Mitogen-induced changes in the actin cytoskeleton are accompanied by changes in the tyrosine phosphorylation of several proteins in focal adhesions. In this study, we have investigated the role of RAFTK (also termed Pyk2/CAK-β), a cytoplasmic tyrosine kinase related to focal adhesion kinase (FAK), in heregulin-mediated signal transduction in breast cancer cells. Stimulation of T47D cells with heregulin (HRG) induced the tyrosine phosphorylation of RAFTK and the formation of a multiprotein complex. Maximal phosphorylation of the proteins participating in this complex occurred within 2 h of HRG stimulation. Analyses of the members of the HRG-stimulated complex revealed that RAFTK associated with p190 RhoGAP (p190), RasGAP, c-Abl as well as with the focal adhesion molecules p130cas and paxillin. c-Abl was found to be associated with RAFTK through the region of RAFTK containing amino acids 419-1009. Site-directed mutagenesis of Y881 aa within the RAFTK sequence abolished the binding of RAFTK to c-Abl, indicating that the tyrosine residue 881 of RAFTK is the c-Abl binding site within the RAFTK molecule. Overexpression of wild-type RAFTK significantly enhanced breast cancer cell invasion, while overexpression of the mutants Tyr402 or Tyr881 of RAFTK inhibited this migration. Therefore, RAFTK serves as a mediator and an integration point between focal adhesion molecules in HRG-mediated signaling in T47D breast cancer cells.

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