Signal Peptide Peptidase-catalyzed Cleavage of Hepatitis C Virus Core Protein Is Dispensable for Virus Budding but Destabilizes the Viral Capsid

BmSPP is a virus resistance gene in Bombyx mori

Introduction

Signal peptide peptidase (SPP) is an intramembrane protease involved in a variety of biological processes, it participates in the processing of signal peptides after the release of the nascent protein to regulate the endoplasmic reticulum associated degradation (ERAD) pathway, binds misfolded membrane proteins, and aids in their clearance process. Additionally, it regulates normal immune surveillance and assists in the processing of viral proteins. Although SPP is essential for many viral infections, its role in silkworms remains unclear. Studying its role in the silkworm, Bombyx mori , may be helpful in breeding virus-resistant silkworms.

Methods

First, we performed RT-qPCR to analyze the expression pattern of BmSPP. Subsequently, we inhibited BmSPP using the SPP inhibitor 1,3-di-(N-carboxybenzoyl-L-leucyl-L-leucylaminopropanone ((Z-LL)2-ketone) and downregulated the expression of BmSPP using CRISPR/Cas9 gene editing. Furthermore, we assessed the impact of these interventions on the proliferation of Bombyx mori nucleopolyhedrovirus (BmNPV).

Results

We observed a decreased in the expression of BmSPP during viral proliferation. It was found that higher concentration of the inhibitor resulted in greater inhibition of BmNPV proliferation. The down-regulation of BmSPP in both in vivo and in vitro was found to affect the proliferation of BmNPV. In comparison to wild type silkworm, BmSPPKO silkworms exhibited a 12.4% reduction in mortality rate.

Discussion

Collectively, this work demonstrates that BmSPP plays a negative regulatory role in silkworm resistance to BmNPV infection and is involved in virus proliferation and replication processes. This finding suggests that BmSPP servers as a target gene for BmNPV virus resistance in silkworms and can be utilized in resistance breeding programs.

Signal peptide peptidase: A potential therapeutic target for parasitic and viral infections

INTRODUCTION

Signal peptide peptidase (SPP) is a GxGD-type intramembrane-cleaving aspartyl protease responsible for clearing accumulating signal peptides in the endoplasmic reticulum. SPP is conserved among all kingdoms and is essential for maintaining cell homeostasis. Inhibition of SPP with selective inhibitors and the structurally similar HIV protease inhibitors results in signal peptide accumulation and subsequent cell death. Identification of SPP homologues in major human parasitic infections has opened a new therapeutic opportunity. Moreover, the essentiality of mammalian SPP-mediated viral protein processing during infection is emerging.

AREAS COVERED

This review introduces the discovery and biological function of human SPP enzymes and identify parasitic homologues as pharmacological targets of both SPP and HIV protease inhibitors. Later, the role of mammalian SPP during viral infection and how disruption of host SPP can be employed as a novel antiviral therapy are examined and discussed.

EXPERT OPINION

Parasitic and viral infections cause severe health and economic burden, exacerbated by the lack of new therapeutics in the pipeline. SPP has been shown to be essential for malaria parasite growth and encouraging evidence in other parasites demonstrates broad essentiality of these proteases as therapeutic targets. As drug resistant parasite and viruses emerge, SPP inhibition will provide a new generation of compounds to counter the growing threat of antimicrobial resistance.

Very-long-chain fatty acid metabolic capacity of 17-beta-hydroxysteroid dehydrogenase type 12 (HSD17B12) promotes replication of hepatitis C virus and related flaviviruses

Flaviviridae infections represent a major global health burden. By deciphering mechanistic aspects of hepatitis C virus (HCV)-host interactions, one could discover common strategy for inhibiting the replication of related flaviviruses. By elucidating the HCV interactome, we identified the 17-beta-hydroxysteroid dehydrogenase type 12 (HSD17B12) as a human hub of the very-long-chain fatty acid (VLCFA) synthesis pathway and core interactor. Here we show that HSD17B12 knockdown (KD) impairs HCV replication and reduces virion production. Mechanistically, depletion of HSD17B12 induces alterations in VLCFA-containing lipid species and a drastic reduction of lipid droplets (LDs) that play a critical role in virus assembly. Oleic acid supplementation rescues viral RNA replication and production of infectious particles in HSD17B12 depleted cells, supporting a specific role of VLCFA in HCV life cycle. Furthermore, the small-molecule HSD17B12 inhibitor, INH-12, significantly reduces replication and infectious particle production of HCV as well as dengue virus and Zika virus revealing a conserved requirement across Flaviviridae virus family. Overall, the data provide a strong rationale for the advanced evaluation of HSD17B12 inhibition as a promising broad-spectrum antiviral strategy for the treatment of Flaviviridae infections.

Role of cleavage at the core-E1 junction of hepatitis C virus polyprotein in viral morphogenesis

In hepatitis C virus (HCV) polyprotein sequence, core protein terminates with E1 envelope signal peptide. Cleavage by signal peptidase (SP) separates E1 from the complete form of core protein, anchored in the endoplasmic reticulum (ER) membrane by the signal peptide. Subsequent cleavage of the signal peptide by signal-peptide peptidase (SPP) releases the mature form of core protein, which preferentially relocates to lipid droplets. Both of these cleavages are required for the HCV infectious cycle, supporting the idea that HCV assembly begins at the surface of lipid droplets, yet SPP-catalyzed cleavage is dispensable for initiation of budding in the ER. Here we have addressed at what step(s) of the HCV infectious cycle SP-catalyzed cleavage at the core-E1 junction is required. Taking advantage of the sole system that has allowed visualization of HCV budding events in the ER lumen of mammalian cells, we showed that, unexpectedly, mutations abolishing this cleavage did not prevent but instead tended to promote the initiation of viral budding. Moreover, even though no viral particles were released from Huh-7 cells transfected with a full-length HCV genome bearing these mutations, intracellular viral particles containing core protein protected by a membrane envelope were formed. These were visualized by electron microscopy as capsid-containing particles with a diameter of about 70 nm and 40 nm before and after delipidation, respectively, comparable to intracellular wild-type particle precursors except that they were non-infectious. Thus, our results show that SP-catalyzed cleavage is dispensable for HCV budding per se, but is required for the viral particles to acquire their infectivity and secretion. These data support the idea that HCV assembly occurs in concert with budding at the ER membrane. Furthermore, capsid-containing particles did not accumulate in the absence of SP-catalyzed cleavage, suggesting the quality of newly formed viral particles is controlled before secretion.

Modulation of triglyceride and cholesterol ester synthesis impairs assembly of infectious hepatitis C virus

In hepatitis C virus infection, replication of the viral genome and virion assembly are linked to cellular metabolic processes. In particular, lipid droplets, which store principally triacylglycerides (TAGs) and cholesterol esters (CEs), have been implicated in production of infectious virus. Here, we examine the effect on productive infection of triacsin C and YIC-C8-434, which inhibit synthesis of TAGs and CEs by targeting long-chain acyl-CoA synthetase and acyl-CoA:cholesterol acyltransferase, respectively. Our results present high resolution data on the acylglycerol and cholesterol ester species that were affected by the compounds. Moreover, triacsin C, which blocks both triglyceride and cholesterol ester synthesis, cleared most of the lipid droplets in cells. By contrast, YIC-C8-434, which only abrogates production of cholesterol esters, induced an increase in size of droplets. Although both compounds slightly reduced viral RNA synthesis, they significantly impaired assembly of infectious virions in infected cells. In the case of triacsin C, reduced stability of the viral core protein, which forms the virion nucleocapsid and is targeted to the surface of lipid droplets, correlated with lower virion assembly. In addition, the virus particles that were released from cells had reduced specific infectivity. YIC-C8-434 did not alter the association of core with lipid droplets but appeared to decrease production of infectious virus particles, suggesting a block in virion assembly. Thus, the compounds have antiviral properties, indicating that targeting synthesis of lipids stored in lipid droplets might be an option for therapeutic intervention in treating chronic hepatitis C virus infection.

N-terminal AH2 segment of protein NS4B from hepatitis C virus. Binding to and interaction with model biomembranes

HCV NS4B, a highly hydrophobic protein involved in the alteration of the intracellular host membranes forming the replication complex, plays a critical role in the HCV life cycle. NS4B is a multifunctional membrane protein that possesses different regions where diverse and significant functions are located. One of these important regions is the AH2 segment, which besides being highly conserved has been shown to play a significant role in NS4B functioning. We have carried out an in-depth biophysical study aimed at the elucidation of the capacity of this region to interact, modulate and disrupt membranes, as well as to study the structural and dynamic features relevant for that disruption. We show that a peptide derived from this region, NS4BAH2, is capable of specifically binding phosphatidyl inositol phosphates with high affinity, and its interfacial properties suggest that this segment could behave similarly to a pre-transmembrane domain partitioning into and interacting with the membrane depending on the membrane composition and/or other proteins. Moreover, NS4BAH2 is capable of rupturing membranes even at very low peptide-to-lipid ratios and its membrane-activity is modulated by lipid composition. NS4BAH2 is located in a shallow position in the membrane but it is able to affect the lipid environment from the membrane surface down to the hydrophobic core. The NS4B region where peptide NS4BAH2 resides might have an essential role in the membrane replication and/or assembly of the viral particle through the modulation of the membrane structure and hence the replication complex.

High-throughput screening and rapid inhibitor triage using an infectious chimeric Hepatitis C virus

The recent development of a Hepatitis C virus (HCV) infectious virus cell culture model system has facilitated the development of whole-virus screening assays which can be used to interrogate the entire virus life cycle. Here, we describe the development of an HCV growth assay capable of identifying inhibitors against all stages of the virus life cycle with assay throughput suitable for rapid screening of large-scale chemical libraries. Novel features include, 1) the use of an efficiently-spreading, full-length, intergenotypic chimeric reporter virus with genotype 1 structural proteins, 2) a homogenous assay format compatible with miniaturization and automated liquid-handling, and 3) flexible assay end-points using either chemiluminescence (high-throughput screening) or Cellomics ArrayScan™ technology (high-content screening). The assay was validated using known HCV antivirals and through a large-scale, high-throughput screening campaign that identified novel and selective entry, replication and late-stage inhibitors. Selection and characterization of resistant viruses provided information regarding inhibitor target and mechanism. Leveraging results from this robust whole-virus assay represents a critical first step towards identifying inhibitors of novel targets to broaden the spectrum of antivirals for the treatment of HCV.

Lipid droplet binding of hepatitis C virus core protein genotype 3

Background. Hepatitis C virus (HCV) genotype 3 is known to cause steatosis (fatty liver) that is more frequent and severe than other genotypes. We previously identified sequence elements within genotype 3 HCV Core domain 3 that were sufficient for lipid accumulation. Aims. We examined various genotype 3 Core domains for lipid droplet localization and compared the lipid droplet binding regions of domain 2 with a genotype 1 isolate. Methods. We generated HCV Core domain constructs fused with green fluorescent protein and performed immunofluorescence to visualize lipid droplets. Results. Constructs containing HCV Core domain 2 are appropriately localized to lipid droplets with varying degrees of efficiency. When compared to genotype 1, there are polymorphisms within domain 2 that do not appear to alter lipid droplet localization. Conclusions. In summary, the differences in a steatosis-associated HCV Core genotype 3 isolate do not appear to involve altered lipid droplet localization.

Interaction with membranes of the full C-terminal domain of protein NS4B from hepatitis C virus

Hepatitis C virus (HCV) NS4B protein is a transmembrane highly hydrophobic protein responsible for many key aspects of the viral replication process. The C-terminal part of NS4B is essential for replication and is a potential target for HCV replication inhibitors. In this work we have carried out a study of the binding to and interaction with model biomembranes of a peptide corresponding to the C-terminal domain of NS4B, NS4B(Cter). We show that NS4B(Cter) partitions into phospholipid membranes, is capable of rupturing membranes even at very low peptide-to-lipid ratios and its membrane-activity is modulated by lipid composition. NS4B(Cter) is located in a shallow position in the membrane but it is able to affect the lipid environment from the membrane surface down to the hydrophobic core. Our results identify the C-terminal region of the HCV NS4B protein as a membrane interacting domain, and therefore directly implicated in the HCV life cycle and possibly in the formation of the membranous web.

Production of infectious hepatitis C virus in primary cultures of human adult hepatocytes

BACKGROUND & AIMS

Although hepatitis C virus (HCV) can be grown in the hepatocarcinoma-derived cell line Huh-7, a cell-culture model is needed that supports its complete, productive infection cycle in normal, quiescent, highly differentiated human hepatocytes. We sought to develop such a system.

METHODS

Primary cultures of human adult hepatocytes were inoculated with HCV derived from Huh-7 cell culture (HCVcc) and monitored for expression of hepatocyte differentiation markers and replication of HCV. Culture supernatants were assayed for HCV RNA, core antigen, and infectivity titer. The buoyant densities of input and progeny virus were compared in iodixanol gradients.

RESULTS

While retaining expression of differentiation markers, primary hepatocytes supported the complete infectious cycle of HCV, including production of significant titers of new infectious progeny virus, which was called primary-culture-derived virus (HCVpc). Compared with HCVcc, HCVpc had lower average buoyant density and higher specific infectivity; this was similar to the characteristics of virus particles associated with the very-low-density lipoproteins that are produced during in vivo infection. These properties were lost after re-culture of HCVpc in poorly differentiated Huh-7 cells, suggesting that authentic virions can be produced only by normal hepatocytes that secrete authentic very-low-density lipoproteins.

CONCLUSIONS

We have established a cell-culture-based system that allows production of infectious HCV in physiologically relevant human hepatocytes. This provides a useful tool for the study of HCV interactions with its natural host cell and for the development of antiviral therapies.

Sequential processing of hepatitis C virus core protein by host cell signal peptidase and signal peptide peptidase: a reassessment

Hepatitis C virus (HCV) core protein is believed to play critical roles in the virus morphogenesis and pathogenesis. In HCV polyprotein, core protein terminates with a signal peptide followed by E1 envelope protein. It has remained unclear whether cleavage by host cell signal peptidase (SP) at the core-E1 junction to generate the complete form of core protein, which is anchored in the endoplasmic reticulum membrane, is absolutely required for cleavage within the signal peptide by host cell signal peptide peptidase (SPP) to liberate the mature form of core protein, which is then free for trafficking to lipid droplets. In this study, the possible sources of disagreement in published reports have been examined, and we conclude that a product generated upon inhibition of SP-catalysed cleavage at the core-E1 junction in heterologous expression systems was incorrectly identified as mature core protein. Moreover, inhibition of this cleavage in the most relevant model of human hepatoma cells replicating a full-length HCV genome was shown to abolish interaction of core protein with lipid droplets and production of infectious progeny virus. These results firmly establish that SPP-catalysed liberation of mature core protein is absolutely dependent on prior cleavage by SP at the correct core-E1 site to generate the complete form of core protein, consistent with this obligatory order of processing playing a role in HCV infectious cycle.

Biophysical characterization of the fusogenic region of HCV envelope glycoprotein E1

We have studied the binding and interaction of the peptide E1(FP) with various model membranes. E1(FP) is derived from the amino acid segment 274-291 of the hepatitis C virus envelope glycoprotein E1, which was previously proposed to host the peptide responsible for fusion to target membranes. In the present study we addressed the changes which take place upon E1(FP) binding in both the peptide and the phospholipid bilayer, respectively, through a series of complementary experiments. We show that peptide E1(FP) binds to and interacts with phospholipid model membranes, modulates the polymorphic phase behavior of membrane phospholipids, is localized in a shallow position in the membrane and interacts preferentially with cholesterol. The capability of modifying the biophysical properties of model membranes supports its role in HCV-mediated membrane fusion and suggests that the mechanism of membrane fusion elicited by class I and II fusion proteins might be similar.

A membranotropic region in the C-terminal domain of hepatitis C virus protein NS4B interaction with membranes

We have identified a membrane-active region in the HCV NS4B protein by studying membrane rupture induced by a NS4B-derived peptide library on model membranes. This segment corresponds to one of two previously predicted amphipathic helix and define it as a new membrane association domain. We report the binding and interaction with model membranes of a peptide patterned after this segment, peptide NS4B(H2), and show that NS4B(H2) strongly partitions into phospholipid membranes, interacts with them, and is located in a shallow position in the membrane. Furthermore, changes in the primary sequence cause the disruption of the hydrophobicity along the structure and prevent the resulting peptide from interacting with the membrane. Our results suggest that the region where the NS4B(H2) is located might have an essential role in the membrane replication and/or assembly of the viral particle through the modulation of the replication complex. Our findings therefore identify an important region in the HCV NS4B protein which might be implicated in the HCV life cycle and possibly in the formation of the membranous web.

Characterization of hepatitis C virus core protein multimerization and membrane envelopment: revelation of a cascade of core-membrane interactions

The molecular basis underlying hepatitis C virus (HCV) core protein maturation and morphogenesis remains elusive. We characterized the concerted events associated with core protein multimerization and interaction with membranes. Analyses of core proteins expressed from a subgenomic system showed that the signal sequence located between the core and envelope glycoprotein E1 is critical for core association with endoplasmic reticula (ER)/late endosomes and the core's envelopment by membranes, which was judged by the core's acquisition of resistance to proteinase K digestion. Despite exerting an inhibitory effect on the core's association with membranes, (Z-LL)(2)-ketone, a specific inhibitor of signal peptide peptidase (SPP), did not affect core multimeric complex formation, suggesting that oligomeric core complex formation proceeds prior to or upon core attachment to membranes. Protease-resistant core complexes that contained both innate and processed proteins were detected in the presence of (Z-LL)(2)-ketone, implying that core envelopment occurs after intramembrane cleavage. Mutations of the core that prevent signal peptide cleavage or coexpression with an SPP loss-of-function D219A mutant decreased the core's envelopment, demonstrating that SPP-mediated cleavage is required for core envelopment. Analyses of core mutants with a deletion in domain I revealed that this domain contains sequences crucial for core envelopment. The core proteins expressed by infectious JFH1 and Jc1 RNAs in Huh7 cells also assembled into a multimeric complex, associated with ER/late-endosomal membranes, and were enveloped by membranes. Treatment with (Z-LL)(2)-ketone or coexpression with D219A mutant SPP interfered with both core envelopment and infectious HCV production, indicating a critical role of core envelopment in HCV morphogenesis. The results provide mechanistic insights into the sequential and coordinated processes during the association of the HCV core protein with membranes in the early phase of virus maturation and morphogenesis.

Making the cut: central roles of intramembrane proteolysis in pathogenic microorganisms

Proteolysis in cellular membranes to liberate effector domains from their transmembrane anchors is a well-studied regulatory mechanism in animal biology and disease. By contrast, the function of intramembrane proteases in unicellular organisms has received little attention. Recent progress has now established that intramembrane proteases execute pivotal roles in a range of pathogens, from regulating Mycobacterium tuberculosis envelope composition, cholera toxin production, bacterial adherence and conjugation, to malaria parasite invasion, fungal virulence, immune evasion by parasitic amoebae and hepatitis C virus assembly. These advances raise the exciting possibility that intramembrane proteases may serve as targets for combating a wide range of infectious diseases. This Review focuses on summarizing the advances, evaluating the limitations and highlighting the promise of this newly emerging field.

Signal peptide peptidases: a family of intramembrane-cleaving proteases that cleave type 2 transmembrane proteins

Five genes encode the five human signal peptide peptidases (SPPs), which are intramembrane-cleaving aspartyl proteases (aspartyl I-CLiPs). SPPs have been conserved through evolution with family members found in higher eukaryotes, fungi, protozoa, arachea, and plants. SPPs are related to the presenilin family of aspartyl I-CLiPs but differ in several key aspects. Presenilins (PSENs) and SPPs both cleave the transmembrane region of membrane proteins; however, PSENs cleave type 1 membrane proteins whereas SPPs cleave type 2 membrane proteins. Though the overall homology between SPPs and PSENs is minimal, they are multipass membrane proteins that contain two conserved active site motifs YD and GxGD in adjacent membrane-spanning domains and a conserved PAL motif of unknown function near their COOH-termini. They differ in that the active site YD and GxGD containing transmembrane domains of SPPs are inverted relative to PSENs, thus, orienting the active site in a consistent topology relative to the substrate. At least two of the human SPPs (SPP and SPPL3) appear to function without additional cofactors, but PSENs function as a protease, called gamma-secretase, only when complexed with Nicastrin, APH-1 and Pen-2. The biological roles of SPP are largely unknown, and only a few endogenous substrates for SPPs have been identified. Nevertheless there is emerging evidence that SPP family members are highly druggable and may regulate both essential physiologic and pathophysiologic processes. Further study of the SPP family is needed in order to understand their biological roles and their potential as therapeutic targets.

Screening a peptide library by DSC and SAXD: comparison with the biological function of the parent proteins

We have recently identified the membranotropic regions of the hepatitis C virus proteins E1, E2, core and p7 proteins by observing the effect of protein-derived peptide libraries on model membrane integrity. We have studied in this work the ability of selected sequences of these proteins to modulate the L_β-L_α and L_α-H_II phospholipid phase transitions as well as check the viability of using both DSC and SAXD to screen a protein-derived peptide library. We demonstrate that it is feasible to screen a library of peptides corresponding to one or several proteins by both SAXD and DSC. This methodological combination should allow the identification of essential regions of membrane-interacting proteins which might be implicated in the molecular mechanism of membrane fusion and/or budding.

Intramembrane processing by signal peptide peptidase regulates the membrane localization of hepatitis C virus core protein and viral propagation

Hepatitis C virus (HCV) core protein has shown to be localized in the detergent-resistant membrane (DRM), which is distinct from the classical raft fraction including caveolin, although the biological significance of the DRM localization of the core protein has not been determined. The HCV core protein is cleaved off from a precursor polyprotein at the lumen side of Ala(191) by signal peptidase and is then further processed by signal peptide peptidase (SPP) within the transmembrane region. In this study, we examined the role of SPP in the localization of the HCV core protein in the DRM and in viral propagation. The C terminus of the HCV core protein cleaved by SPP in 293T cells was identified as Phe(177) by mass spectrometry. Mutations introduced into two residues (Ile(176) and Phe(177)) upstream of the cleavage site of the core protein abrogated processing by SPP and localization in the DRM fraction. Expression of a dominant-negative SPP or treatment with an SPP inhibitor, L685,458, resulted in reductions in the levels of processed core protein localized in the DRM fraction. The production of HCV RNA in cells persistently infected with strain JFH-1 was impaired by treatment with the SPP inhibitor. Furthermore, mutant JFH-1 viruses bearing SPP-resistant mutations in the core protein failed to propagate in a permissive cell line. These results suggest that intramembrane processing of HCV core protein by SPP is required for the localization of the HCV core protein in the DRM and for viral propagation.

Hepatitis C virus core protein binding to lipid membranes: the role of domains 1 and 2

We have analysed and identified different membrane-active regions of the Hepatitis C virus (HCV) core protein by observing the effect of 18-mer core-derived peptide libraries from two HCV strains on the integrity of different membrane model systems. In addition, we have studied the secondary structure of specific membrane-interacting peptides from the HCV core protein, both in aqueous solution and in the presence of model membrane systems. Our results show that the HCV core protein region comprising the C-terminus of domain 1 and the N-terminus of domain 2 seems to be the most active in membrane interaction, although a role in protein-protein interaction cannot be excluded. Significantly, the secondary structure of nearly all the assayed peptides changes in the presence of model membranes. These sequences most probably play a relevant part in the biological action of HCV in lipid interaction. Furthermore, these membranotropic regions could be envisaged as new possible targets, as inhibition of its interaction with the membrane could potentially lead to new vaccine strategies.

Interaction of the most membranotropic region of the HCV E2 envelope glycoprotein with membranes. Biophysical characterization

The previously identified membrane-active regions of the hepatitis C virus (HCV) E1 and E2 envelope glycoproteins led us to identify different segments that might be implicated in viral membrane fusion, membrane interaction, and/or protein-protein binding. HCV E2 glycoprotein contains one of the most membranotropic segments, segment 603-634, which has been implicated in CD81 binding, E1/E2 and E2/E2 dimerization, and membrane interaction. Through a series of complementary experiments, we have carried out a study of the binding and interaction with the lipid bilayer of a peptide corresponding to segment 603-634, peptide E2(FP), as well as the structural changes induced by membrane binding that take place in both the peptide and the phospholipid molecules. Here, we demonstrate that peptide E2(FP) binds to and interacts with phospholipid model membranes, modulates the polymorphic phase behavior of membrane phospholipids, is localized in a shallow position in the membrane, and is probably oligomerized in the presence of membranes. These data support the role of E2(FP) in HCV-mediated membrane fusion, and sustain the notion that this segment of the E2 envelope glycoprotein, together with other segments of E2 and E1 glycoproteins, provides the driving force for the merging of the viral and target cell membranes.

The signal peptide of the mouse mammary tumor virus Rem protein is released from the endoplasmic reticulum membrane and accumulates in nucleoli

N-terminal signal sequences mediate endoplasmic reticulum (ER) targeting and insertion of nascent secretory and membrane proteins and are, in most cases, cleaved off by signal peptidase. The mouse mammary tumor virus envelope protein and its alternative splice variant Rem have an unusually long signal sequence, which contains a nuclear localization signal. Although the envelope protein is targeted to the ER, inserted, and glycosylated, Rem has been described as a nuclear protein. Rem as well as a truncated version identical to the cleaved signal sequence have been shown to function as nuclear export factors for intron-containing transcripts. Using transiently transfected cells, we found that Rem is targeted to the ER, where the C-terminal portion is translocated and glycosylated. The signal sequence is cleaved off and accumulates in nucleoli. In a cell-free in vitro system, the generation of the Rem signal peptide depends on the presence of microsomal membranes. In vitro and in cells, the signal peptide initially accumulates in the membrane and is subsequently released into the cytosol. This release does not depend on processing by signal peptide peptidase, an intramembrane cleaving protease that can mediate the liberation of signal peptide fragments from the ER membrane. Our study suggests a novel pathway by which a signal peptide can be released from the ER membrane to fulfill a post-targeting function in a different compartment.

Identification of the membrane-active regions of hepatitis C virus p7 protein: biophysical characterization of the loop region

We have identified the membrane-active regions of the hepatitis C virus p7 protein by performing an exhaustive study of membrane rupture, hemifusion, and fusion induced by a p7-derived peptide library on model membranes having different phospholipid compositions. We report the identification in p7 of a highly membranotropic region located at the loop domain of the protein. Here, we have investigated the interaction of a peptide patterned after the p7 loop (peptide p7(L)), studying its binding and interaction with the lipid bilayer, and evaluated the binding-induced structural changes of the peptide and the phospholipids. We show that positively rich p7(L) strongly binds to negatively charged phospholipids and it is localized in a shallow position in the bilayer. Furthermore, peptide p7(L) exhibits a high tendency to oligomerize in the presence of phospholipids, which could be the driving force for the formation of the active ion channel. Therefore, our findings suggest that the p7 loop could be an attractive candidate for antiviral drug development, because it could be a target for antiviral compounds that may lead to new vaccine strategies.

Evidence for cellular uptake of recombinant hepatitis C virus non-enveloped capsid-like particles

Although the hepatitis C virus (HCV) is an enveloped virus, naked nucleocapsids have been reported in the serum of infected patients, and most recently novel HCV subgenomes with deletions of the envelope proteins have been identified. However the significance of these findings remains unclear. In this study, we used the baculovirus expression system to generate recombinant HCV capsid-like particles, and investigated their possible interactions with cells. We show that expression of HCV core in insect cells can sufficiently direct the formation of capsid-like particles in the absence of the HCV envelope glycoproteins and of the 5' untranslated region. By confocal microscopy analysis, we provide evidence that the naked capsid-like particles could be uptaken by human hepatoma cells. Moreover, our findings suggest that they have the potential to produce cell-signaling effects.

Alanine scanning of the hepatitis C virus core protein reveals numerous residues essential for production of infectious virus

Hepatitis C virus (HCV) is an important human pathogen affecting an estimated 3% of the world's population. Recent advances have enabled in vitro propagation of the virus and allow assembly and egress to be investigated for the first time. As a component of the virion, the HCV core protein likely functions primarily in infectious virus production, although little is known about the determinants of this activity. To investigate the roles of core in the viral life cycle, we performed a comprehensive deletion and alanine scanning mutagenesis study of this protein in the context of a genotype 2a reporter virus. We have confirmed that core protein is essential for infectious virion production and have identified numerous residues required for this role. The infectivity of several assembly-defective core mutants could be rescued by compensatory mutations identified in p7 and NS2, suggesting genetic interactions with core and highlighting the importance of these nonstructural proteins in infectious virion morphogenesis.

The hepatitis C virus life cycle as a target for new antiviral therapies

The burden of disease consequent to hepatitis C virus (HCV) infection has been well described and is expected to increase dramatically over the next decade. Current approved antiviral therapies are effective in eradicating the virus in approximately 50% of infected patients. However, pegylated interferon and ribavirin-based therapy is costly, prolonged, associated with significant adverse effects, and not deemed suitable for many HCV-infected patients. As such, there is a clear and pressing need for the development of additional agents that act through alternate or different mechanisms, in the hope that such regimens could lead to enhanced response rates more broadly applicable to patients with hepatitis C infection. Recent basic science enhancements in HCV cell culture systems and replication assays have led to a broadening of our understanding of many of the mechanisms of HCV replication and, therefore, potential novel antiviral targets. In this article, we have attempted to highlight important new information as it relates to our understanding of the HCV life cycle. These steps broadly encompass viral attachment, entry, and fusion; viral RNA translation; posttranslational processing; HCV replication; and viral assembly and release. In each of these areas, we present up-to-date knowledge of the relevant aspects of that component of the viral life cycle and then describe the preclinical and clinical development targets and pathways being explored in the translational and clinical settings.