BOVINE PAPILLOMAVIRUS E1 PROTEIN AFFECTS THE HOST-CELL CYCLE PHASE FRACTIONS

C127 murine fibroblast cells were electroporated with a bovine papillo mavirus E1 protein expression vector and examined by flow cytometry. E 1 expressing cells (E1+) within the total cell population were disting uished from nonexpressing cells (E1(-)) by immunofluorescent staining with anti-E1 serum and a fluorescein-conjugated second antibody. Under conditions of saturation with the first and second antibodies, the sp ecific green fluorescence reflected the level of intracellular E1 prot ein. Simultaneous staining with a DNA-specific dye, propidium iodide ( PI), enabled the cell cycle distributions for the E1(+) and E1(-) cell populations to be determined. It was found that the E1(+) subpopulati on had a reduced percentage of cells in G1 phase and an increased perc entage of G2+M phase cells, compared to the E1(-) subpopulation. There was no significant difference in overall doubling time or percentage of noncycling cells in the E1(+) vs. E1- populations, indicating that the change in cell cycle distribution was not due to a general activat ion or inhibition of cell growth by E1. Direct measurement of cell cyc le phase fractions confirmed that the G1 phase was decreased and the G 2+M phase was increased in E1 expressing cells. As these observations were made in the absence of other viral proteins or viral DNA replicat ion, it suggests that the E1 protein exerts an effect on the host cell independent of its direct role in viral DNA replication. Thus, E1 may interact directly with the host cell cycle regulatory machinery. (C) 1994 Wiley-Liss, Inc.