GRB2 AND ITS APOPTOTIC ISOFORM GRB3-3 ASSOCIATE WITH HETEROGENEOUS NUCLEAR RIBONUCLEOPROTEIN-C, AND THESE INTERACTIONS ARE MODULATED BY POLY(U) RNA

Grb2 is an adaptor molecule comprising one Src homology (SH) 2 and two SH3 domains. This protein has a natural isoform named Grb3-3 with a d eletion within the SH2 domain. Numerous evidence points to a functiona l connection between SH2- and SH3-containing proteins and molecules im plicated in RNA. biogenesis. In this context, we have examined the bin ding of Grb2 and Grb3-3 to heterogeneous nuclear ribonucleoprotein (hn RNP) C. By the use of an in vivo genetic approach and through in vitro experiments, we furnish evidence that both Grb2 and Grb3-3 interact w ith hnRNP C proteins. Subcellular fractionation studies clearly show t hat Grb2 is partially localized in the nucleus. In addition, coimmunop recipitation experiments demonstrate that Grb2 hnRNP C complexes exist in intact hematopoietic cells. The carboxyl-terminal SH3 domains of G rb2 and Grb3-3 are primarily responsible for the association with hnRN P C. However, although the proline-rich motif of hnRNP C is involved i n the interaction with Grb2, it is not in the binding to Grb3-3. Furth ermore, poly(U) RNA inhibits the association of Grb2 with hnRNP C, whe reas it enhances the interaction between Grb3-3 and hnRNP C. These fin dings suggest that the Grb2/Grb3-3-hnRNP C interactions might fulfill different biological functions.