1. Introduction
Vaccinia virus (family
Poxviridae, genus
Orthopoxvirus) is a dsDNA (~190 kb) virus with a complex architecture and an exclusive cytoplasmic replication [
1]. It was widely used as a vaccine to eradicate the genetically closely related and highly pathogenic variola virus from the human population. A major 11 kDa phosphoprotein [
2] present in purified virions was identified in 1991 as the product of the expression of the
F17R gene by Zhang and Moss, who constructed a vaccinia virus mutant of the WR strain (designated as iF17 in this work), which requires the addition of IPTG to culture media of infected cell cultures to induce expression of the
F17R gene (initially designated
F18R [
3,
4]). When BSC-1 cells were infected with iF17 in the absence of IPTG, viral DNA and late protein synthesis proceeded normally except that the A10 (precursor of P4a) and A3 (precursor of P4b) proteins were not processed and non-infectious immature particles with an aberrant morphology are formed (designated as iF17
− particles in this work). The production of these particles indicates an inhibition in morphogenesis that correlates with impairment of proteolytic processing of the major viral structural proteins P4a (encoded by
A10L) and P4b (encoded by
A3L) [
5]. Using this F17 IPTG-dependent expression mutant, Wickramasekera and Traktman [
4] developed a transient F17 complementation system for an extensive structure/function analysis of the F17 phosphoprotein. This system helped reveal that several charged or hydrophobic residues are essential for the production of infectious virus and that the mutation of two serine sites (S52 and S62, phosphorylated by cellular proline-directed kinases) had no apparent impact on virion morphogenesis but led to the assembly of virions with significantly reduced infectivity. Furthermore, non-infectious iF17
− viral particles produced in cells infected with the iF17 mutant virus in the absence of IPTG could be purified and were found to contain wild-type levels of viral DNA and several core proteins (L4, I1, A30 and A5). However, these particles were not active for in vitro transcription [
4,
6]. In addition infection of cells using these particles did not reveal any detectable CPE or induce the expression of the early I3 protein but the attachment/entry steps were not investigated [
4].
Several published studies of vaccinia virus proteins refer to an abundant 11 kDa phosphoprotein (designated as 11K (F17) in this paper) as the likely product of the
F17R gene [
5]. The first publications on 11K (F17) suggested that is present in the core, associated with viral DNA [
7]. However, Traktman and collaborators [
6] found the F17 protein localised between the outer membrane and the lucent core of the virion. Pedersen and collaborators [
8] also reported that the protein F17 localised into the space between the outer viral membrane and the core. Furthermore, Schmidt and collaborators [
9] identified F17 in the lateral bodies, together with H1 and G4 proteins. However, F17 is also very close to proteins P4a, P4b and P39 (encoded by
A4L), as expected for a structural virion protein of the core wall [
10].
Phosphorylation of an 11 kDa protein was shown in earlier reports [
11,
12,
13,
14] and was characterised as the product of the
F17R gene [
5]. Kao and collaborators [
15] found that core-associated phosphorylated 11K (F17) binds dsDNA, albeit with a very low affinity. F17 was found to be hyper-phosphorylated in the virions devoid of the viral H1 phosphatase, suggesting regulation of the F17 phosphorylated state [
16]. In the MS/Phosphoproteome studies, 5 sites of phosphorylation (S52, S53, S61, S62, S64) were found in virion-associated F17 [
17,
18]. The S52 and S61 sites are phosphorylated by cellular proline-directed kinases in vitro and in vivo and mutation of both of these phosphorylation sites led to the assembly of normal-looking virions but with significantly reduced infectivity [
4]. In contrast, Schmidt and collaborators [
9] found only three F17 phosphorylation sites in virions: S3, S40 and S52.
Earlier work suggested that VP13.8K presumably encoded by
F17R, is in a highly polymerised form via disulfide bonds, as is the case for other viral proteins [
19]. The core P4a/
A10L and P25/
L4R proteins are reduced soon after infection, concomitant with the delivery of the cores into the cytoplasm, whereas the viral membrane proteins remain disulfide bonded [
20]. F17 and several core proteins (VP8/
L4R, P39/
A4, P4a/
A10L, P4b/
A3L) are disulfide cross-linked into high-molecular-weight complexes in the virion but F17 is rapidly reduced after delivery of viral cores into the host cytosol and also in the cores prepared in vitro [
9].
Until recently, the F17 protein was thought to be only involved in morphogenesis; however, Meade and collaborators used the iF17 virus to show that F17 synthesised during late stages of infection in quiescent human fibroblasts binds to the Raptor and Rictor proteins (associated with mTOR). This binding leads to inhibition of the interferon response and allows late protein synthesis in these quiescent cells [
21]. Furthermore, mimicks of phosphorylated forms of F17 are expressed in primary normal human dermal fibroblasts (NHDFs) and increase the phosphorylated state of p70S6K and 4EBP1, thus activating protein synthesis [
22].
Paradoxically, early studies on vaccinia virus infection demonstrated that an inhibitor of cell protein synthesis is present in vaccinia virions [
23,
24]. In addition, purified cores inhibited protein synthesis when added to reticulocyte lysates [
25], as also suggested in a study of a coupled cell-free transcription and translation system directed by vaccinia cores [
26]. The formation of the ribosomal 40S-Met-tRNA initiation complex in a reticulocyte lysate was inhibited by purified cores [
27,
28]. An inhibitor of in vitro protein synthesis was found to detach from purified viral cores after an in vitro protein kinase reaction [
29] and was purified to apparent homogeneity, yielding a basic protein of 11 kDa [
30] presumably encoded by the
F17R gene. This 11K (F17) protein inhibited protein synthesis when added to a reticulocyte lysate at a stoichiometric ratio of approximately one protein molecule/ribosome and inhibited the formation of the 40S ribosomal subunit–Met-tRNAi ribosomal initiation complex after incubation in reticulocyte lysates or in Ehrlich ascites tumour cell lysates [
30]. This inhibition was reversed by a reticulocyte cell supernatant factor and by a partially purified eIF2 preparation [
31]. In contrast, Damaso and collaborators [
32] concluded that proteins released from the viral cores were probably not directly involved in protein synthesis inhibition in vitro [
33].
The recent and paradoxical demonstration that F17 can activate late protein synthesis by binding to mTOR subunits [
21] or when expressed in non-infected cells [
22], prompted us to investigate whether the previously described inhibitory 11K (F17) protein [
30] could be mediated by F17. To this aim, we investigated if non-infectious vaccinia virus (VV) particles devoid of F17 protein [
4] are inefficient in inhibiting protein synthesis. Here, we show that these iF17
− particles contain all the major core proteins (except F17) and that cells can be infected with such particles although the infection is non-productive. Their viral DNA was injected into the cytoplasm and the iF17
− particles stimulated the expression of the mCherry fluorescence upon co-infection of the non-permissive BSC40 cells with an MVA which expresses mCherry. Importantly, despite their ability to inject their content into the cell cytoplasm, the iF17
− particles failed to inhibit protein synthesis even at a very high multiplicity of infection (MOI), and the infectious iF17 virions (produced in the presence of IPTG) contained approximately threefold less F17 protein and were 3 to 8 times less efficient to inhibit protein synthesis than their wild-type parent, thus confirming that virion-associated F17 is essential to inhibit host cell protein synthesis, immediately after entry of the cores into the cytoplasm.
3. Discussion
We previously isolated a general protein synthesis inhibitor from purified cores derived from vaccinia virions and containing an 11 kDa protein as a major component [
30]. Its physico-chemical properties corresponded to those of the abundant virion structural F17 phosphoprotein [
5] but it was not characterised. Recently, the F17 protein (synthesised late) was shown to counter cGAS activation to reduce the interferon response and concomitantly activate the translation system [
21,
22]. To resolve this paradox, we investigated the properties of the iF17
− non-infectious viral particles devoid of the F17 protein [
4].
Because the iF17
− particles have structural defects, and differences in protein composition, and are defective in viral RNA synthesis (compared to wild-type virus), it is difficult to absolutely conclude that the absence or diminished amounts of F17 are directly responsible for the observed inhibition of protein synthesis. As an example, previous studies revealed that despite having a full set of transcription enzymes, L4-deficient vaccinia virus particles (devoid of P25 protein) were deficient for early transcription. Therefore, upon infection of cells, electron micrographs revealed a gap between cores and the surrounding membrane. In this paper we show that defective iF17
− particles do not significantly inhibit protein synthesis. We also identified a strong correlation between the magnitude of protein synthesis inhibition and the amount of F17 protein present in infectious iF17 virions (as compared to parental WR virions). These observations are in agreement with the previously described properties of an inhibitor of protein synthesis, which we isolated from VV cores, that match those of F17 [
30].
Composition of the iF17− particles. We confirmed that non-infectious iF17
− viral particles could be purified when the expression of F17 protein is inhibited and contains viral DNA [
4]. It was previously shown by Western blot that they contained the A5, A30, I1 and the major L4 proteins [
4]. Our analyses of Coomassie blue-stained core proteins (and lateral body) fractions obtained after detergent plus DTT treatment of iF17
− particles strongly suggested that the purified iF17
− particles contained the major structural proteins, but lacked F17.
Importantly, we found that the 11 kDa protein content of iF17 virions was lower by about three times when compared to the amount present in wild-type WR. Additionally, iF17 virions exhibited a similar reduction of the corresponding Particle/PFU ratio. This strongly suggests that the vaccinia virus can accommodate variable amounts of F17 protein, likely because this protein is mainly associated with the lateral bodies [
9]. Interestingly, this is also the case for the protein phosphatase H1, also associated with the lateral bodies [
9], because wild-type WR virion appears to contain sixfold more H1 protein than that of a corresponding IPTG-dependent mutant produced in the presence of IPTG [
16]. Importantly, this lower content in F17 correlated with a lower efficiency in inhibiting protein synthesis, as shown below.
Entry/fusion of the iF17− particles. Because the iF17
− particles are defective for early transcription despite containing viral DNA [
4], it was necessary to investigate if they are capable of entering target cells. We showed that the iF17
− particles did attach to BSC40 cells and injected their viral DNA into the cytoplasm. Furthermore, we found that in the non-permissive BSC40 cells co-infected with MVA-T7g and iF17
− particles, the latter strongly stimulated the expression of the mCherry reporter as evidenced by the fluorescence intensities, providing a second and independent argument showing that the iF17
− viral particles could enter cells.
The inhibition of protein synthesis is a direct function of the amount of virion-associated F17 protein injected into the cell. First, we showed that the iF17
− viral particles (devoid of F17 protein) could infect cells but did not significantly inhibit protein synthesis, even at very high multiplicities of infection (4500 particles per cell, equivalent to 40 to 80 PFU/cell), as compared to iF17 or WR viruses. Second, we showed that the infectious iF17 virions were approximately threefold less inhibitory for protein synthesis, in agreement with their threefold and up to fourfold lower content of F17 protein. Altogether, these results strongly suggest a stoichiometric inhibition of protein synthesis that is proportional to the amounts of F17 present in the lateral bodies, in agreement with our previous demonstration that the 11 kDa inhibitor (presumably F17) solubilised from purified virions inhibited protein synthesis in a reticulocyte lysate [
30].
Paradoxical effects of F17 on protein synthesis. In contrast with our results showing that virion-associated F17 protein inhibits protein synthesis, it was recently shown that F17 synthesised during late stages of infection, deregulates mTOR by binding to its subunits, resulting in the activation of the protein synthesis initiation factors [
21]. Furthermore, a transiently expressed F17 protein in NHDF cells was found to activate mTOR in these cells [
22]. These results strongly suggest that F17 is a double-edged sword for protein synthesis.
Phosphorylation of the F17 at distinct residues might provide an explanation of the paradoxical observations on protein synthesis. Indeed, only 3 sites of phosphorylation S53, S2 and S40 were identified in virion-associated F17, whereas the previously demonstrated S62 phosphorylation of late synthesised F17 [
4] was never observed [
9]. It was also suggested that phosphorylation may partition F17’s functions as a structural protein and mTOR regulator [
22].
Physiological significance of the inhibition of protein synthesis by virion-associated F17. The virion-associated F17 protein is very rapidly dispersed into the cytoplasm after injection of the cores [
8,
9], concomitant to the rapid inhibition of protein synthesis after VV infection of HeLa cells at high MOI [
23]. Furthermore, this inhibition at high MOI has only been shown when actively growing cells were infected. However, rapid and efficient inhibition of protein synthesis occurred in EAT or L cells in suspension upon amino-acid starvation (exposed to amino acid analogues) when infected by vaccinia virus at a productive MOI in the same actively growing cells, suggesting that cells with a reduced rate of protein synthesis might be very sensitive to F17 inhibition [
46]. This hypothesis is also supported by the observations that resting B lymphocytes need to be activated to become permissive for a productive VV infection [
47] and that lymphocyte infection with VV is toxic, even with UV-inactivated virus [
48]. Therefore, inhibition of protein synthesis by virion-associated F17 may be beneficial for the vaccinia virus to counter host immunity immediately after entry, well before the early expression of several viral proteins dedicated to this function [
49]. In conclusion, our findings provide important insight into a novel functional role of the F17 protein.