Relevance of Rab Proteins for the Life Cycle of Hepatitis C Virus

Abstract

Although potent direct-acting antiviral drugs for the treatment of chronic hepatitis C virus (HCV) infection are licensed, there are more than 70 million individuals suffering from chronic HCV infection. In light of the limited access to these drugs, high costs, and a lot of undiagnosed cases, it is expected that the number of HCV cases will not decrease worldwide in the next years. Therefore, and due to the paradigmatic character of HCV for deciphering the crosstalk between viral pathogens and the host cell, characterization of HCV life cycle remains a challenge. HCV belongs to the family of Flaviviridae. As an enveloped virus HCV life cycle depends in many steps on intracellular trafficking. Rab GTPases, a large family of small GTPases, play a central role in intracellular trafficking processes controlling fusion, uncoating, vesicle budding, motility by recruiting specific effector proteins. This review describes the relevance of various Rab proteins for the different steps of the HCV life cycle.

Keywords: hepatitis C virus; morphogenesis; rab GTPases; release; replication.

Figure 1

Schematic model of the HCV life cycle and involved Rab proteins. The entry of the HCV lipoviroparticles (LVPs) occurs in a coordinated way by binding of the LVPs to specific (co)receptors. The LVPs initially bind to heparan sulfate proteoglycans (HSPG) and the low density-lipoprotein receptor (LDLR) resulting in a local accumulation. After binding to scavenger receptor class B member 1 (SR-BI) a conformational change of E2 enables the binding to CD81 which then activates signal transduction via the epidermal growth factor receptor (EGFR). The LVP-loaded CD81 migrates by lateral diffusion to the tight junctions (TJ) where interaction with the TJ proteins occluding (OCLN) and claudin-1 (CLDN1) takes place and the LVPs are internalized via clathrin mediated endocytosis [Additional (co)receptors are: L-SIGN, DC-SIGN, ephidrin type A receptor 2 (EPHA2), Niemann-Pick C1-like receptor 1 (NPC1L1) and transferrin receptor (TnfR)]. After maturation of early endosomes to late endosomes the release of the viral nucleocapsid into the cytoplasm occurs in a pH-dependent manner. Hereby, Rab5A is required for acidification as it transports the vacuolar (H+)-ATPase (V-ATPase) ATP6v0a1 from the trans-Golgi (TGN) network toward endocytotic vesicles. The trafficking from early to late endosomes and finally lysosomes for degradation is regulated by Rab5 and Rab7. The TJ proteins OCLN and CLDN1 are recycled under control of Rab13. The uncoated positive-strand RNA is transported to the rough endoplasmic reticulum (ER) where it serves as template for the viral polyprotein synthesis. HCV infection leads to a massive rearrangement of intracellular ER derived membranes leading to the formation of a specific microenvironment, the membranous web (MW). The MW consists of double-membrane vesicles (DMVs) and multi-membrane vesicles (MMVs) embedded in a membranous matrix where HCV replication occurs in replication complexes (RCs). Rab5A and Rab7 are associated with the RCs. Another Rab protein that controls HCV replication is Rab27a. HCV assembly occurs in close proximity to the RCs. Replication and assembly sites are linked via cLDs. The Rab GTPases Rab18 and Rab32 are involved in HCV particle assembly by tethering LDs toward ER derived membranes and facilitating the trafficking of HCV Core and NS5A toward membrane structures involved in particle assembly. The exact mechanism of LVP release remains enigmatic. There is growing evidence that viral particles are released via the endosomal pathway rather than the canonical secretory route. This novel pathway includes the Rab1b-dependent transport of HCV particles through the Golgi compartment. Here, the Rab proteins Rab8b, 13, 23, 32, 33 are involved. Rab11a, a classical marker for recycling endosomes further directs the particles toward late endosomes (Rab7a-dependent) where the particles are sorted either for release by fusion of multivesicular bodies (MVBs) with the plasma membrane (Rab9a and Rab7a-dependent) in parallel to the release of exosomes (Rab3d- and Rab27a-dependent) or for autolysosomal degradation. For this, Rab9 together with TIP47 is required.