The bZIP domain of the HBZ-SP1 protein is involved in the cell cycle arrest induced by transfection of HeLa cells with pEGFP-HBZ-SP1. (A) Cell cycle of HeLa cells transfected with different mutants of the HBZ-SP1 protein. HeLa cells were transfected with pNLS-EGFP, pEGFP-HBZ-SP1, pEGFP-HBZΔAD, or pEGFP-HBZΔADΔbZIP. HeLa cells were arrested using a double thymidine block and restimulated with 10% FCS for the indicated times, harvested, and their DNA content was then analyzed by flow cytometry. Bars show the percentage of cells in each phase of the cell cycle. The data represent results from one of three independent experiments; the other two experiments showed similar results. (B) Effects of EGFP-HBZ-SP1 and the mutants on HPV-18 transcription. HeLa cells were cotransfected with pLCR-Luc and pNLS-EGFP, pEGFP-HBZ-SP1, pEGFP-HBZΔAD, or pEGFP-HBZΔADΔbZIP. The luciferase values are expressed as levels of fold activation relative to luciferase activity measured in cells transfected with pcDNA3.1(-)/Myc-His in the presence of the luciferase reporter gene without the early promoter P105. The total amount of DNA in each series of transfection was equal, through the addition of pEGFP-C2 acting as filler DNA. Luciferase values were normalized for β-galactosidase activity. Values represent the mean ± S.D. (n = 3).