In vivo assembly of a biological membrane of defined size, shape, and lipid composition.

At restrictive temperature, mutant ts1 of bacteriophage PM2 makes membrane vesicles inside infected Alteromonas espejiana. A shift from restrictive to permissive temperature resulted in rapid maturation to infectious virions. The membrane vesicles were isolated from cellular membranes by sucrose density gradient centrifugation. Analysis of the unique peak at rho = 1.190 g/cm3 showed spheres of two diameters, 50 nm and 54 nm. The wild-type virus is icosahedral with an average diameter of 60 nm. Gel electrophoresis indicated the absence in the vesicles of the coat and spike proteins. sp27 and sp43, respectively, and the presence of only one viral structural protein, sp6.6. DNA was also present. The lipid in the vesicles was composed of phosphatidylglycerol and phosphatidylethanolamine in a proportion similar to that of the wild-type virus, whose ratio is nearly the inverse of that found in the host membrane. Thus, membrane vesicles made by mutant ts1 resembled the membrane of the wild-type virus in size, shape, and lipid composition, but contained only one of the four structural proteins of the virus. This hydrophobic protein, sp6.6 may be responsible for stimulating membrane morphogenesis.     

Characterization of temperature-sensitive mutants of bacteriophage PM2: Membrane mutants

Summary In an effort to understand the genetic regulation of membrane morphogenesis, twenty-nine temperature-sensitive mutants of the membrane-containing bacteriophage PM2 were isolated. Characterization at restrictive temperature revealed groups showing no lysis (Groups I–IV), partial lysis (Groups V–VIII), and full lysis (Groups IX–XII) of the host Pseudomonas BAL-31. When the cell lysis data are considered in conjunction with data on stimulation of viral DNA synthesis, at least six mutant groups are defined. Analysis by gel electrophoresis of the pattern of viral proteins synthesized under restrictive conditions further divides the mutants into twelve groups. Temperature shift experiments delineate early, intermediate and late mutants. Complementation data support some of these groupings. The observed low levels of complementation and recombination are discussed in terms of gene product/genome restriction, bound to the membrane at the site of infection.It is of particular interest to membrane morphogenesis that under restrictive conditions late mutants in Groups II, III and IV make empty-appearing vesicles inside the cell that are the size of virus membranes as seen in thin sections of cells in the electron microscope. Mutants ts 1 (Group II) and ts 12 (Group III) show defects in their ability to incorporate into membranes viral structural proteins sp 13 and sp 6.6. The possibility is discussed that either of these proteins control the size and shape of the viral membrane.     

Mutant ts1 of bacteriophage PM2 is defective in the major capsid protein and fails to package its DNA.

Infection of Alteromonas espejiana at restrictive temperature with mutant ts1 of bacteriophage PM2 resulted in the intracellular accumulation of virus-sized empty-appearing membrane vesicles. The DNA associated with purified vesicles was fully susceptible to digestion with DNase. Sedimentation analysis and electron microscopy suggested a full-length linear form of the normally circular viral genome. A pulse-chase-shift experiment suggested that [3H]thymidine-labeled DNA made under restrictive conditions is assembled into virions after shift to permissive temperature. A defective structural protein in the ts1 virion appears to be the cause of a rapid rate of thermal inactivation of infectivity. Analysis of the proteins of ts1 by isoelectric focusing indicated a more alkaline isoelectric mobility of the major capsid protein, sp27. Six spontaneous revertants of ts1 showed reversion to the wild-type isoelectric form of sp27. These results identify sp27 as the defective gene product of ts1. Taken together, these results suggest that the membrane of PM2 is formed without the aid of an inner core or an outer scaffolding.     

Replication of herpesvirus DNA. V. Maturation of concatemeric DNA of pseudorabies virus to genome length is related to capsid formation.

The maturation of pseudorabies virus DNA from the replicative concatemeric form to molecules of genome length was examined using nine DNA+ temperature-sensitive mutants of pseudorabies virus, each belonging to a different complementation group. At the nonpermissive temperature, cells infected with each of the mutants synthesized concatemeric DNA. Cleavage of the concatemeric DNA to genome-length viral DNA was defective in all the DNA+ ts mutants tested, indicating that several viral gene products are involved in the DNA maturation process. In none of the ts mutant-infected cells were capsids with electron-dense cores (containing DNA) formed. Empty capsids with electron-translucent cores were, however, formed in cells infected with six of the nine temperature-sensitive mutants; in cells infected with three of the mutants, no capsid assembly occurred. Because these three mutants are deficient both in maturation of DNA and in the assembly of viral capsids, we conclude that maturation of viral DNA is dependent upon the assembly of capsids. In cells infected with two of the mutants (tsN and tsIE13), normal maturation of viral DNA occurred after shiftdown of the cells to the permissive temperature in the presence of cycloheximide, indicating that the temperature-sensitive proteins involved in DNA maturation became functional after shiftdown. Furthermore, because cycloheximide reduces maturation of DNA in wild-type-infected cells but not in cells infected with these two mutants, we conclude that a protein(s) necessary for the maturation of concatemeric DNA, which is present in limiting amounts during the normal course of infection, accumulated in the mutant-infected cells at the nonpermissive temperature. Concomitant with cleavage of concatemeric DNA, full capsids with electron-dense cores appeared after shiftdown of tsN-infected cells to the permissive temperature, indicating that there may be a correlation between maturation of DNA and formation of full capsids. The number of empty and full capsids (containing electron-dense cores) present in tsN-infected cells incubated at the nonpermissive temperature, as well as after shiftdown to the permissive temperature in the presence of cycloheximide, was determined by electron microscopy and by sedimentation analysis in sucrose gradients. After shiftdown to the permissive temperature in the presence of cycloheximide, the number of empty capsids present in tsN-infected cells decreased with a concomitant accumulation of full capsids, indicating that empty capsids are precursors to full capsids.     

Temperature-Sensitive Mutants of Simian Virus 40: Infection of Permissive Cells

Ten temperature-sensitive mutants of simian virus 40 have been isolated and characterized in permissive cells. The mutants could be divided into three functional groups and two complementation groups. Seven mutants produced T antigen, infectious viral deoxyribonucleic acid (DNA), and structural viral antigen but predominantly the empty shell type of viral particles. Two mutants produced T antigen and infectious viral DNA, but, although viral structural protein(s) could be detected immunologically, no V antigen or viral particles were found. These two functional groups of mutants did not complement each other. A single mutant was defective in the synthesis of viral DNA, viral structural antigens, and viral particles. T antigen could be detected in infected cells by fluorescent antibody but was reduced by complement fixation assay. This mutant stimulated cell DNA synthesis at the restrictive temperature and complemented the other two functional groups of mutants.     

Genetic and phenotypic analysis of herpes simplex virus type 1 mutants conditionally resistant to immune cytolysis

Nine temperature-sensitive (ts) mutants of herpes simplex virus type 1 selected for their inability to render cells susceptible to immune cytolysis after infection at the nonpermissive temperature have been characterized genetically and phenotypically. The mutations in four mutants were mapped physically by marker rescue and assigned to functional groups by complementation analysis. In an effort to determine the molecular basis for cytolysis resistance, cells infected with each of the nine mutants were monitored for the synthesis of viral glycoprotein in total cell extracts and for the presence of these glycoproteins in plasma membranes. The four mutants whose ts mutations were mapped were selected with polypeptide-specific antiserum to glycoproteins gA and gB; however, three of the four mutations mapped to DNA sequences outside the limits of the structural gene specifying these glycoproteins. Combined complementation and phenotypic analysis indicates that the fourth mutation also lies elsewhere. The ts mutations in five additional cytolysis-resistant mutants could not be rescued with single cloned DNA fragments representing the entire herpes simplex virus type 1 genome, suggesting that these mutants may possess multiple mutations. Complementation tests with the four mutants whose ts lesions had been mapped physically demonstrated that each represents a new viral gene. Examination of mutant-infected cells at the nonpermissive temperature for the presence of viral glycoproteins in total cell extracts and in membranes at the cell surface demonstrated that (i) none of the five major viral glycoproteins was detected in extracts of cells infected with one mutant, suggesting that this mutant is defective in a very early function; (ii) cells infected with six of the nine mutants exhibited greatly reduced levels of all the major viral glycoproteins at the infected cell surface, indicating that these mutants possess defects in the synthesis or processing of viral glycoproteins; and (iii) in cells infected with one mutant, all viral glycoproteins were precipitable at the surface of the infected cell, despite the resistance of these cells to cytolysis. This mutant is most likely mutated in a gene affecting a late stage in glycoprotein processing, leading to altered presentation of glycoproteins at the plasma membrane. The finding that the synthesis of both gB and gC was affected coordinately in cells infected with six of the nine mutants suggests that synthesis of these two glycoproteins, their transport to the cell surface, or their insertion into plasma membranes is coordinately regulated.     

Resolution and characterization of intracytoplasmic forms of reverse transcriptase from Rauscher leukemia virus-producing cells.

We have investigated the association of viral DNA with cell DNA in chicken embryo kidney (CEK) cells productively infected with chicken embryo lethal orphan (CELO) virus and in human (HEK) cells infected with mutants ts36 and ts125 of human adenovirus type 5 under permissive and restrictive conditions. Cell and viral DNA molecules were separated after CELO virus infection of CEK cells by alkaline sucrose gradient centrifugation, network formation, and CsCl density gradient centrifugation, methods that rely on different properties of the DNA. The cell DNA was then tested for viral sequences by DNA reannealing kinetics. Between 500 and 1,000 viral genome equivalents per cell were found at 36 h postinfection associated with cell DNA purified by each method. These values greatly exceeded the amount of free viral DNA found contaminating cell DNA prepared by the same methods from uninfected cells to which CELO virus DNA had been added. Quantitative agreement in the amounts of viral DNA found associated with cell DNA purified by these different methods suggests that CELO virus DNA is integrated into chick cell DNA during lytic infection. Similar experiments in HEK cells using mutants ts36 and ts125 of adenovirus type 5 at both restrictive and permissive temperatures showed that the same proportion of viral DNA is associated with cell DNA in the absence of viral DNA replication, and this suggests that the difference in the frequency with which cells are transformed by these mutants is not due to a difference in the frequency integration.     

Temperature-Sensitive Mutants of Adenovirus Type 12 Defective in Viral DNA Synthesis

Ten temperature-sensitive (ts) mutants of adenovirus type 12 which produce plaques at 31 but not at 38.5 C have been isolated after mutagenesis with nitrosoguanidine or nitrous acid. The mutants have been classified into six separate complementation groups. DNA-DNA hybridizations have shown that at 38.5 C the ts 401 and 406 mutants of groups B and E, respectively, synthesized less than 10% of the normal level of viral DNA. The two mutants were also defective in the production of late proteins at the nonpermissive temperature, as shown by fluorescent-antibody tests and analysis by sodium dodecyl sulfatepolyacrylamide gel electrophoresis. Genetic recombination between the ts viruses 401 and 406 has been demonstrated; the recombination frequency for the wild-type virus production was 17.7%. Both mutants induced an increase in thymidine kinase activity at 38.5 C. Moreover, the two viral DNA-defective mutants shut off host DNA synthesis at the restrictive temperature. It is striking that at 38.5 C ts virus 401 transformed two to eight times more hamster cells than the wild-type virus, whereas ts virus 406 transformed at a frequency similar to the wild-type virus.     

Genetic Analysis of Simian Virus 40 III. Characterization of a Temperature-Sensitive Mutant Blocked at an Early Stage of Productive Infection in Monkey Cells

A temperature-sensitive mutant of simian virus 40 (SV40), ts(*)101, has been characterized during productive infection in monkey kidney cells. The mutant virion can adsorb to and penetrate the cell normally at the restrictive temperature, but cannot induce the synthesis of cellular deoxyribonucleic acid (DNA) nor initiate the synthesis of SV40-specific tumor, virion, or U antigens or viral DNA. First-cycle infection with purified ts(*)101 DNA is normal at the restrictive temperature, but the resulting progeny virions are still temperature-sensitive. The mutant neither complements nor inhibits other temperature-sensitive SV40 mutants or wild-type virions. The affected protein in the ts(*)101 mutant may be a regulatory structural protein, possibly a core protein, that is interacting with the viral DNA.     

Imaging of the Alphavirus Capsid Protein during Virus Replication

Alphaviruses are enveloped viruses with highly organized structures. The nucleocapsid (NC) core contains a capsid protein lattice enclosing the plus-sense RNA genome, and it is surrounded by a lipid bilayer containing a lattice of the E1 and E2 envelope glycoproteins. Capsid protein is synthesized in the cytoplasm and particle budding occurs at the plasma membrane (PM), but the traffic and assembly of viral components and the exit of virions from host cells are not well understood. To visualize the dynamics of capsid protein during infection, we developed a Sindbis virus infectious clone tagged with a tetracysteine motif. Tagged capsid protein could be fluorescently labeled with biarsenical dyes in living cells without effects on virus growth, morphology, or protein distribution. Live cell imaging and colocalization experiments defined distinct groups of capsid foci in infected cells. We observed highly motile internal puncta that colocalized with E2 protein, which may represent the transport machinery that capsid protein uses to reach the PM. Capsid was also found in larger nonmotile internal structures that colocalized with cellular G3BP and viral nsP3. Thus, capsid may play an unforeseen role in these previously observed G3BP-positive foci, such as regulation of cellular stress granules. Capsid puncta were also observed at the PM. These puncta colocalized with E2 and recruited newly synthesized capsid protein; thus, they may be sites of virus assembly and egress. Together, our studies provide the first dynamic views of the alphavirus capsid protein in living cells and a system to define detailed mechanisms during alphavirus infection.     

Partial reversion of conditional transformation correlates with a decrease in the sensitivity of rat cells to killing by the parvovirus minute virus of mice but not in their capacity for virus production: effect of a temperature-sensitive v-src oncogene.

The cytolytic effect of the autonomous parvovirus minute virus of mice, prototype strain (MVMp), was studied in cultures of ts 339/NRK rat cells that display a temperature-sensitive transformed phenotype as a result of their transformation with a Rous sarcoma virus strain matured in the v-src oncogene. A shift from restrictive (39.5 degrees C) to permissive (34.5 degrees C) temperature was associated with a marked sensitization of these cells to killing by MVMp. In contrast, ts 339/NRK cell derivatives supertransformed with a wild-type src oncogene were sensitive to MVMp at both temperatures, suggesting that the expression of a functional oncogene product may determine, at least in part, the extent of the parvoviral cytopathic effect. Although ts 339/NRK cells were quite resistant to parvoviral attack at 39.5 degrees C, they were similarly proficient in MVMp uptake, viral DNA and protein synthesis, and infectious particle production at both permissive and restrictive temperatures. Consistently, electron microscopic examination of infected ts 339/NRK cultures incubated at 39.5 degrees C revealed the presence, in the majority of the cells, of numerous full and empty virions that were predominantly located in autophagic-type vacuoles. Thus, in this system, the reversion of transformed and MVMp-sensitive phenotypes appears to correlate with the setting up of a noncytocidal mode of parvovirus production. These results raise the possibility that the physiological state of host cells may affect their susceptibility to parvoviruses by modulating not only their capacity for virus replication but also cellular processes controlling the cytopathic effect of viral products.     

Assembly of bacteriophage PRD1: particle formation with wild-type and mutant viruses

Bacteriophage PRD1 contains DNA, 17 proteins, and lipid. The assembly pathway involves the formation of empty particles that contain lipid and all of the proteins of mature virions, with the possible exception of one. The major and minor capsid proteins, P3 and P5, occur as soluble multimers before they appear in the empty particles. Nonsense mutants of PRD1 that involve structural proteins of the virion other than P3 form particles that are missing only the defective protein. Those mutants that are unable to form P3 do not form particles. Mutations in two other genes that code for nonstructural proteins (P10, which is membrane bound, and P17, which is soluble) result in the absence of particles. Protein P2 is necessary for adsorption to host cells. Protein P9 is necessary for particle filling with DNA, whereas P20 and P22 are necessary for stable DNA packaging. Electron micrographs of infected cells confirmed the gradient analysis of particle formation. No free vesicles were observed in mutants that could not form complete empty particles, indicating that there are no free intermediate particles before the empty virions.     

Canine parvovirus host range is determined by the specific conformation of an additional region of the capsid.

We analyzed a region of the capsid of canine parvovirus (CPV) which determines the ability of the virus to infect canine cells. This region is distinct from those previously shown to determine the canine host range differences between CPV and feline panleukopenia virus. It lies on a ridge of the threefold spike of the capsid and is comprised of five interacting loops from three capsid protein monomers. We analyzed 12 mutants of CPV which contained amino acid changes in two adjacent loops exposed on the surface of this region. Nine mutants infected and grew in feline cells but were restricted in replication in one or the other of two canine cell lines tested. Three other mutants whose genomes contain mutations which affect one probable interchain bond were nonviable and could not be propagated in either canine or feline cells, although the VP1 and VP2 proteins from those mutants produced empty capsids when expressed from a plasmid vector. Although wild-type and mutant capsids bound to canine and feline cells in similar amounts, infection or viral DNA replication was greatly reduced after inoculation of canine cells with most of the mutants. The viral genomes of two host range-restricted mutants and two nonviable mutants replicated to wild-type levels in both feline and canine cells upon transfection with plasmid clones. The capsids of wild-type CPV and two mutants were similar in susceptibility to heat inactivation, but one of those mutants and one other were more stable against urea denaturation. Most mutations in this structural region altered the ability of monoclonal antibodies to recognize epitopes within a major neutralizing antigenic site, and that site could be subdivided into a number of distinct epitopes. These results argue that a specific structure of this region is required for CPV to retain its canine host range.     

The PM2 virion has a novel organization with an internal membrane and pentameric receptor binding spikes

Biological membranes are notoriously resistant to structural analysis. Excellent candidates to tackle this problem in situ are membrane-containing viruses where the membrane is constrained by an icosahedral capsid. Cryo-EM and image reconstruction of bacteriophage PM2 revealed a membrane bilayer following the internal surface of the capsid. The viral genome closely interacts with the inner leaflet. The capsid, at a resolution of 8.4 A, reveals 200 trimeric capsomers with a pseudo T = 21 dextro organization. Pentameric receptor-binding spikes protrude from the surface. It is evident from the structure that the PM2 membrane has at least two important roles in the life cycle. First, it acts as a scaffold to nucleate capsid assembly. Second, after host recognition, it fuses with the host outer membrane to promote genome entry. The structure also sheds light on how the viral supercoiled circular double-stranded DNA genome might be packaged and released.     

Isolation and preliminary characterization of a phosphonoacetic acid-resistant and temperature-sensitive mutant of herpes simplex virus type 1.

A group of 43 phosphonoacetic acid (PAA)-resistant mutants of herpes simplex virus type 1 was isolated after the mutagenesis of infected cells with nitrosoguanidine. One of these mutants, designated PAA1rts1, was found to be temperature sensitive (ts), that is, unable to replicate at 39.5 degrees C, the nonpermissive temperature. Recombination analysis of PAA1rts1 indicated that the PAA1r mutation and the ts1 mutation are loosely linked and are located on two separate genes. PAA1rts1 showed a defect in viral DNA synthesis at 39.5 degrees C, which presumably can be attributed to the production of a PAA-resistant and thermolabile DNA polymerase. PAA1rts1 was also defective in the shutoff of host DNA synthesis at the restrictive temperature.     

Role of early and late replication events in induction of apoptosis by baculoviruses.

Autographa californica nuclear polyhedrosis virus (AcMNPV) mutants that lack the apoptotic suppressor gene p35 cause apoptosis in Spodoptera frugiperda SF21 cells. To identify a viral signal(s) that induces programmed cell death, we first defined the timing of apoptotic events during infection. Activation of a P35-inhibitable caspase, intracellular fragmentation of host and AcMNPV DNA, and cell membrane blebbing coincided with the initiation of viral DNA synthesis between 9 and 12 h after infection and thus suggested that apoptotic signaling begins at or before this time. Virus entry was required since binding of budded virus to host cell receptors alone was insufficient to induce apoptosis. To therefore determine the contribution of early and late replication events to apoptotic signaling, we used the AcMNPV mutant ts8 with a temperature-sensitive lesion in the putative helicase gene p143. At the nonpermissive temperature at which viral DNA synthesis was conditionally blocked, ts8 caused extensive apoptosis of the SF21 cell line p3576D, which dominantly interferes with anti-apoptotic function of viral P35. Confirming that apoptosis can be induced in the absence of normal viral DNA synthesis, parental SF21 cells also underwent apoptosis when infected with a ts8 p35 deletion mutant at the nonpermissive temperature. However, maximum levels of ts8 p35 deletion mutant-induced apoptosis required a temperature-sensitive event(s) that included the initiation of viral DNA synthesis. Collectively, these data suggested that baculovirus-induced apoptosis can be triggered by distinct early (pre-DNA synthesis) and late replicative events, including viral DNA synthesis or late gene expression.     

Transport and assembly of gag proteins into Moloney murine leukemia virus.

We have studied the process of Moloney murine leukemia virus (M-MuLV) assembly by characterization of core (gag) protein mutants and analysis of wild-type (wt) gag proteins produced by cells in the presence of the ionophore monensin. Our genetic studies involved examination of linker insertion mutants of a Gag-beta-galactosidase (Gag-beta-gal) fusion protein, GBG2051, which is incorporated into virus particles when expressed in the presence of wt viral proteins. Analysis indicated that the amino-terminal two-thirds of the gag matrix domain is essential for targeting of proteins to the plasma membrane; mutant proteins localized to the cytoplasm or were trapped on intracellular membranes. Mutations through most of the coding region of the gag capsid domain generated proteins which were released from cells in membrane vesicles but not in virions. In contrast, linker insertions into p12gag or carboxy-terminal portions of the matrix or capsid coding regions did not affect assembly of fusion proteins into virus particles. Monensin, which blocks vesicular transport, inhibited gag protein intracellular transport and release from cells. Our results suggest that a significant proportion of M-MuLV myristylated gag proteins travel via vesicles to the cell surface. Specific matrix protein polypeptide regions and myristic acid modification are both necessary for appropriate gag protein transport, while capsid protein interactions appear to mediate the final phase of virion formation.     

The UL25 gene product of herpes simplex virus type 1 is involved in uncoating of the viral genome

Studies on the herpes simplex virus type 1 UL25-null mutant KUL25NS have shown that the capsid-associated UL25 protein is required at a late stage in the encapsidation of viral DNA. Our previous work on UL25 with the UL25 temperature-sensitive (ts) mutant ts1204 also implicated UL25 in a role at very early times in the virus growth cycle, possibly at the stage of penetration of the host cell. We have reexamined this mutant and discovered that it had an additional ts mutation elsewhere in the genome. The ts1204 UL25 mutation was transferred into wild-type (wt) virus DNA, and the UL25 mutant ts1249 was isolated and characterized to clarify the function of UL25 at the initial stages of virus infection. Indirect immunofluorescence assays and in situ hybridization analysis of virus-infected cells revealed that the mutant ts1249 was not impaired in penetration of the host cell but had an uncoating defect at the nonpermissive temperature. When ts1249-infected cells were incubated initially at the permissive temperature to allow uncoating of the viral genome and subsequently transferred to the restrictive temperature, a DNA-packaging defect was evident. The results suggested that ts1249, like KUL25NS, had a block at a late stage of DNA packaging and that the packaged genome was shorter than the full-length genome. Examination of ts1249 capsids produced at the nonpermissive temperature revealed that, in comparison with wt capsids, they contained reduced amounts of UL25 protein, thereby providing a possible explanation for the failure of ts1249 to package full-length viral DNA.