We have used the yeast two-hybrid system to isolate proteins that interact with the carboxy-terminal SH3-SH2-SH3 region of Vav. One of the clones encoded heterogeneous nuclear ribonucleoprotein K (hnRNP K), a poly(rC)-specific RNA-binding protein. The interaction between Vav and hnRNP K involves the binding of the most carboxy-terminal SH3 domain of Vav to two proline-rich sequences present in the central region of hnRNP K. Overexpression of Vav in mouse fibroblasts leads to the formation of a stable complex with the endogenous hnRNP K and to the preferential redistribution of this protein to the cytoplasmic fraction. More importantly, Vav and hnRNP K proteins also interact in hematopoietic cells. In addition, Vav associates in vitro with a second 45-kDa poly(rC)-specific RNA-binding protein via its SH3-SH2-SH3 region. These results suggest that Vav plays a role in the regulation of the late steps of RNA biogenesis by modulating the function of poly(rC)-specific ribonucleoproteins.