CREB1 and ATF1 bind the X2 box of HLA-A and HLA-B and are partners in the CIITA-induced transactivation.

(A) EMSA analysis of nuclear extracts from HeLa cells incubated with the X2 probes of HLA-A and HLA-B (X2A and X2B probes). Arrows indicate protein/DNA complexes. Supershifts were observed for ATF/CREB, ATF1, and CREB1 (indicated by asterisks). (B) Transient cotransfection assay of HLA-A and HLA-B promoter–driven luciferase reporter constructs (1 μg/well) with Rc/RSV-CREB (1 μg/well), Rc/RSV-ATF1 (1 μg/well), and/or pREP4-CIITA (0.5 μg/well) in Tera-2 cells. Normalized luciferase activity values are expressed as mean fold induction ± SD of n = 4. (C) Transient cotransfection assay of HLA-A and HLA-B promoter constructs with a range of CIITA concentrations (0, 0.01, 0.03, 0.13, 0.5, and 2 μg/well) in Tera-2 cells. Normalized luciferase activity values are expressed as mean fold induction ± SD of n = 4.