Antiphosphotyrosine immunoblot of the chimeric I-IIA-IIA receptor in COS-1 cells transfected with I-IIA-IIA alone, I-IIA-IIA plus FcγRIIB2, and I-IIA-IIA plus Trun-B2. Lanes 1 and 2, I-IIA-IIA transfectants; lanes 3 and 4, I-IIA-IIA plus Trun-B2 cotransfectants; lanes 5 and 6, I-IIA-IIA plus FcγRIIB2 cotransfectants. Transfected cells were either not stimulated (−) or stimulated (+) with EA. The arrow shows the position of the tyrosine phosphorylated I-IIA-IIA chimera (60 kD). The expression of I-IIA-IIA was similar in each transfection. FcγRIIB and Trun-B2 were also expressed at similar levels (MFI for FcγRIIB was 114 and 104 for Trun-B2).