Pleckstrin homology domains interact with filamentous actin

A fraction of Bruton's tyrosine kinase (Btk) co-localizes with actin fibers upon stimulation of mast cells via the high affinity IgE receptor (Fc epsi lon RI). In this study, a molecular basis of the Btk co-localization with a ctin fibers is presented. Btk and other Tec family tyrosine kinases have a pleckstrin homology (PH) domain at their N termini. The PH domain is a shor t peptide module frequently found in signal-transducing proteins and cytosk eletal proteins. Filamentous actin (F-actin) is shown to be a novel ligand for a subset of PH domains, including that of Btk. The actin-binding site w as mapped to a 10-residue region of the N-terminal region of Btk. Basic res idues in this short stretch are demonstrated to be involved in actin bindin g. Isolated PH domains induced actin filament bundle formation. Consistent with these observations, Btk binds F-actin in vitro and in vitro. Wild-type Btk protein is in part translocated to the cytoskeleton upon FceRI cross-l inking, whereas Btk containing a mutated PH domain is not. Phosphatidylinos itol 3,4,5-trisphosphate-mediated membrane translocation of Btk was enhance d in cytochalasin D-pretreated, Fc epsilon RI-stimulated mast cells. These data indicate that PH domain-mediated F-actin binding plays a role in Btk c o-localization with actin filaments.