In vitro replication of dengue virus requires the presence of cis-acting elements within the 5' end and the 3' UTR of the viral genome. Some, like the putative cyclization sites (PCS), may promote interaction at both ends of the viral genome. To investigate whether viral or cellular proteins could be involved in this interaction, UV-induced cross-linking assays using extracts from the monocytic cell line U937 were performed. Our data demonstrate that the 5' end and the 3' UTR with the PCS interact with five cellular proteins with the same molecular weight. When both regions were differentially labeled, with biotin and 32P, respectively, the interaction of at least seven proteins with both ends could be demonstrated. Immunoprecipitation assays also demonstrate that La protein binds to the 5' end and with the 3' UTR. Moreover, these proteins also interact with the nonstructural proteins NS5 and NS3. The role of the NS5-La and NS3-La interaction in U937 cells remains to be established.