The N-terminal half of the core protein of hepatitis C virus is sufficient for nucleocapsid formation

Signal peptide peptidase promotes the formation of hepatitis C virus non-enveloped particles and is captured on the viral membrane during assembly

The maturation of the core protein (C) of Hepatitis C virus (HCV) is controlled by the signal peptidase (sp) and signal peptide peptidase (spp) of the host. To date, it remains unknown whether spp cleavage influences viral infectivity and/or the assembly process. Here, evidence is provided that cleavage by spp is not required for assembly of nucleocapsid-like particles (NLPs) in yeast (Pichia pastoris). The immature NLPs (not processed by spp) show a density of 1.11 g ml(-1) on sucrose gradients and a diameter of 50 nm. Co-expression of human spp (hspp) with C generates the 21 kDa mature form of the protein and promotes the accumulation of non-enveloped particles. The amount of non-enveloped particles accumulating in the cell was correlated directly with the expression level of hspp. Furthermore, immunocapture studies showed that hspp was embedded in the membranes of enveloped particles. These results suggest that maturation of the C protein can occur after formation of the enveloped particles and that the abundance of hspp influences the types of particle accumulating in the cells.

Hepatitis C virus core protein is a dimeric alpha-helical protein exhibiting membrane protein features

The building block of hepatitis C virus (HCV) nucleocapsid, the core protein, together with viral RNA, is composed of different domains involved in RNA binding and homo-oligomerization. The HCV core protein 1-169 (C(HCV)169) and its N-terminal region from positions 1 to 117 (C(HCV)117) were expressed in Escherichia coli and purified to homogeneity suitable for biochemical and biophysical characterizations. The overall conformation and the oligomeric properties of the resulting proteins C(HCV)169 and C(HCV)117 were investigated by using analytical centrifugation, circular dichroism, intrinsic fluorescence measurements, and limited proteolysis. Altogether, our results show that core protein (C(HCV)169) behaves as a membranous protein and forms heterogeneous soluble micelle-like aggregates of high molecular weight in the absence of detergent. In contrast, it behaves, in the presence of mild detergent, as a soluble, well-folded, noncovalent dimer. Similar to findings observed for core proteins of HCV-related flaviviruses, the HCV core protein is essentially composed of alpha-helices (50%). In contrast, C(HCV)117 is soluble and monodispersed in the absence of detergent but is unfolded. It appears that the folding of the highly basic domain from positions 2 to 117 (2-117 domain) depends on the presence of the 117-169 hydrophobic domain, which contains the structural determinants ensuring the binding of core with cellular membranes. Finally, our findings provide valuable information for further investigations on isolated core protein, as well as for attempts to reconstitute nucleocapsid particles in vitro.

Screening of hepatocyte proteins binding to F protein of hepatitis C virus by yeast two-hybrid system

AIM: To investigate the biological function of F protein by yeast two-hybrid system.

METHODS: We constructed F protein bait plasmid by cloning the gene of F protein into pGBKT7, then recombinant plasmid DNA was transformed into yeast AH109 (a type). The transformed yeast AH109 was mated with yeast Y187 (α type) containing liver cDNA library plasmid in 2×YPDA medium. Diploid yeast was plated on synthetic dropout nutrient medium (SD/-Trp-Leu-His-Ade) containing X-α-gal for selection and screening. After extracting and sequencing plasmids from positive (blue) colonies, we underwent sequence analysis by bioinformatics.

RESULTS: Thirty-six colonies were selected and sequenced. Among them, 11 colonies were zymogen granule protein, 5 colonies were zinc finger protein, 4 colonies were zinc-α-2-glycoprotein, 1 colony was sialyltransferase, 1 colony was complement control protein factor I, 1 colony was vitronectin, and 2 colonies were new genes with unknown function.

CONCLUSION: The yeast two-hybrid system is an effective method for identifying hepatocyte proteins interacting with F protein of hepatitis C virus. F protein may bind to different proteins.

Analysis of self-association of West Nile virus capsid protein and the crucial role played by Trp 69 in homodimerization

The understanding of capsid (C) protein interactions with itself would provide important data on how the core is organized in flaviviruses during assembly. In this study, West Nile (WN) virus C protein was shown to form homodimers using yeast two-hybrid analysis in conjunction with mammalian two-hybrid and in vivo co-immunoprecipitation assays. To delineate the region on the C protein which mediates C-C dimerization, truncation studies were carried out. The results obtained clearly showed that the internal hydrophobic segment flanked by helix I and helix III of WN virus C protein is essential for the self-association of C protein. The crucial role played by Trp 69 in stabilizing the self-association of C protein was also demonstrated by mutating Trp to Gly/Arg/Phe. Substitution of the Trp residue with Gly/Arg abolished the dimerization, whereas substitution with Phe decreased the self-association significantly. The results of this study pinpoint a critical residue in the C protein that potentially plays a role in stabilizing the homotypic interaction.

In vitro assembly into virus-like particles is an intrinsic quality of Pichia pastoris derived HCV core protein

Different variants of hepatitis C virus core protein (HCcAg) have proved to self-assemble in vitro into virus-like particles (VLPs). However, difficulties in obtaining purified mature HCcAg have limited these studies. In this study, a high degree of monomeric HCcAg purification was accomplished using chromatographic procedures under denaturing conditions. Size exclusion chromatography and sucrose density gradient centrifugation of renatured HCcAg (in the absence of structured RNA) under reducing conditions suggested that it assembled into empty capsids. The electron microscopy analysis of renatured HCcAg showed the presence of spherical VLPs with irregular shapes and an average diameter of 35nm. Data indicated that HCcAg monomers assembled in vitro into VLPs in the absence of structured RNA, suggesting that recombinant HCcAg used in this work contains all the information necessary for the assembly process. However, they also suggest that some cellular factors might be required for the proper in vitro assembly of capsids.

Nucleic acid binding properties and intermediates of HCV core protein multimerization in Pichia pastoris

Little is known about the in vivo assembly pathway or structure of the hepatitis C virus nucleocapsid. In this work the intermediates of HCcAg multimerization in Pichia pastoris cells and the nucleic acid binding properties of structured nucleocapsid-like particles (NLPs) were studied. Extensive cross-linking was observed for HCcAg after glutaraldehyde treatment. Data suggest that HCcAg exists in dimeric forms probably representing P21-P21, P21-P23, and P23-P23 dimers. In addition, the presence of HCcAg species that might represent trimers and multimers was observed. After sucrose equilibrium density gradient purification and nuclease digestion, NLPs were shown to contain both RNA and DNA molecules. Finally, the analysis by electron microscopy indicated that native NLPs were resistant to nuclease treatment. These results indicated that HCcAg assembles through dimers, trimers, and multimers' intermediates into capsids in P. pastoris cells. Assembly of NLPs in its natural environment might confer stability to these particles by adopting a compact structure.