Transport and processing of the Rous sarcoma virus Gag protein in the endoplasmic reticulum

Specificity of plasma membrane targeting by the rous sarcoma virus gag protein

Budding of C-type retroviruses begins when the viral Gag polyprotein is directed to the plasma membrane by an N-terminal membrane-binding (M) domain. While dispersed basic amino acids within the M domain are critical for stable membrane association and consequent particle assembly, additional residues or motifs may be required for specific plasma membrane targeting and binding. We have identified an assembly-defective Rous sarcoma virus (RSV) Gag mutant that retains significant membrane affinity despite having a deletion of the fourth alpha-helix of the M domain. Examination of the mutant protein's subcellular distribution revealed that it was not localized to the plasma membrane but instead was mistargeted to intracytoplasmic membranes. Specific plasma membrane targeting was restored by the addition of myristate plus a single basic residue, by multiple basic residues, or by the heterologous hydrophobic membrane-binding domain from the cellular Fyn protein. These results suggest that the fourth alpha-helix of the RSV M domain promotes specific targeting of Gag to the plasma membrane, either through a direct interaction with plasma membrane phospholipids or a membrane-associated cellular factor or by maintaining the conformation of Gag to expose specific plasma membrane targeting sequences.

Insertion of capsid proteins from nonenveloped viruses into the retroviral budding pathway

Retroviral Gag proteins direct the assembly and release of virus particles from the plasma membrane. The budding machinery consists of three small domains, the M (membrane-binding), I (interaction), and L (late or "pinching-off") domains. In addition, Gag proteins contain sequences that control particle size. For Rous sarcoma virus (RSV), the size determinant maps to the capsid (CA)-spacer peptide (SP) sequence, but it functions only when I domains are present to enable particles of normal density to be produced. Small deletions throughout the CA-SP sequence result in the release of particles that are very large and heterogeneous, even when I domains are present. In this report, we show that particles of relatively uniform size and normal density are released by budding when the size determinant and I domains in RSV Gag are replaced with capsid proteins from two unrelated, nonenveloped viruses: simian virus 40 and satellite tobacco mosaic virus. These results indicate that capsid proteins of nonenveloped viruses can interact among themselves within the context of Gag and be inserted into the retroviral budding pathway merely by attaching the M and L domains to their amino termini. Thus, the differences in the assembly pathways of enveloped and nonenveloped viruses may be far simpler than previously thought.

A block to human immunodeficiency virus type 1 assembly in murine cells

Human immunodeficiency virus type 1 (HIV-1) does not replicate in murine cells. We investigated the basis of this block by infecting a murine NIH 3T3 reporter cell line that stably expressed human CD4, CCR5, and cyclin T1 and contained a transactivatable HIV-1 long terminal repeat (LTR)-green fluorescent protein (GFP) cassette. Although the virus entered efficiently, formed provirus, and was expressed at a level close to that in a highly permissive human cell line, the murine cells did not support M-tropic HIV-1 replication. To determine why the virus failed to replicate, the efficiency of each postentry step in the virus replication cycle was analyzed using vesicular stomatitis virus G pseudotypes. The murine cells supported reverse transcription and integration at levels comparable to those in the human osteosarcoma-derived cell line GHOST.R5, and human cyclin T1 restored provirus expression, consistent with earlier findings of others. The infected murine cells contained nearly as much virion protein as did the human cells but released less than 1/500 the amount of p24(gag) into the culture medium. A small amount of p24(gag) was released and was in the form of fully infectious virus. Electron microscopy suggested that aberrantly assembled virion protein had accumulated in cytoplasmic vesicular structures. Virions assembling at the cell membrane were observed but were rare. The entry of M-tropic JR.FL-pseudotyped reporter virus was moderately reduced in the murine cells, suggesting a minor reduction in coreceptor function. A small reduction in the abundance of full-length viral mRNA transcripts was also noted; however, the major block was at virion assembly. This could have been due to a failure of Gag to target to the cell membrane. This block must be overcome before a murine model for HIV-1 replication can be developed.

A subset of porcine reproductive and respiratory syndrome virus GP3 glycoprotein is released into the culture medium of cells as a non-virion-associated and membrane-free (soluble) form

The GP3 protein of the IAF-Klop strain of porcine reproductive and respiratory syndrome virus (PRRSV) was expressed in 293 cells by a recombinant human type 5 adenovirus carrying the open reading frame 3 gene. The protein exhibited a molecular mass of 42 kDa and comigrated with GP3 expressed in PRRSV-infected MARC-145 cells. Removal of N-linked glycans from GP3 resulted in a 27-kDa protein (P3), confirming its highly glycosylated nature. Pulse-chase experiments carried out either in the context of PRRSV infection or upon individual expression of GP3 in 293 cells showed that the protein remains completely endo-beta-N-acetylglucosaminidase H-sensitive even after 4 h of synthesis. Thus, the transport of GP3 was restricted to the premedial Golgi compartment, presumably the endoplasmic reticulum (ER). However, a minor fraction of GP3 was found to be secreted in the culture medium as a soluble membrane-free form. This released protein (sGP3) was readily identified upon individual expression of GP3 in 293 cells as well as in the context of PRRSV infection, albeit at lower levels. The sGP3 migrated as a smear and displayed a molecular mass ranging from 43 to 53 kDa. The unglycosylated form of sGP3 comigrated with its intracellular deglycosylated counterpart, suggesting that the release from the cell of a subset of GP3 did not result from cleavage of a putative membrane-anchor sequence. Strikingly, unlike GP3, the sGP3 acquired Golgi-specific modifications of its carbohydrate side chains and folded into a disulfide-linked homodimer. Brefeldin A treatment completely abolished the release of sGP3, suggesting that the ER-to-Golgi compartment is an obligatory step in cellular secretion of sGP3. In contrast, 10 mM monensin did not prevent sGP3 release but inhibited the terminal glycosylation that confers on the protein its diffuse pattern. Since GP3 was found to be nonstructural in the case of the North American strain, secretion of a minor fraction of GP3 might be an explanation for its high degree of immunogenicity in infected pigs. Furthermore, this secreted protein might be relevant as a model for further studies on the cellular subcompartments involved in the sorting of proteins to the extracellular milieu.

Efficient photoreceptor-targeted gene expression in vivo by recombinant adeno-associated virus

We describe a general approach for achieving efficient and cell type-specific expression of exogenous genes in photoreceptor cells of the mammalian retina. Recombinant adeno-associated virus (rAAV) vectors were used to transfer the bacterial lacZ gene or a synthetic green fluorescent protein gene (gfp) to mouse or rat retinas after injection into the subretinal space. Using a proximal murine rod opsin promoter (+86 to -385) to drive expression, reporter gene product was found exclusively in photoreceptors, not in any other retinal cell type or in the adjacent retinal pigment epithelium. GFP-expressing photoreceptors typically encompassed 10-20% of the total retinal area after a single 2-microl injection. Photoreceptors were transduced with nearly 100% efficiency in the region directly surrounding the injection site. We estimate approximately 2.5 million photoreceptors were transduced as a result of the single subretinal inoculation. This level of gene transfer and expression suggests the feasibility of genetic therapy for retinal disease. The gfp-containing rAAV stock was substantially free of both adenovirus and wild-type AAV, as judged by plaque assay and infectious center assay, respectively. Thus, highly purified, helper virus-free rAAV vectors can achieve high-frequency tissue-specific transduction of terminally differentiated, postmitotic photoreceptor cells.

Targeting of Moloney murine leukemia virus gag precursor to the site of virus budding

Retrovirus Moloney murine leukemia virus (M-MuLV) matures by budding at the cell surface. Central to the budding process is the myristoylated viral core protein precursor Gag which, even in the absence of all other viral components, is capable of associating with the cytoplasmic leaflet of the plasma membrane and assembling into extracellular virus-like particles. In this paper we have used heterologous, Semliki Forest virus-driven, expression of M-MuLV Gag to study the mechanism by which this protein is targeted to the cell surface. In pulse-chase experiments, BFA, monensin, and 20 degrees C block did not affect incorporation of Gag into extracellular particles thereby indicating that the secretory pathway is not involved in targeting of Gag to the cell surface. Subcellular fractionation studies demonstrated that newly synthesized Gag became rapidly and efficiently associated with membranes which had a density similar to that of plasma membrane-derived vesicles. Protease-protection studies confirmed that the Gag-containing membranes were of plasma membrane origin, since in crude cell homogenates, the bulk of newly synthesized Gag was protease-resistant as expected of a protein that binds to the cytoplasmic leaflet of the plasma membrane. Taken together these data indicate that targeting of M-MuLV Gag to the cell surface proceeds via direct insertion of the protein to the cytoplasmic side of the plasma membrane. Furthermore, since the membrane insertion reaction is highly efficient and specific, this suggests that the reaction is dependent on as-yet-unidentified cellular factors.