ORF3 protein of hepatitis E virus interacts with the Bβ chain of fibrinogen resulting in decreased fibrinogen secretion from HuH-7 cells

The ORF3 protein of hepatitis E virus (HEV), the precise cellular functions of which remain obscure, was used in a yeast two-hybrid screen to identify its cellular binding partners. One of the identified interacting partners was fibrinogen B beta protein. The ORF3-fibrinogen B beta interaction was verified by co-immunoprecipitation and fluorescence resonance energy transfer in mammalian cells. Fibrinogen is a hepatic acute-phase protein and serves as a central molecule that maintains host homeostasis and haemostasis during an acute-phase response. Metabolic labelling of ORF3-transfected HuH-7 cells showed that secreted as well as intracellular levels of fibrinogen were decreased in these cells compared with vector-transfected controls. Northern hybridization and RT-PCR analyses revealed that the mRNA levels of all three chains of fibrinogen, A alpha, B beta and gamma, were transcriptionally downregulated in ORF3-transfected cells. The constitutive expression of fibrinogen genes can be significantly upregulated by interleukin (IL)-6, an important mediator of liver-specific gene expression during an acute-phase response. Transcription of fibrinogen genes after IL-6 stimulation was less in ORF3-expressing cells compared with controls. This report adds one more biological function to, and advances our understanding of, the cellular role of the ORF3 protein of HEV. The possible implications of these findings in the virus life cycle are discussed.