One of the curious aspects of viral serine/threonine kinases is that they appear to be encoded exclusively by large and evolutionarily old DNA viruses, such as herpesviruses, poxviruses, and baculoviruses.
Trying to explain this requires some speculation. One possibility would be that viral kinases merely are an “optional feature” of viruses, encoded only by those viral species that have a large enough genome to be able to afford them. Alternatively, they may be evolutionary relics, present in many of the evolutionarily ancient, complex DNA viruses and selected away in viruses of more recent origin. The fact that all studied viral kinases constitute important virulence factors argues against this possibility in favor of a third possibility: viral serine/threonine kinases may actually be one of the key elements of these old, large DNA viruses and may have aided them in still being around after millions of years of host evolution.
Indeed, evolutionarily very distinct members of the herpesviruses, with natural hosts ranging from koi to humans, are predicted to encode viral kinases (5
). Throughout the tremendous evolution of their hosts, herpesviruses have successfully adapted themselves and kept their ability to establish lifelong infections. Apparently, viral kinases were not selected away in the evolutionary process but instead may have aided in the adaptation of these viruses to the increasing complexity of their host.
Intrinsic to the limited size of their genome, in order to be able to replicate and spread in a host, viruses have to do a lot with a little. Hence, it comes as no surprise that many viral proteins are multifunctional. This is especially true for viral kinases, which have the ability to phosphorylate, and thereby alter, several viral and cellular targets, which may result in a plethora of different effects. Some of these effects are intentional and beneficial for viral replication, and others may actually be side effects that are biologically irrelevant and thereby artifactual. One of the main tasks in the field of viral kinase research will be to better discriminate these possibilities. This is especially challenging since the major roles of viral kinases in pathogenesis do not appear to translate easily to obvious phenotypes in vitro
. Indeed, as indicated above, many kinases, including the herpesvirus kinases, are often not absolutely required for growth in cell culture, although lack of viral kinase activity results in strongly reduced virulence. One important issue may be that, in contrast to the typical immortalized cell cultures, many of the viral target cells in vivo
comprise highly differentiated, nonproliferating cells. These in vivo
target cells are therefore less suitable for viral replication, which may increase the importance of viral kinases, especially since many of these kinases have been reported to promote viral replication and gene expression. As an example to support this idea, mutation of the CHPK kinase of VZV does not substantially affect viral growth in cell culture, whereas it is essential for replication in human T cells and human thymus/liver xenografts in SCID-hu mice (150
). Also indicative of the caution needed when interpreting results from immortalized cell lines is that SV40 T antigens have been reported to induce lamin A/C phosphorylation in 293T cells, thereby compensating for the defect in viral egress observed for a CHPK-null EBV recombinant in 293 cells (137
In order to meet the need to better define the biological importance of individual functions of viral kinases, several approaches may be followed. Studies of important primary target cells and/or organ cultures may highlight which kinase-dependent phenotypes manifest themselves in these cells. Since many of the kinase targets are cellular proteins, homologous studies, using primary cells of the natural host of the virus, will be particularly informative. In addition, a search for mutations in the viral kinases that abrogate their interaction with specific, but not all, substrates may be vital in revealing the importance of individual functions of viral kinases. Alternatively, complementing kinase deficiency with cellular or viral factors that restore only selected functions of the kinase should also yield important data in this respect. For example, replacing the US3 gene in alphaherpesviruses with specific antiapoptotic factors will allow us to better define the importance of the antiapoptotic activity of this kinase in virus biology.
There are several indications that viral serine/threonine protein kinases may be promising future targets for the development of antiviral drugs. First, although sometimes not associated with obvious phenotypes in cell culture, as described above, all viral protein kinases identified appear to play important and frequently essential roles in virus virulence. Second, and perhaps equally important, most viral serine/threonine protein kinases are evolutionarily very distinct from cellular protein kinases, which should facilitate the selection of drugs that do not interfere with cellular kinase activity.
A first proof of principle that viral kinases may represent interesting drug targets is maribavir. Maribavir is a drug directed against the UL97 kinase of HCMV and displays potent antiviral activity against HCMV (21
). It is important to keep in mind that the drug of preference used against HCMV is GCV and that UL97 plays an important role in activating GCV by phosphorylating it to its monophosphate form (123
). Hence, the two treatment strategies may interfere with each other (38
). Maribavir successfully passed phase I and phase II clinical trials (214
). Although this demonstrates the potential of viral kinases as therapeutic targets, unfortunately, maribavir very recently did not pass phase III clinical trials conducted by Viropharma. Despite this mishap, viral serine/threonine protein kinases should still be considered one of the promising avenues for the design of novel antiviral drugs (3
). One example may be the B1 kinase, which is of critical importance in the poxvirus replication cycle. A first indication for the potential of B1 as a target for antiviral therapy was demonstrated, since interfering RNAs that target B1 effectively reduced vaccinia and monkeypox virus yields and plaques in in vitro
assays, especially when administered together with the proven antiviral drug cidofovir (220
An aspect of viral kinases that may warrant further research is their effect on cellular signaling networks and cascades. Phosphorylation and dephosphorylation arguably represent the most important regulatory switches of signaling proteins, affecting virtually every aspect of the biology of a cell and the host organism (174
). Up to now, relatively little information has been available on the potential effect of viral serine/threonine kinases on cellular signaling cascades. This is even more relevant when considering that most, if not all, of these viral kinases are incorporated in the virus particle. Although largely unexplored at this time, there is thus the possibility that viral kinases may affect cellular signaling cascades at the very early stages of infection, during viral entry, thereby preparing and perhaps steering the otherwise hostile cellular environment to their own benefit and needs.
Obviously, there is a need for more research on viral protein kinases to fully address their invariably multifunctional and pivotal roles in viral replication, spread, and immune evasion. This information, together with rational drug design strategies, is needed to explore the therapeutic potential of these viral enzymes.