Mapping the Jak1 interaction domain of v-Abl in vitro and in vivo. (A) GST fusion proteins spanning various domains of v-Abl, including the src homology 2 and the protein tyrosine kinase domains (SH2-PTK, aa 237 to 645), the proline-rich and the DNA binding domains (aa 706 to 1080), and the F-actin binding domain (aa 1081 to 1244), or smaller GST fusion proteins spanning aa 706 to 857, 858 to 981, and 982 to 1080 were expressed in bacteria and bound to glutathione beads. (B and C) Equal amounts of fusion proteins were incubated with 1 mg of whole-cell extracts prepared from a murine pre-B-cell line (clone K) (B) or 1 to 3 μg of purified Jak1 protein (C). Bound materials were washed extensively, eluted off the beads, fractionated by SDS-PAGE (7% polyacrylamide), and blotted with an antibody to Jak1. The GST moiety alone and the SH2 domain of Crk were used as controls for the specificity of the GST fusion protein-Jak1 interaction. Jak1 immunoprecipitates from the same whole-cell extracts (panel B, lane 6) were included as control for the Jak1 protein. (D) IL-3-dependent BAF/3 cells were stably transfected with v-Abl expression constructs encoding the wild-type p160 v-Abl or the Jak1 binding mutant Δ858–1080. A 1-mg portion of total protein extracts from the transfectants was immunoprecipitated (IP) with an antibody to Jak1. Immune complexes were fractionated by SDS-PAGE (7% polyacrylamide) and subjected to Western blotting (WB) with an Abl antibody. The blot was stripped and probed with a Jak1-specific antibody to control for the amount of Jak1 immunoprecipitated.