Early steps in the assembly of vesicular stomatitis virus nucleocapsids in infected cells.

The assembly of nucleocapsids is an essential step in the replicative cycle of vesicular stomatitis virus (VSV). In this study, we have examined the early events of vesicular stomatitis virus nucleocapsid assembly in BHK-21 cells. Nuclease-resistant intracellular nucleocapsids were isolated at various stages of assembly and analyzed for RNA and protein contents. The smallest ribonucleoprotein complex formed during nucleocapsid assembly contains the 5'-terminal 65 nucleotides of nascent viral RNA complexed with the viral proteins N and NS. Elongation of the assembling nucleocapsids proceeds unidirectionally towards the 3' terminus by the sequential addition of viral proteins which incrementally protect short stretches of the growing RNA chain. Pulse-chase studies show that the assembling nucleocapsids can be chased into full-length nucleocapsids which are incorporated into mature virions. Our results also suggest an involvement of the cytoskeletal framework during nucleocapsid assembly.     

Role of the vesicular stomatitis virus matrix protein in maintaining the viral nucleocapsid in the condensed form found in native virions.

Vesicular stomatitis virus was extracted with 60 mM octylglucoside in the absence of salts and in the presence of 0.5 M NaCl. The resulting extracted virus particles were examined by electron microscopy, and the proteins present were identified by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Extraction in the absence of salts yielded subviral structures which we cell "skeletons" as originally suggested by Cartwright et al. (J. Gen. Virol. 7:19-32, 1970). The skeletons contained the viral N, M, and L proteins, but they lacked the glycoprotein (G) entirely. Morphologically, the skeletons resembled intact vesicular stomatitis virus but they were slightly longer and smaller in diameter. Like native vesicular stomatitis virus, skeletons were found to have lateral striations spaced 5.0 to 6.0 nm apart along the length of the structure. In contrast to extraction in the absence of NaCl, extraction of vesicular stomatitis virus with 60 mM octylglucoside in the presence of 0.5 M NaCl yielded highly extended viral nucleocapsids in which N was the predominant protein; no M or G proteins could be detected. These results support the view that the M protein is involved in maintaining the nucleocapsid in the compact form found in native virions.     

Maturation of viral proteins in cells infected with temperature-sensitive mutants of vesicular stomatitis virus.

Maturation of viral proteins in cells infected with mutants of vesicular stomatitis virus was studied by surface iodination and cell fractionation. The movement of G, M, and N proteins to the virion bud appeared to be interdependent. Mutations thought to be in G protein prevented its migration to the cell surface, allowed neither M nor N protein to become membrane bound, and blocked formation of viral particles. Mutant G protein appeared not to leave the endoplasmic reticulum at the nonpermissive temperature, but this defect was partially reversible. In cells infected with mutants that caused N protein to be degraded rapidly or prevented its assembly into nucleocapsids, M protein did not bind to membranes and G protein matured to the cell surface, but never entered structures with the density of virions. Mutations causing M protein to be degraded prevented virion formation, and G protein behaved as in cells infected by mutants in N protein. These results are consistent with a model of virion formation involving coalescence of soluble nucleocapsid and soluble M protein with G protein already in the plasma membrane.     

Ribonucleic Acid Species of Intracellular Nucleocapsids and Released Virions of Vesicular Stomatitis Virus

Infection of chicken embryo cells with vesicular stomatitis (VS) virus resulted in variable production of three classes of intracellular viral ribonucleocapsids with sedimentation coefficients of approximately 140S, 110S, and 80S, as well as three corresponding classes of released virions designated B, LT, and T. Intracellular nucleocapsids of each class contained three proteins of which the major N protein was firmly bound, and the minor L and NS1 proteins were readily dissociated with 0.5 m NaCl. The ribonucleic acid (RNA) species extracted from B, LT, and T virions, and from corresponding intracellular nucleocapsids, contained RNA species with approximate molecular weights of 3.2 x 10(6), 2.0 x 10(6), and 10(6), respectively, as determined by polyacrylamide gel electrophoresis. These values are roughly equivalent to sedimentation coefficients of 42S, 28S, and 23S for each of the virion and nucleocapsid RNA species. Cells infected at high multiplicity with undiluted passage VS virus gave rise primarily to virions and nucleocapsids containing 23S RNA, whereas cells productively infected with purified B virions produced predominantly B and LT virions and nucleocapsids. At late stages in the productive cycle of infection, more virions containing 42S RNA were produced, but the intracellular pool of nucleocapsids containing 28S and 23S RNA remained relatively constant. Additional studies by more refined techniques are required to test the hypothesis that nucleocapsids containing 28S and 23S RNA are precursors of the 42S RNA in infectious VS-B virions and that production of defective T and LT virions results from failure of ligation of the RNA precursors.     

Plasma membrane microdomains containing vesicular stomatitis virus M protein are separate from microdomains containing G protein and nucleocapsids

Immunogold electron microscopy and analysis were used to determine the organization of the major structural proteins of vesicular stomatitis virus (VSV) during virus assembly. We determined that matrix protein (M protein) partitions into plasma membrane microdomains in VSV-infected cells as well as in transfected cells expressing M protein. The sizes of the M-protein-containing microdomains outside the virus budding sites (50 to 100 nm) were smaller than those at sites of virus budding (approximately 560 nm). Glycoprotein (G protein) and M protein microdomains were not colocalized in the plasma membrane outside the virus budding sites, nor was M protein colocalized with microdomains containing the host protein CD4, which efficiently forms pseudotypes with VSV envelopes. These results suggest that separate membrane microdomains containing either viral or host proteins cluster or merge to form virus budding sites. We also determined whether G protein or M protein was colocalized with VSV nucleocapsid protein (N protein) outside the budding sites. Viral nucleocapsids were observed to cluster in regions of the cytoplasm close to the plasma membrane. Membrane-associated N protein was colocalized with G protein in regions of plasma membrane of approximately 600 nm. In contrast to the case for G protein, M protein was not colocalized with these areas of nucleocapsid accumulation. These results suggest a new model of virus assembly in which an interaction of VSV nucleocapsids with G-protein-containing microdomains is a precursor to the formation of viral budding sites.     

Stereo images of vesicular stomatitis virus assembly.

Viral assembly was studied by viewing platinum replicas of cytoplasmic and outer plasma membrane surfaces of baby hamster kidney cells infected with vesicular stomatitis virus. Replicas of the cytoplasmic surface of the basilar plasma membrane revealed nucleocapsids forming bullet-shaped tight helical coils. The apex of each viral nose cone was anchored to the membrane and was free of uncoiled nucleocapsid, whereas tortuous nucleocapsid was attached to the base of tightly coiled structures. Using immunoelectron microscopy, we identified the nucleocapsid (N) viral protein as a component of both the tight-coil and tortuous nucleocapsids, whereas the matrix (M) protein was found only on tortuous nucleocapsids. The M protein was not found on the membrane. Using immunoreagents specific for the viral glycoprotein (G protein), we found that the amount of G protein per virion varied. The G protein was consistently localized at the apex of viral buds, whereas the density of G protein on the shaft was equivalent to that in the surrounding membrane. These observations suggest that G-protein interaction with the nucleocapsid via its cytoplasmic domain may be necessary for the initiation of viral assembly. Once contact is established, nucleocapsid coiling proceeds with nose cone formation followed by formation of the helical cylinder. M protein may function to induce a nucleocapsid conformation favorable for coiling or may cross-link adjacent turns in the tight coil or both.     

Phosphate Acceptor Amino Acid Residues in Structural Proteins of Rhabdoviruses

Partial acid hydrolysates of the [(32)P]phosphate- or [(3)H]serine-labeled proteins of purified vesicular stomatitis, rabies, Lagos bat, Mokola, or spring viremia of carp virions and of purified intracellular nucleocapsids of these viruses have been analyzed by paper electrophoresis for the presence of phosphorylated amino acids. Both phosphoserine and phosphothreonine, with the former predominant, were present in virion and nucleocapsid preparations that contained phosphoproteins. An exception was the fish rhabdovirus, which contained only phosphoserine. When vesicular stomatitis or rabies virus proteins were phosphorylated in a cell-free system by the virion-associated protein kinase and analyzed for the presence of phosphorylated amino acid residues, phosphoserine was again found to be more abundant than phosphothreonine. After in vitro protein phosphorylation, another phospho-compound, possibly a third phosphoamino acid, was detected in the partial acid hydrolysates of these viruses.     

Protein Composition of the Structural Components of Vesicular Stomatitis Virus

Digitonin, a sterol glycoside which complexes with cholesterol, stripped off the envelope of vesicular stomatitis (VS) virions and liberated two viral structural proteins, 83% of P6 and 53% of P4. Deoxycholate also disrupted VS virions but released nucleocapsid cores which could be identified by higher buoyant density, ratio of incorporated (3)H-uridine to (14)C-protein, and electron microscopy. The major nucleocapsid protein was P5 but varying amounts of the minor protein aggregate P2 were present, depending on the concentration of urea used for extraction. P2 appeared to be a polymer of P5. Two other minor structural proteins, P1 and P3, could not be located in the virion. From these data, we conclude that the three microscopically identifiable structures of VS virions are each composed primarily of a single major protein, as follows: P6 = envelope protein, P4 = protein of underlying "shell," and P5 = nucleocapsid protein.     

Separate pathways of maturation of the major structural proteins of vesicular stomatitis virus.

Cell fractionation and protein electrophoresis were used to study the intracellular sites of synthesis and intermediate structures in the assembly of the virion proteins of vesicular stomatitis virus. Each of the three major virion proteins assembled into virions through a separable pathway. The nucleocapsid (N) protein was first a soluble protein and later incorporated into free, cytoplasmic nucleocapsids. A small amount of N protein was bound to membranes at later times, presumably representing either nucleocapsids in the process of budding or completed virions attached to the cell surface. The matrix (M) protein also appeared to be synthesized as a soluble protein, but was then directly incorporated into membranous structures with the same density as whole virus. Very little M protein was ever found in membranes banding at the density of plasma membranes. The M protein entered extracellular virus very quickly, as though it moved directly from a soluble state into budding virus. In contrast, the glycoprotein (G) was always membrane bound; it appeared to be directly inserted into membranes during its synthesis. Glycosylation of the G protein was completed only in smooth membrane fractions, possibly in the Golgi apparatus. After a minimum time of 15 min following its synthesis, G protein was incorporated into the surface plasma membrane, from which it was slowly shed into virions. These multiple processing steps probably account for its delayed appearance in virus. From this work it appears that the three major structural proteins come into the surface budding structure through independent pathways and together they coalesce at the plasma membrane to form the mature virion.     

Temperature-sensitive defect of vesicular stomatitis virus in complementation group II.

The prototype member of the complementation group II temperature-sensitive (ts) mutants of vesicular stomatitis virus, ts II 052, has been investigated. In ts II 052-infected HeLa cells at the restrictive temperature (39.5 degrees C), reduced viral RNA synthesis was observed by comparison with infections conducted at the permissive temperature (30 degrees C). It was found that for an infection conducted at 39.5 degrees C, no 38S RNA or intracytoplasmic nucleocapsids were present. For nucleocapsids isolated from ts II 052 purified virions or from ts II 052-infected cells at 30 degrees C, the RNA was sensitive to pancreatic RNase after an exposure at 39.5 degrees C in contrast to the resistance observed for wild-type virus. The nucleocapsid stability of wild-type virus when heated to 63 degrees C or submitted to varying pH was not found in nucleocapsids extracted from ts II 052 purified virions. The data suggest that for ts II 052 there is an altered relationship between the viral 38S RNA and the nucleocapsid protein(s) by comparison with wild-type virus. Such results argue for the complementation group II gene product being N protein, so that the ts defect in ts II 052 represents an altered N protein.     

Pardaxin, a hydrophobic toxin of the Red Sea flatfish, disassembles the intact membrane of vesicular stomatitis virus

Reaction of vesicular stomatitis virus with pardaxin, the hydrophobic toxin of the Red Sea flatfish, resulted in a profound morphological change of many virions and dissociation of their membrane and nucleocapsid into components readily separable by density gradient centrifugation. The basic matrix protein and acidic pardaxin segregated largely with the high density nucleocapsid. The dissociated virion membrane formed lipoprotein vesicles which retained glycoprotein spikes and a certain amount of N protein but no appreciable amounts of other nucleocapsid proteins and little if any RNA. Iodination of the tyrosine residue of the glycoprotein tail fragment provided supporting evidence that the COOH terminus of the glycoprotein extends beyond the inner layer of the membrane into the interior of the virion. These data indicate that pardaxin may serve as a probe for studying the organization of viral membranes, and, hopefully, other biological membranes.     

Interactions of wild-type and mutant M protein of vesicular stomatitis virus with viral nucleocapsid and envelope in intact virions. Evidence from [125I]iodonaphthyl azide labeling and specific cross-linking

Four different temperature-sensitive M protein mutants (tsM) of vesicular stomatitis virus (VSV) were characterized with regard to the association of the mutated M protein either with nucleocapsids or with membranes in the intact virions. Virions were labeled with the photoreactive hydrophobic probe [125I]iodonaphthyl azide (INA) to assess interactions between viral proteins and the lipid envelope. In wild type (wt) virions, the three major structural proteins--G, M, and N--were labeled in the ratio ca. 1.0:0.4:0.2. INA labeled only the membrane-associated peptide of G protein, both in the intact virion and in reconstituted G protein--viral lipid vesicles, demonstrating the specificity of INA for lipid bilayer regions. Labeling of tsM virions with INA resulted in a 2--3-fold greater incorporation into M protein than was found for wt virions, suggesting increased M--membrane associations in the mutant virions. Temperature-stable revertants from tsM possessed wt labeling characteristics. Interaction of the M protein with nucleocapsids was assessed from the abundance of disulfide-linked M--N complexes found after disruption of the virions by sodium dodecyl sulfate solution under nonreducing conditions. The abundance of such complexes was 30--80% less from tsM virions than from wt virions, suggesting decreased M--nucleocapsid interactions in tsM virions. Temperature-stable revertants from tsM resembled wt in the abundance of M--N complex formed. We conclude that the mutations alter M protein in such a way as simultaneously to increase its association with membrane and to decrease its affinity for nucleocapsids in the intact virion.     

Organization of the vesicular stomatitis virus glycoprotein into membrane microdomains occurs independently of intracellular viral components

The glycoprotein (G protein) of vesicular stomatitis virus (VSV) is primarily organized in plasma membranes of infected cells into membrane microdomains with diameters of 100 to 150 nm, with smaller amounts organized into microdomains of larger sizes. This organization has been observed in areas of the infected-cell plasma membrane that are outside of virus budding sites as well as in the envelopes of budding virions. These observations raise the question of whether the intracellular virion components play a role in organizing the G protein into membrane microdomains. Immunogold-labeling electron microscopy was used to analyze the distribution of the G protein in arbitrarily chosen areas of plasma membranes of transfected cells that expressed the G protein in the absence of other viral components. Similar to the results with virus-infected cells, the G protein was organized predominantly into membrane microdomains with diameters of approximately 100 to 150 nm. These results indicate that internal virion components are not required to concentrate the G protein into membrane microdomains with a density similar to that of virus envelopes. To determine if interactions between the G protein cytoplasmic domain and internal virion components were required to create a virus budding site, cells infected with recombinant VSVs encoding truncation mutations of the G protein cytoplasmic domain were analyzed by immunogold-labeling electron microscopy. Deletion of the cytoplasmic domain of the G protein did not alter its partitioning into the 100- to 150-nm microdomains, nor did it affect the incorporation of the G protein into virus envelopes. These data support a model for virus assembly in which the G protein has the inherent property of partitioning into membrane microdomains that then serve as the sites of assembly of internal virion components.