The life cycle of human papillomaviruses (HPVs) is dependent on epithelial differentiation. Among the viral proteinsexpressed in differentiated epithelial cells are the viral capsid proteins, L1 and L2, as well as the E1^E4 fusion proteins. In this study, the expression and intracellular localization of the E1^E4 proteins of HPV type 31b were examined in both monolayer and raft cultures of the CIN-612 cell line which maintains episomal copies of HPV-31b. In this cell line, a high level of E1^E4 protein expression was observed in the cytoplasm of a small percentage of cells in monolayer culture. A large increase in E1^E4 protein levels was observed upon stratification of the CIN-612 cell line in raft cultures, with E1^E4 protein expression limited to the uppermost layers of the epithelium. A diffuse, slightly grainy cytoplasmic localization of E1^E4 protein was observed in both monolayer and raft culture systems. Although virion synthesis is entirely dependent upon phorbol ester or synthetic diacylglycerol treatment of raft cultures, E1^E4 expression was observed in both treated and untreated monolayer and raft cultures of the CIN-612 cell line. In monolayer cultures of two simian virus 40-transformed cell lines, cos-7 and MK-6, transiently transfected with an E1^E4 expression vector, the distribution of E1^E4 protein was found to differ substantially from that in the CIN-612 cells. In these cell lines E1^E4 protein was found to exhibit a total collapse into either cytoplasmic inclusion granules in the cos-7 cells or a perinuclear halo-like structure in the MK-6 cell line. The host cell, its differentiation state, and the amount of expression can therefore significantly affect the distribution of the E1^E4 proteins.
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