Protein tyrosine kinases, including receptor tyrosine kinases (RTKs), are a major focus of research due to their diverse and essential functions during regulation of cell growth and development. While less studied, it is now clear that protein tyrosine phosphatases (PTPs) also participate in a myriad of cellular and developmental processes. The Drosophila PTP Corkscrew (Csw) plays an essential, positive role in RTK signaling. The importance of Csw function is underscored in csw mutant animals where developmental defects result in death. The molecular mechanisms of Csw function are being revealed, and unlike other members of the RTK signaling cassette, whose mechanisms of action are seemingly invariant, the molecular mechanism of Csw function differs depending upon the RTK. In a screen for Csw binding proteins, D-Importin 7/RanBP7 (Dim-7), a member of the Importin B superfamily of nuclear import proteins, was identified. In regard to Dim-7, preliminary studies suggest a two-part model whereby I. Interaction between Csw and Dim-7 constitutes part of a mechanism whereby the import process can be regulated at the level of the activated receptor, and II. Dim-7 is the nuclear transport protein that imports activated map kinase (D-ERK) into the nucleus. To test the first part of this model, the molecular mechanism by which RTK activation is able to regulate nuclear import will be determined. Molecular studies will define residues and/or domains within Csw essential for interaction with Dim-7. Likewise, molecular studies will define residues and/or domains within Dim-7 essential for interaction with Csw. To test the second part of this model, genetic studies with the six newly isolated dim-7 mutations will determine the requirement for Dim-7 throughout development. Further, the molecular mechanism by which Dim-7 functions to import D-ERK into the nucleus will be revealed by defining the essential molecular elements of the interaction between Dim-7 and D-ERK, as well as defining the molecular interactions between Dim-7 and known components of the nuclear import machinery. Finally, the specificity of Dim-7 will be addressed by determining the array of nuclear proteins that depend upon Dim-7 for their import. Experiments will test whether Dim-7 imports other members of the map kinase family and two hybrid screens will identify additional Dim-7 interacting proteins. It is apparent that nuclear import is not a passive process during signal transduction. Elucidation of the mechanism by which specific components of signaling pathways (e.g. Csw) directly participate in the process of nuclear import will contribute to a greater understanding of how signaling pathways control cellular growth and differentiation.
