Differential Signaling after β1 Integrin Ligation Is Mediated Through Binding of CRKL to p120^CBL and p110^HEF1*

CRKL is an SH2-SH3-SH3 adapter protein that is a major substrate of the BCR/ABL oncogene. The function of CRKL in normal cells is unknown. In cells transformed by BCR/ABL we have previously shown that CRKL is associated with two focal adhesion proteins, tensin and paxillin, suggesting that CRKL could be involved in integrin signaling. In two hematopoietic cell lines, MO7e and H9, we found that CRKL rapidly associates with tyrosine-phosphorylated proteins after cross-linking of β1 integrins with fibronectin or anti-β1 integrin monoclonal antibodies. The major tyrosine-phosphorylated CRKL-binding protein in the megakaryocytic MO7e cells was identified as p120^CBL, the cellular homolog of the v-Cbl oncoprotein. However, in the lymphoid H9 cell line, the major tyrosine-phosphorylated CRKL-binding protein was p110^HEF1. In both cases, this binding was mediated by the CRKL SH2 domain. Interestingly, although both MO7e and H9 cells express p120^CBLand p110^HEF1, β1 integrin cross-linking induces tyrosine phosphorylation of p120^CBL (but not p110^HEF1) in MO7e cells and of p110^HEF1 (but not p120^CBL) in H9 cells. In both cell types, CRKL is constitutively complexed to C3G, SOS, and c-ABL through its SH3 domains, and the stoichiometry of these complexes does not change upon integrin ligation. Thus, in different cell types CRKL and its SH3-associated proteins may form different multimeric complexes depending on whether p120^CBL or p110^HEF1 is tyrosine-phosphorylated after integrin ligation. The shift in association of CRKL and its SH3-associated proteins from p120^CBL to p110^HEF1 could contribute to different functional outcomes of “outside-in” integrin signaling in different cells.