Cell cycle-dependent inhibition of Kirsten Murine sarcoma-leukemia virus release by cytochalasin B

Type D retrovirus capsid assembly and release are active events requiring ATP

Mason-Pfizer monkey virus (M-PMV), the prototype type D retrovirus, differs from most other retroviruses by assembling its Gag polyproteins into procapsids in the cytoplasm of infected cells. Once assembled, the procapsids migrate to the plasma membrane, where they acquire their envelope during budding. Because the processes of M-PMV protein transport, procapsid assembly, and budding are temporally and spatially unlinked, we have been able to determine whether cellular proteins play an active role during the different stages of procapsid morphogenesis. We report here that at least two stages of morphogenesis require ATP. Both procapsid assembly and procapsid transport to the plasma membrane were reversibly blocked by treating infected cells with sodium azide and 2-deoxy-D-glucose, which we show rapidly and reversibly depletes cellular ATP pools. Assembly of procapsids in vitro in a cell-free translation/assembly system was inhibited by the addition of nonhydrolyzable ATP analogs, suggesting that ATP hydrolysis and not just ATP binding is required. Since retrovirus Gag polyproteins do not bind or hydrolyze ATP, these results demonstrate that cellular components must play an active role during retrovirus morphogenesis.

Autographa californica M nuclear polyhedrosis virus and cytochalasin D: antagonists in the regulation of protein synthesis

During infection by the baculovirus Autographa californica multiple nuclear polyhedrosis virus (AcMNPV), synthesis of host proteins in IPLB-Sf-21 cells is inhibited. We report here that cytochalasin D (CD), a drug that specifically interacts with actin, behaved as an antagonist in the virus-mediated shutdown of host proteins actin and tubulin. In uninfected cells, CD caused an increase in actin synthesis but had no apparent effect on tubulin synthesis. In infected cells, CD similarly enhanced actin synthesis early in infection and delayed the virus shutoff of actin synthesis by 14 hr. The shutoff of tubulin synthesis was delayed by 8 hr. Addition of CD to infected cells after host protein synthesis ceased resulted in an induction of actin synthesis reversing viral inhibitory effects. Similarly, the removal of CD resulted in virus-induced inhibition of actin synthesis. Treatment of infected cells with CD caused a delay in the onset and/or shutoff of at least five viral proteins and inhibited the amplification of polyhedrin synthesis by at least 8 hr.

Autographa californica MNPV nucleocapsid assembly: inhibition by cytochalasin D

Autographa californica nuclear polyhedrosis virus (AcMNPV) is a large, double-stranded DNA virus of lepidopteran insects. It replicates in the nucleus of host cells and produces two different infectious forms. One of them buds from the plasma membrane and is responsible for systemic infection in insects and for the spread of infection in cell culture. Earlier we reported that cytochalasin D (CD) inhibited the production of this form of the virus in infected IPLB-SF-21 cells by a mechanism other than the prevention of budding [L. E. Volkman, P. A. Goldsmith, and R. T. Hess (1987). Virology 156, 32-39]. Herein we present evidence that CD interferes with proper nucleocapsid assembly which indicates that microfilaments are involved in this nuclear process. The observation that phalloidin localizes in the nuclei of 24-hr infected cells in the absence, but not in the presence, of CD supports this hypothesis. Viral DNA replication and synthesis of the major capsid antigen and its transport to the nucleus are not inhibited by CD.

Interaction of frog virus 3 with the cytomatrix. III. Role of microfilaments in virus release

The role of microfilaments in the release of frog virus 3 (FV3) from the plasma membrane was studied. Scanning electron microscopic study of FV3-infected baby hamster kidney (BHK) cells showed that late in infection (15 hr), numerous microvillus-like projections containing virions and microfilaments occur on the cell surface. Two microfilament-disrupting drugs, cytochalasin B and cytochalasin D, inhibited both the formation of microvillus-like projections and virus release. In the drug-treated cells, the virions accumulated in large numbers beneath the plasma membrane (transmission electron microscopy), suggesting that both drugs affected the release of the virus at the level of plasma membrane rather than the traverse of the virus to the plasma membrane. Two-dimensional gel analysis of actin from FV3-infected and uninfected cells revealed the following. There was no difference in the synthesis of actin in infected versus uninfected cells. However, the actin of infected cells is more acidic than its counterpart in uninfected cells. Temporally, the change in actin preceded the formation of microvilli-like projections involved in virus release. The change in actin is virus induced and is linked to virus maturation since a ts mutant of FV3 (ts9467), which is deficient in virus production at the restrictive temperature (30 degrees), did not modify actin. The mutant, at the permissive temperature (25 degrees), produced virions and altered the actin. Together, the above results attribute an active role for microfilaments in virus release.

Cytochalasin B inhibits the maturation of measles virus

The release of measles virus was studied in the presence of cytochalasin B (CB), a drug that disrupts actin microfilaments. In the presence of CB, infected cells accumulated infectious virus while virus released from these cultures decreased drastically (up to 99% inhibition). Electron micrographs showed that viral buds were reduced and had an unusual distribution along the cell membrane in CB-treated cultures. CB inhibition of released virus occurred rapidly (within 30 min) and to a full extent even when the drug was added during the final 2 hr of a 48-hr replicative cycle. CB inhibition of cellular functions is reversible and, similarly, inhibition of virus release could be almost completely reversed within 30 min after the drug was removed. Since CB can also inhibit sugar transport and protein glycosylation, 2-deoxy-D-glucose (DG) was used to study the manifestations of glycosylation inhibition. DG inhibited virus production only when added during the first one-third of the replicative cycle and inhibited cell-associated and released virus to an equal extent. Cytochalasin D, which disrupts microfilaments without affecting protein glycosylation, caused an inhibition of virus release analogous to the inhibition caused by CB. Thus, alteration of microfilament structure alters the normal budding process of measles virions. This suggests that microfilaments may play a role in the release of budding virions.

Retrovirus gene expression during the cell cycle. I. Virus production, synthesis, and expression of viral proteins in Rauscher murine leukemia virus-infected mouse cells

Synchronized mouse cells (JLS-V9) chronically infected with Rauscher murine leukemia virus were used to study virus production, the synthesis of gag and env precursor proteins, and the expression of env protein on the cell surface during the cell cycle. The amount of virus released into the medium by synchronized cells during a 30-min interval was determined by using the XC plaque assay and by measuring reverse transcriptase activity. The results show that virus production occurs during mitosis. Labeling of the cell surface of synchronized cells with 125I or with fluorescein-conjugated antiserum shows that the amount of gp 70env on the cell surface parallels cellular growth. Therefore, the cell cycle-dependent release of virus is not accompanied by similar variations in the amount of viral envelope protein on the cell surface. Immunoprecipitation of cells labeled with [35S]methionine, followed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, was used to measure viral protein synthesis during the cell cycle. The rate of synthesis of gag precursor proteins show three maximums corresponding to the G1, middle S, and late S to G2 phases of the cell cycle. The rate of synthesis of env precursor proteins does not change, suggesting that in these cells the synthesis of these two gene products is controlled separately.

Efficient production of mammalian RNA tumor viruses in serum-free culture medium allows rapid RNA subunit purification

A wide variety of infected mammalian cell cultures were observed to produce high levels of RNA tumor virus particles in the absence of serum for at least 12 h. Virus production was measured by yields of 50-70 S virus RNAs isolated directly from serum-free culture media by chromatography on oligo(dT)-cellulose. Yields of RNAs from viruses produced in serum-free medium were comparable to yields obtained from purified viruses produced in serum-containing medium. Subunits of viral RNAs were thermally dissociated and separated by a new sedimentation system using sucrose gradients with resolving power (in the relevant size range) equivalent to that obtained with electrophoresis in polyacrylamide gels. RNA subunits isolated directly from serum-free medium appeared to be intact as judged by poly(A) content and resedimentation. The overall approach developed here represents dramatic savings in time and effort over previous ways of producing purified RNA subunits from tumor viruses.