find that enhanced Carma1 S645 phosphorylation augments CBM complex formation, NF-kappa B activation and IL-2 or IFN-gamma production after stimulation of Jurkat T cells or murine Th1 cells. Thus, our data define PP2A-mediated dephosphorylation of Carma1 as a critical step to limit T-cell activation and effector cytokine production. The EMBO Journal (2011) 30, 594-605. doi:10.1038/emboj.2010.331; Published online 14 December 2010