Identification of syntaxin 4 as an essential factor for the hepatitis C virus life cycle

Although there is evidence that multivesicular bodies (MVBs) are involved in the release of hepatitis C virus (HCV), many aspects of HCV release are still not fully understood. The amount of α-taxilin that prevents SNARE (soluble N-ethylmaleimidesensitive factor attachment protein receptor) complex formation by binding to free syntaxin 4 is reduced in HCV-positive cells. Therefore, it was analyzed whether the t-SNARE protein syntaxin 4 which mediates vesicles fusion is involved in the HCV life cycle. HCV-positive cells possess an increased amount of syntaxin 4 protein, although the amount of syntaxin 4-specific transcripts is decreased in HCV-positive Huh7.5 cells and in HCV-infected primary human hepatocytes. In HCV-positive cells a significant longer half-life of syntaxin 4 was found that overcompensates for the decreased expression and leads to the elevated level of syntaxin 4. Overexpression of syntaxin 4 reduces the intracellular amount of infectious viral particles by facilitating viral release, while silencing of syntaxin 4 expression using specific siRNAs inhibits the release of HCV particles and so leads to an increase in the intracellular amount of infectious viral particles. This indicates that HCV uses a SNARE-dependent pathway for viral release. Confocal immunofluorescence microscopy revealed a colocalization of syntaxin 4 with a MVB-specific marker, exosomes and HCV core, which suggests a fraction of syntaxin 4 is associated with exosomes loaded with HCV. Altogether, it is assumed that syntaxin 4 is a novel essential cellular factor for the release of HCV.