Identification of drosophila bicoid-interacting proteins using a custom two-hybrid selection

Bicoid directs pattern formation in the developing Drosophila embryo, and does so by performing two seemingly unrelated tasks; it activates transcription and represses translation. To understand how Bicoid carries out this dual role, we sought to identify Bicoid-ancillary proteins that might mediate Bicoid's function in transcription or translation. We used a customized version of the two-hybrid method and found two Bicoid-interacting proteins, Bin1 and Bin3, both of which interact with Bicoid in vitro. Bin1 is similar to a human protein (SAP18) involved in transcription regulation, and Bin3, described in this paper, is similar to a family of protein methyltransferases that modify RNA-binding proteins. Given that Bicoid's role as a translation regulator requires RNA binding, we suggest that the Bicoid-interacting methyltransferase might be important for that role. The custom two-hybrid method we used, in which Bicoid is bound to DNA via its own DNA binding domain, rather than via a fusion-protein tether, should be generally applicable to other DNA binding proteins.