Human and mouse Abelson interacting proteins (Abi) are SH3-domain containing proteins that bind to the proline-rich motifs of the Abelson protein tyrosine kinase, We report a new member of this gene family, a Drosophila Abi (dAbi) that is a substrate for Abl kinase and that coimmunoprecipitates with Abl if the Abi SH3 domain is intact. We have identified a new function for both dAbi and human Abi-2 (hAbi-2), Both proteins activate the kinase activity of Abl as assayed by phosphorylation of the Drosophila Enabled (Ena) protein. Removal of the dAbi SH3 domain eliminates dAbi's activation of Abl kinase activity. dAbi is an unstable protein in cells and is present at low steady state levels but its protein level is increased coincident with phosphorylation by Abl kinase, Expression of the antisense strand of dAbi reduces dAbi protein levels and abolishes activation of Abl kinase activity. Modulation of Abi protein levels may be an important mechanism for regulating the level of Abl may level kinase activity in the cell.
