PKC isoforms and CARMA1 play crucial roles in immunoreceptor-dependent NF-kappaB activation. We tested whether PKC-dependent phosphorylation of CARMA1 directly regulates this signaling cascade. B cell antigen receptor (BCR) engagement led to the progressive recruitment of CARMA1 into lipid rafts and to the association of CARMA1 with, and phosphorylation by, PKCbeta. Furthermore, PKCbeta interacted with the serine-rich CARMA1 linker, and both PKCbeta and PKCtheta phosphorylated identical serine residues (S564, S649, and S657) within this linker. Mutation of two of these sites ablated the functional activity of CARMA1. In contrast, deletion of the linker resulted in constitutive, receptor- and PKC-independent NF-kappaB activation. Together, our data support a model whereby CARMA1 phosphorylation controls NF-kappaB activation by triggering a shift from an inactive to an active CARMA1 conformer. This PKC-dependent switch regulates accessibility of the CARD and CC domains and controls assembly and full activation of the membrane-associated IkappaB kinase (IKK) signalosome.
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