Entry of Tiger Frog Virus (an Iridovirus) into HepG2 Cells via a pH-Dependent, Atypical, Caveola-Mediated Endocytosis Pathway

Tiger frog virus (TFV), in the genus Ranavirus of the family Iridoviridae, causes high mortality of cultured tiger frog tadpoles in China. To explore the cellular entry mechanism of TFV, HepG2 cells were treated with drugs that inhibit the main endocytic pathways. We observed that TFV entry was inhibited by NH4Cl, chloroquine, and bafilomycin, which can all elevate the pH of acidic organelles. In contrast, TFV entry was not influenced by chlorpromazine or overexpression of a dominant-negative form of Esp15, which inhibit the assembly of clathrin-coated pits. These results suggested that TFV entry was not associated with clathrin-mediated endocytosis, but was related to the pH of acidic organelles. Subsequently, we found that endocytosis of TFV was dependent on membrane cholesterol and was inhibited by the caveolin-1 scaffolding domain peptide. Dynamin and actin were also required for TFV entry. In addition, TFV virions colocalized with the cholera toxin subunit B, indicating that TFV enters as caveola-internalized cargo into the Golgi complex. Taken together, our results demonstrated that TFV entry occurs by caveola-mediated endocytosis with a pH-dependent step. This atypical caveola-mediated endocytosis is different from the clathrin-mediated endocytosis of frog virus 3 (FV3) by BHK cells, which has been recognized as a model for iridoviruses. Thus, our work may help further the understanding of the initial steps of iridovirus infection in lower vertebrates.