The IL-13Rα′ chain associates with IL-4β. cDNA for IL-4Rβ (lanes 1, 5, and 9), IL4Rβ plus IL-13Rα′ (lanes 2, 6, and 10), IL-4Rβ plus IL-2Rγ_c (lanes 3, 7, and 11), and all three chains (α′, β, and γ_c; lanes 4, 8, and 12) was transfected to CHO-K1 cells. Transfected cells were incubated with 1 nmol/L of ¹²⁵I–IL-4.¹²⁵I–IL-4/IL-4R cross-linked complex was immunoprecipitated from the cell lysate at 4°C by incubating with protein A (G) sepharose beads that had been preincubated with anti–IL-4Rβ (P7), γ_c, or c-Myc antibodies. The resulting complex was washed five times with lysing buffer, resuspended with reducing buffer, and analyzed by 8% SDS-PAGE as described previously. The molecular weight markers are shown on the left. The position of different receptor chains is indicated (IL-4Rβ, black arrow; IL-13Rα′, white arrowhead; IL-2Rγ_c, black arrowhead; and approximately 55 kD, white arrow).