PKCβ modulates antigen receptor signaling via regulation of Btk membrane localization

Mutations in Bruton's tyrosine kinase (Btk) result in X‐linked agammaglobulinemia (XLA) in humans and X‐linked immunodeficiency (xid) in mice. While targeted disruption of the protein kinase C‐β (PKCβ) gene in mice results in an immunodeficiency similar to xid, the overall tyrosine phosphorylation of Btk is significantly enhanced in PKCβ‐deficient B cells. We provide direct evidence that PKCβ acts as a feedback loop inhibitor of Btk activation. Inhibition of PKCβ results in a dramatic increase in B‐cell receptor (BCR)‐mediated Ca 2+ signaling. We identified a highly conserved PKCβ serine phosphorylation site in a short linker within the Tec homology domain of Btk. Mutation of this phosphorylation site led to enhanced tyrosine phosphorylation and membrane association of Btk, and augmented BCR and FcϵRI‐mediated signaling in B and mast cells, respectively. These findings provide a novel mechanism whereby reversible translocation of Btk/Tec kinases regulates the threshold for immunoreceptor signaling and thereby modulates lymphocyte activation.