Intracellular infectious hepatitis C virus (HCV) particles display a distinctly higher buoyant density than do secreted virus particles, suggesting that the characteristic low density of extracellular HCV particles is acquired during viral egress. We took advantage of this difference to examine the determinants of assembly, maturation, degradation, and egress of infectious HCV particles. The results demonstrate that HCV assembly and maturation occur in the endoplasmic reticulum (ER) and post-ER compartments, respectively, and that both depend on microsomal transfer protein and apolipoprotein B, in a manner that parallels the formation of very-low-density lipoproteins (VLDL). In addition, they illustrate that only low-density particles are efficiently secreted and that immature particles are actively degraded, in a proteasome-independent manner, in a post-ER compartment of the cell. These results suggest that by coopting the VLDL assembly, maturation, degradation, and secretory machinery of the cell, HCV acquires its hepatocyte tropism and, by mimicry, its tendency to persist.Hepatitis C virus (HCV) establishes persistent infection in Ͼ70% of infected individuals (25), and over 170 million people are persistently infected worldwide. Persistent HCV infection is associated with a chronic inflammatory disease (hepatitis) that ultimately leads to hepatic fibrosis, cirrhosis, and hepatocellular carcinoma (25). Chronic infection is also associated with disorders of lipid metabolism (54), with abnormal accumulation of lipids in the liver parenchymal cells (steatosis) and reduced serum beta-lipoprotein levels (52). Currently, the only approved antiviral therapy for HCV is the administration of type I interferon combined with ribavirin. However, this therapy is toxic and is effective in only a fraction of cases (43).HCV is the sole member of the genus Hepacivirus, which belongs to the Flaviviridae family. The virus is enveloped, and the single-stranded positive-strand RNA genome contains a single open reading frame flanked by untranslated regions (5Ј UTR and 3Ј UTR) that contain RNA sequences essential for RNA translation and replication (17,18). Translation of the single open reading frame is driven by an internal ribosomal entry site (IRES) sequence present within the 5Ј UTR (24), and the resulting polyprotein, of approximately 3,000 amino acids in length, is processed by cellular and viral proteases into its individual components (44). The nonstructural proteins NS3, NS4A, NS4B, NS5A, and NS5B are sufficient to support efficient HCV RNA replication in membranous compartments in the cytosol (10,35,39). Overexpression of the core, E1, and E2 proteins is sufficient for the formation of virus-like structures in insect cells (6), and expression of the viral polyprotein leads to the formation of virus-like particles in HeLa (38) and Huh-7 (23) cells. It has been proposed that infectious particles are assembled when genomic RNA-containing core particles bud through the endoplasmic reticulum (ER) membrane (47), acquiring the v...

A conventional affinity protein purification system often requires a separate protease to separate the target protein from the affinity tag. This paper describes a unique protein purification system in which the target protein is fused to the C-terminus of a modified protein splicing element (intein). A small affinity tag is inserted in a loop region of the endonuclease domain of the intein to allow affinity purification. Specific mutations at the C-terminal splice junction of the intein allow controllable C-terminal peptide bond cleavage. The cleavage is triggered by addition of thiols such as dithiothreitol or free cysteine, resulting in elution of the target protein while the affinity-tagged intein remains immobilized on the affinity column. This system eliminates the need for a separate protease and allows purification of a target protein without the N-terminal methionine. We have constructed general cloning vectors and demonstrated single-column purification of several proteins. In addition, we discuss several factors that may affect the C-terminal peptide bond cleavage activity.

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A major portion of newly synthesized apolipoprotein B (apoB) is degraded intracellularly. This degradation has been demonstrated to be mediated largely by the ubiquitin-proteasome pathway. We examined whether nascent apoB polypeptides or full-length apoB is selectively retrotranslocated from the endoplasmic reticulum into the cytosol for degradation. Herein, we found that full-length apoB as well as partial-length apoB peptides are ubiquitinated in HepG2 cells, and ubiquitination is an exclusively cytosolic process. Calnexin, which binds specifically to glycoproteins, has been postulated to promote apoB folding and complete translocation; we found that ubiquitinated apoB is bound to calnexin, suggesting that ubiquitinated apoB is glycosylated. In addition to calnexin binding, we have other pieces of evidence that the full-length intracellular ubiquitinated apoB is glycosylated, because (i) it binds to concanavalin A, and (ii) glycan can be demonstrated in the fulllength ubiquitinated apoB by a chemical detection method involving oxidation of adjacent hydroxyl groups in the glycan moiety. Because glycosylation occurs inside the endoplasmic reticulum, the full-length glycosylated apoB must have been retrotranslocated into the cytosol for ubiquitination and proteasome-mediated degradation. Next we synchronized translation in HepG2 cells by puromycin treatment. A pulse-chase experiment using [35 S]methionine labeling of intracellular apoB in these synchronized cells demonstrated that nascent partial-length apoB peptides are also ubiquitinated cotranslationally. We conclude that the ubiquitin proteasome-mediated degradation of apoB targets both nascent peptides cotranslationally before translocation as well as full-length apoB after its translocation into the endoplasmic reticulum.

A variety of stress stimuli, including ischemia-reperfusion (I/R) injury, induce the transcriptional repressor ATF3 in the kidney. The functional consequences of this upregulation in ATF3 after renal I/R injury are not well understood. Here, we found that ATF3-deficient mice had higher renal I/R-induced mortality, kidney dysfunction, inflammation (number of infiltrating neutrophils, myeloperoxidase activity, and induction of IL-6 and P-selectin), and apoptosis compared with wild-type mice. Furthermore, gene transfer of ATF3 to the kidney rescued the renal I/R-induced injuries in the ATF3-deficient mice. Molecular and biochemical analysis revealed that ATF3 interacted directly with histone deacetylase 1 (HDAC1) and recruited HDAC1 into the ATF/NF-B sites in the IL-6 and IL-12b gene promoters. The ATF3-associated HDAC1 deacetylated histones, which resulted in the condensation of chromatin structure, interference of NF-B binding, and inhibition of inflammatory gene transcription after I/R injury. Taken together, these data demonstrate epigenetic regulation mediated by the stress-inducible gene ATF3 after renal I/R injury and suggest potential targeted approaches for acute kidney injury.