THE NCK SH2 SH3 ADAPTER PROTEIN IS PRESENT IN THE NUCLEUS AND ASSOCIATES WITH THE NUCLEAR-PROTEIN SAM68/

SH2/SH3 adaptor proteins are essential components of the signal transd uction pathways initiated by tyrosine kinases. Nck is a ubiquitously e xpressed adaptor protein whose function has been enigmatic. We perform ed confocal microscopy to localize Nck in NIH3T3 and A431 cells. Surpr isingly, Nck was identified in the nucleus as well as the cytoplasm wi th no visible change in localization due to PDGF or EGF stimulation. W estern blot analysis of nuclear and cytosolic fractions confirmed that there was no translocation in response to growth factor and that tyro sine phosphorylation was specific to only cytosolic Nck. Far Western b lot analysis with either Nck, the SH2 domain, or the SH3 domains revea led differential binding in nuclear and cytosolic lysates, indicating specific binding partners for each subcellular location. The major tar get of c-Src during mitosis is SAM68, a RNA-binding protein ordinarily localized to the nucleus. SAM68 was identified as a nuclear specific binding partner of Nck in both nonmitotic and mitotic cells. Several t yrosine kinases can be found in the nucleus but their signal transduct ion remains undefined. The discovery of an adaptor protein in the nucl eus suggests there are signal transduction mechanisms within the nucle us that recapitulate those found in the cytoplasm.