JAK2 and JAK1 constitutively associate with hIL-5Rα and βc, respectively. (A) Unstimulated or IL-5–stimulated TF-h5Rα cells were lysed and immunoprecipitated with anti–hIL-5Rα, KM1266, or control Ab. The immunoprecipitated proteins were analyzed on SDS-PAGE and transferred to Immobilon membranes. The membranes were probed with anti-JAK2 or anti-JAK1 Ab and reprobed with anti–hIL-5Rα MoAb KM1074. (B) Cell lysates from unstimulated or IL-5–stimulated TF-h5Rα cells were immunoprecipitated with anti-βc MoAb or control Ab and separated on SDS-PAGE. The membrane was probed with anti-JAK1 Ab and reprobed with anti-βc polyclonal Ab.