Vaccinia virus G9 protein is an essential component of the poxvirus entry-fusion complex

Resistance of a vaccinia virus A34R deletion mutant to spontaneous rupture of the outer membrane of progeny virions on the surface of infected cells

The extracellular form of vaccinia virus is referred to as an enveloped virion (EV) because it contains an additional lipoprotein membrane surrounding the infectious mature virion (MV) that must be discarded prior to cell fusion and entry. Most EVs adhere to the surface of the parent cell and mediate spread of the infection to adjacent cells. Here we show that some attached EVs have ruptured envelopes. Rupture was detected by fluorescence microscopy of unfixed and unpermeabilized cells using antibodies to the F13 and L1 proteins, which line the inner side of the EV membrane and the outer side of the MV membrane, respectively. The presence of ruptured EV membranes was confirmed by immunogold transmission electron microscopy. EVs with broken membranes were present on several cell lines examined including one deficient in glycosaminoglycans, which are thought to play a role in breakage of the EV membrane prior to fusion of the MV. No correlation was found between EVs with ruptured membranes and actin tail formation. Studies with several mutant viruses indicated that EV membranes lacking the A34 protein were unbroken. This result was consistent with other properties of A34R deletion mutants including resistance of the EV membrane to polyanions, small plaque formation and low infectivity that can be increased by disruption of the EV membrane by freezing and thawing.

Vaccinia virus temperature-sensitive mutants in the A28 gene produce non-infectious virions that bind to cells but are defective in entry

The vaccinia virus temperature-sensitive mutations Cts6 and Cts9 were mapped by marker rescue and DNA sequencing to the A28 gene. Cts6 and Cts9 contain an identical 2-bp deletion truncating the A28 protein and removing the fourth conserved cysteine near the C-terminus. Cts9 mutant virions produced at 40 degrees C were non-infectious and unable to cause cytopathic effect. However, the mutant A28 protein localized to purified mature virions (MV) at 31 degrees C and 40 degrees C. MV of Cts9 produced at 40 degrees C bound to cells but did not enter cells. Low pH treatment of Cts9-infected cells at 18 h p.i. failed to produce fusion from within at 40 degrees C, but gave fusion at 31 degrees C. Adsorption of Cts9 mutant virions to cells followed by low pH treatment showed a defect in fusion from without. The Cts9 phenotype suggests that the A28 protein is involved in both virus entry and cell-cell fusion, and supports the linkage between the two processes.

Association of vaccinia virus fusion regulatory proteins with the multicomponent entry/fusion complex

The proteins encoded by the A56R and K2L genes of vaccinia virus form a heterodimer (A56/K2) and have a fusion regulatory role as deletion or mutation of either causes infected cells to form large syncytia spontaneously. Here, we showed that syncytia formation is dependent on proteins of the recently described entry fusion complex (EFC), which are also required for virus-cell fusion and low-pH-triggered cell-cell fusion. This finding led us to consider that A56/K2 might prevent fusion by direct or indirect interaction with the EFC. To test this hypothesis, we made a panel of recombinant vaccinia viruses that have a tandem affinity purification tag attached to A56, K2, or the A28 EFC protein. Interaction between A56/K2 and the EFC was demonstrated by their copurification from detergent-treated lysates of infected cells and identification by mass spectrometry or Western blotting. In addition, a purified soluble transmembrane-deleted form of A56/K2 was shown to interact with the EFC. Tagged A56 did not interact with the EFC in the absence of K2, nor did tagged K2 interact with the EFC in the absence of A56. The finding that both A56 and K2 are required for efficient binding to the EFC fits well with prior experiments showing that mutation of either A56 or K2 results in spontaneous fusion of infected cells. Because A56 and K2 are located on the surface of infected cells, they are in position to interact with the EFC of released progeny virions and prevent back-fusion and syncytia formation.

Vaccinia virus 4c (A26L) protein on intracellular mature virus binds to the extracellular cellular matrix laminin

Vaccinia virus intracellular mature virus (IMV) binds to glycosaminoglycans (GAGs) on cells via three virion proteins, H3L, A27L, and D8L. In this study, we demonstrated that binding of IMV to BSC40 cells was competitively inhibited by soluble laminin but not by fibronectin or collagen V, suggesting that this cell surface extracellular matrix (ECM) protein may play a role in vaccinia virus entry. Moreover, IMV infection of GAG(-) sog9 cells was also inhibited by laminin, demonstrating that virion binding to laminin does not involve a prior interaction with GAGs. Furthermore, comparative envelope protein analyses of wild-type vaccinia virus strain Western Reserve, which binds to laminin, and of a mutant virus, IA27L, which does not, showed that the A26L open reading frame (ORF), encoding an envelope protein, was mutated in IA27L, resulting in A26L being absent from the IMV. Expression of the wild-type A26L ORF in IA27L resulted in laminin binding activity. Moreover, recombinant A26L protein bound to laminin in vitro with a high affinity, providing direct evidence that A26L is the laminin binding protein on IMV. In summary, these results reveal a novel role for the vaccinia viral envelope protein A26L in binding to the ECM protein laminin, an association that is proposed to facilitate IMV entry.

Vaccinia virus F9 virion membrane protein is required for entry but not virus assembly, in contrast to the related L1 protein

All sequenced poxviruses encode orthologs of the vaccinia virus L1 and F9 proteins, which are structurally similar and share about 20% amino acid identity. We found that F9 further resembles L1 as both proteins are membrane components of the mature virion with similar topologies and induce neutralizing antibodies. In addition, a recombinant vaccinia virus that inducibly expresses F9, like a previously described L1 mutant, had a conditional-lethal phenotype: plaque formation and replication of infectious virus were dependent on added inducer. However, only immature virus particles are made when L1 is repressed, whereas normal-looking intracellular and extracellular virions formed in the absence of F9. Except for the lack of F9, the polypeptide components of such virions were indistinguishable from those of wild-type virus. These F9-deficient virions bound to cells, but their cores did not penetrate into the cytoplasm. Furthermore, cells infected with F9-negative virions did not fuse after a brief low-pH treatment, as did cells infected with virus made in the presence of inducer. In these respects, the phenotype associated with F9 deficiency was identical to that produced by the lack of individual components of a previously described poxvirus entry/fusion complex. Moreover, F9 interacted with proteins of that complex, supporting a related role. Thus, despite the structural relationships of L1 and F9, the two proteins have distinct functions in assembly and entry, respectively.