JAK2 association with the β_IT chain and subsequent phosphorylation by stimulation with GM-CSF. (A) Cell lysates from COS-7 cells transfected with either water or GMRα + β_IT DNA (αβ_IT) were immunoprecipitated (IP) with an anti-JAK2 antibody, and the proteins were separated on SDS-PAGE, transferred to nitrocellulose, and probed using antibodies to the extracellular portion of the β chain (DC-9) or JAK2. (B) COS-7 cells transfected with either GMRα + β_C(αβ_C) or GMRα + β_IT(αβ_IT) were stimulated without (−) or with (+) saturating concentrations of GM-CSF, lysed, and immunoprecipitated with an anti-JAK2 antibody. After SDS-PAGE and transfer to nitrocellulose, the blots were probed with an antiphosphotyrosine antibody (4G10) or anti-JAK2 antibody.