From Structure to Function: New Insights into Hepatitis C Virus RNA Replication

Abstract

With an estimated number of about 180 million affected people worldwide and a limited therapy option, infection with the hepatitis C virus (HCV) 2 is an important medical problem. Initial limitations in propagating HCV in cell culture have been overcome step-by-step in the past few years and culminated in the recent development of a system allowing efficient propagation of infectious HCV in tissue culture. Nevertheless, structural and biochemical studies of viral proteins as well as molecular analysis of viral RNA replication are still major challenges. The recent resolution of the three-dimensional structures of some HCV proteins or domains along with elegant reverse genetic and cell biological studies provided first insights into how HCV amplifies its RNA, exploits the cellular machinery, and eventually overcomes innate immune responses. Organization of the Viral GenomeHCV is a positive strand RNA virus that has been classified as the genus Hepacivirus in the Flaviviridae family. The HCV genome is an uncapped, linear molecule with a length of ϳ9600 nucleotides (nt; Fig. 1A). It carries a long open reading frame that is flanked at the 5Ј-and 3Ј-ends by short highly structured non-translated regions (NTRs). The 5Ј-NTR has a length of about 340 nt and contains an internal ribosome entry site (IRES) required for translation of the HCV genome (1). This RNA element binds the 40 S ribosomal subunit in the absence of other translation initiation factors in a way that the initiation codon is placed in the immediate vicinity of the P site (2). Part of the IRES (domain II) overlaps with RNA signals essential for viral replication (Fig. 1A) arguing for a possible role of domain II in regulating a translation-RNA replication switch. The 3Ј-NTR has a tripartite structure composed of an ϳ40-nt-long variable region downstream of the HCV coding sequence, a poly(U/UC) tract of heterogeneous length, and a highly conserved 98-nt-long sequence designated X-tail (Fig. 1A). Genetic studies have shown that a poly(U/UC) tract of at least ϳ25 nt as well as the complete X-tail are required for RNA replication in cell culture and for infectivity of the viral genome in vivo. An additional cis-acting RNA element (CRE) has been identified in the 3Ј-terminal coding region of NS5B (Fig. 1A). This CRE (designated 5BSL3.2) (3) forms a long distance RNA-RNA interaction with SL2 in the X-tail (4), which is indispensable for RNA replication.Expression of the viral proteins from the monocistronic genome is primarily achieved by production of a polyprotein that is proteolytically cleaved into the structural proteins (core, envelope proteins E1 and E2), the hydrophobic peptide p7, and the non-structural (NS) proteins NS2, NS3, NS4A, NS4B, NS5A, and NS5B (Fig. 1A) (1, 5). Processing of the core to p7 region is mediated by host cell signalases, and in the case of the core protein, in addition by signal peptide peptidase. All remaining cleavages are carried out by two viral proteases: the NS2/3 protease mediating cleavage between NS2 and NS3 and the NS3...

Sensitivity of hepatitis C virus to cyclosporine A depends on nonstructural proteins NS5A and NS5B

“…Nevertheless, chimpanzee experiments have shown this region to be important for in vivo replication and possibly for interferon resistance (Enomoto et al 34 and reviewed in Appel et al 35 ). Additionally, this region of NS5A binds NS5B.…”

Hepatitis C Virus NS5A Protein Is a Substrate for the Peptidyl-prolyl cis/trans Isomerase Activity of Cyclophilins A and B

“…The HCV viral genome (ϳ9.6 kb) codes for a unique polyprotein of ϳ3000 amino acids (recently reviewed in Refs. [2][3][4]. Following processing via viral and cellular proteases, this polyprotein gives rise to at least 10 viral proteins, divided into structural (core, E1, and E2 envelope glycoproteins) and nonstructural proteins (p7, NS2, NS3, NS4A, NS4B, NS5A, NS5B).…”

Genetic and catalytic efficiency structure of an HCV protease quasispecies†

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