G protein βγ subunits act on the catalytic domain to stimulate Bruton's agammaglobulinemia tyrosine kinase

Abstract

G proteins are critical cellular signal transducers for a variety of cell surface receptors. Both α and β subunits of G proteins are able to transduce receptor signals. Several direct effect molecules for Gβγ subunits have been reported; yet the biochemical mechanism by which Gβγ executes its modulatory role is not well understood. We have shown that Gβγ could directly increase the kinase activity of Bruton's tyrosine kinase (Btk) whose defects are responsible for X chromosome-linked agammaglobulinemia in patients. The well characterized interaction of Gβγ with the PH (pleckstrin homology)/TH (Tec-homology) module of Btk was proposed to be the underlying activation mechanism. Here we show that Gβγ also interacts with the catalytic domain of Btk leading to increased kinase activity. Furthermore, we showed that the PH/TH module is required for Gβγ-induced membrane translocation of Btk. The membrane anchorage is also dependent on the interaction of Btk with phosphatidylinositol 3,4,5-trisphosphate, the product of phosphoinositide 3-kinase. These data support a dual role for Gβγ in the activation of Btk signaling function, namely membrane translocation and direct regulation of Btk catalytic activity.