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.