Bruton's tyrosine kinase (Btk) associates with protein kinase C μ

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Abstract
Bruton's tyrosine kinase (Btk) is considered an essential signal transducer in B-cells. Mutational defects are associated with a severe immunodeficiency syndrome, X-chromosome linked agammaglobulinemia (XLA). Here we show by coimmunoprecipitation that a member of the protein kinase C (PKC) family, PKCμ, is constitutively associated with Btk. Neither antigen receptor (Ig) crosslinking nor stimulation of B-cells with phorbol ester or H2O2 affected Btk/PKCμ interaction. GST precipitation analysis revealed association of the Btk pleckstrin/Tec homology domain with PKCμ. Transient overexpression of PKCμ deletion mutants as well as expression of selected PKCμ domains in 293T cells revealed that both the kinase domain and the regulatory C1 region are independently capable of binding to the Btk PH-TH domain. These data show the existence of a PKCμ/Btk complex in vivo and identify two PKCμ domains that participate in Btk interaction.