Binding of STAT5 to bacterially expressed Crkl. Lysates from starved (−) TF-1 cells or cells stimulated with 100 ng/mL of GM-CSF for 10 minutes (+) were analyzed for binding to GST or various GST-Crkl constructs. Crkl constructs include full-length Crkl, full-length Crkl with a mutation of Arg to Lys in the SH2 domain FLVRES sequence (K39), the SH2 domain, and the N-terminal SH3 domain (SH3n) of Crkl. Bound proteins were separated by SDS-PAGE, transferred to PVDF membranes, and immunoblotted with STAT5 antisera.