Schwannoma-derived growth factor (SDGF) is a secreted mitogen and a member of the EGF family of growth factors. Secreted SDGF interacts with cell surface EGF receptors and must be transported into the nucleus in order to act as a mitogen. SDGF also localizes in the nucleus of the cells which produce it. Most studies on the mitogenic activity of growth factors of the EGF family have concentrated on their induction of tyrosine phosphorylation, and there are little knowledge about their transport into the nucleus and their nuclear function. To address these problems, we will investigate the nuclear transport process of SDGF by using SDGF and its mutants which are defective in the nuclear targeting signals. We will also examine the potential requirements for nuclear transport for the mitogenic activity of SDGF by subcellular localization analysis and direct injection of SDGF into the cell nucleus. SDGF binds to DNA at a preferred sequence. Since the DNA binding specificity of SDGF is for a sequence used in attaching DNA to the nuclear matrix, the involvement of this sequence in SDGF nuclear function will be studied by gel retardation and competition assays. SDGF transported into the nucleus also binds to a nuclear protein. We will clone the cDNA of this nuclear protein by both expression cloning and hybridization cloning based on the amino acid sequences of purified proteins. Once we obtain this nuclear protein, we will investigate its DNA binding activity and the biological activity of its complex with SDGF. Finally. the biological function of SDGF transported into the nucleus will be studied by making cell lines which inducibly express SDGF and its mutants. The proposed experiments address new and potentially important mechanisms for growth factor function, and will help clarify the biological basis of diseases in which growth factors are involved.
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