The SH3 domain of Bruton's tyrosine kinase displays altered ligand bindingproperties when auto-phosphorylated in vitro

Btk is a member of the Tec family of protein tyrosine kinases expressed in B cells, it is stimulated following cross-linking of the B cell receptor wh ich leads to the autophosphorylation of a specific residue in the SH3 domai n, Y223. Previous work using Btk-derived fusion proteins has shown that the Btk SH3 domain binds to c-Cbl and Wiskott-Aldrich syndrome protein (WASP) in vitro. We show here that when the Btk SH3 domain fusion protein is autop hosphorylated, its ability to take part in protein interactions is altered as compared to the nonphosphorylated fusion protein. Although the phosphory lated Btk SH3 domain stilt binds c-Cbl, it no longer binds WASP and instead acquires a high affinity for kinase-active Syk. The region of Syk responsi ble for this interaction is contained within its C terminus, suggesting a n ovel mechanism by which these proteins may interact.