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A historical overview of protein kinase PKR… NIH3T3 E3L cells formed solid tumours when injected in
nude mice. Overall, our findings reveal that interference of
effects of IFN and is able to rescue the replication of IFN- E3L protein with several cellular pathways results in
sensitive viruses such as VSV and EMCV following co- promotion of cellular growth, impairment of antiviral
infection. Earlier findings suggested that VV resistance to activity, and resistance to apoptosis (86). These results
IFN was related to interference between the virus and the (which we published with my contribution as the first
IFN system (80). Later studies showed a more complex author in Oncogen journal in 2002) together with the
landscape, with the VV genome encoding secreted proteins previously described works, led me to writing my doctoral
that bind to receptors and ligands of cytokines and thesis “Mechanism of action and regulation of Protein
chemokines (1, 2, 81), and at least two proteins, K3L and Kinase induced by Interferon: PKR/ Mecanismo de acción
E3L, with the ability to inhibit intracellular IFN-induced y regulación de la Proteina Quinasa inducida por
pathways. More recently, another protein (K1) has been Interferón:PKR”, which received the Special Award
identified as a possible PKR inhibitor (82). The K3L (Premio Extraordinario) of the Autonomous University of
protein is expressed early in VV infection. K3L protein Madrid.
binds directly to PKR in vitro, and yeast two-hybrid
interaction assays have localised the K3L protein-binding Recently, in collaboration with the group of Dr Rivas,
site to the C-terminal half of the PKR kinase domain. we have described the regulation of the E3L protein by
Competition binding experiments and sequence homology small ubiquitin-like modifier proteins. E3L interacts with
between K3L and the N-terminal one-third of eIF-2a (72% SUMO1 through a small ubiquitin-like modifier (SUMO)-
similarity and 28% identity) suggest that PKR recognises interacting motif (SIM). SIM integrity is required for
K3L and eIF-2a by a common mechanism (1, 2). In this maintaining the stability of the viral protein and for the
way, K3L protein inhibits autophosphorylation of PKR, covalent conjugation of E3 to SUMO1 or SUMO2, a
blocking the subsequent inhibition of protein synthesis (1, modification that has a negative effect on the E3L
2, 83). The role of the E3L gene as an inhibitor of transcriptional transactivation of several apoptotic genes
apoptosis was first detected after infection of HeLa cells (87). My collaboration with Dr Rivas started in 2003, and
with an E3L deletion mutant of VV by Dr Esteban´s group since then we have made several significant contributions
(1, 68). The VV E3L gene encodes two proteins, p25 and to the knowledge about the link between tumour
p20, expressed early in infection. E3L is a host range gene, suppressors and antiviral activity, resulting in some of the
necessary for efficient VV replication in several cell lines, works described in the present review.
and is required for VV pathogenesis. The E3L protein is a
dsRNA-binding protein where the carboxy-terminal Kaposi's sarcoma herpesvirus (KSHV). In
domain of E3L encodes the conserved motif that binds collaboration with Dr Rivas, we showed that the viral
dsRNA. The N-terminal domain required for protein LANA2 codified by KSHV inhibits apoptosis and
neurovirulence is involved in the direct inhibition of PKR the PKR-mediated translational block (88), and we
activation, nuclear localisation, and Z-DNA binding (1, 2, identified a nuclear export signal with important
84). E3L also inhibits PKR by direct interaction with PKR, implications for the function of this viral protein (89).
leading to heterodimer formation (1, 85). Our contribution
in the study of E3L protein showed that E3L expression in Hepatitis C virus (HCV). We looked deeper into the
NIH 3T3 cells conferred antiapoptotic and oncogenic mechanisms of action of other viral proteins modulating
properties. To analyse E3L effects over cellular PKR and, in order to analyse the effects of hepatitis C
metabolism in a virus-free system, we generated stable virus (HCV) on the antiviral response of the host, we
mouse 3T3 cell lines expressing E3L (86). Expression of developed a novel vaccinia virus (VV)-based delivery
E3L resulted in inhibition of eIF-2a phosphorylation and system polyprotein expression (VT7-HCV7.9), where
IkBa degradation in response to dsRNA. Antiviral structural and nonstructural (except part of NS5B) proteins
responses induced by IFN-a/ß were partially impaired in of HCV ORF from genotype 1b were efficiently expressed
3T3-E3L cells, as we determined by a viability assay upon and produced, and timely regulated in mammalian cell
VSV infection. E3L expression also conferred resistance to lines. HCV polyprotein expression caused a severe
dsRNA-triggered apoptosis. Interestingly, cells expressing cytopathological effect in human cells as a result of the
E3L grew faster than control cells, and showed increased inhibition of protein synthesis and apoptosis induction
expression of cyclin A and decreased levels of p27Kip1. triggered by the activation of the IFN-induced enzymes
E3L cooperated with H-ras in a focus formation assay, and PKR and RNase L systems (90).
@Real Academia Nacional de Farmacia. Spain 151
