Glycine Zipper Motifs in Hepatitis C Virus Nonstructural Protein 4B Are Required for the Establishment of Viral Replication Organelles

Abstract

Hepatitis C virus (HCV) RNA replication occurs in tight association with remodeled host cell membranes, presenting as cytoplasmic accumulations of single, double and multi membrane vesicles in infected cells. Formation of these so-called replication organelles is mediated by a complex interplay of host cell factors and viral replicase proteins. Of these, nonstructural protein 4B (NS4B), an integral transmembrane protein, appears to play a key role, but little is known about the molecular mechanisms how this protein contributes to organelle biogenesis. Using forward and reverse genetics we identified glycine-zipper motifs within transmembrane helices 2 and 3 of NS4B that are critically involved in viral RNA replication. Foerster resonance energy transfer analysis revealed the importance of the glycine-zippers in NS4B homo and heterotypic self-interactions. Additionally, ultrastructural analysis using electron microscopy unraveled a prominent role of glycine-zipper residues for the subcellular distribution and the morphology of HCV-induced double membrane vesicles. Notably, loss-of-function NS4B glycine-zipper mutants prominently induced single membrane vesicles with secondary invaginations that might represent an arrested intermediate state in double membrane vesicle formation. These findings highlight a so far unknown role of glycine residues within the membrane integral core domain for NS4B self-interaction and functional as well as structural integrity of HCV replication organelles.