Phosphorylation represents one the most abundant and important posttranslational modifications of proteins, including viral proteins. Virus-encoded serine/threonine protein kinases appear to be a feature that is unique to large DNA viruses. Although the importance of these kinases for virus replication in cell culture is variable, they invariably play important roles in virus virulence. The current review provides an overview of the different viral serine/threonine protein kinases of several large DNA viruses and discusses their function, importance, and potential as antiviral drug targets.

The US3 protein is a viral serine/threonine kinase that is conserved among all members of the Alphaherpesvirinae. The US3 protein of different alphaherpesviruses causes dramatic alterations in the actin cytoskeleton, such as the disassembly of actin stress fibers and formation of cell projections, which have been associated with increased intercellular virus spread. Here, we find that inhibiting group A p21-activated kinases (PAKs), which are key regulators in Cdc42/Rac1 Rho GTPase signaling pathways, impairs US3-mediated actin alterations. By using PAK1 ؊/؊ and PAK2 ؊/؊ mouse embryo fibroblasts (MEFs), we show that US3-mediated stress fiber disassembly requires PAK2, whereas US3-mediated cell projection formation mainly is mediated by PAK1, also indicating that PAK1 and PAK2 can have different biological effects on the organization of the actin cytoskeleton. In addition, US3 was found to bind and phosphorylate group A PAKs. Lack of group A PAKs in MEFs was correlated with inefficient virus spread. Thus, US3 induces its effect on the actin cytoskeleton via group A PAKs.herpesvirus ͉ Rho GTPase signaling ͉ stress fibers ͉ projections ͉ viral kinase A lphaherpesviruses constitute the largest subfamily of the herpesviruses and contain different, closely related pathogens of humans and animals. In humans, herpes simplex virus (HSV) causes cold sores and genital lesions but may also cause keratitis, blindness, and encephalitis, and varicella-zoster virus (VZV) causes chickenpox and shingles. The closely related porcine alphaherpesvirus pseudorabies virus (PRV) is often used as a model organism to study general aspects of alphaherpesvirus biology (1).Recently, we and others have found that the US3 protein of PRV is able to reorganize the actin cytoskeleton of an infected host cell (2-6). This actin reorganization consists of the disassembly of actin stress fibers and the formation of actincontaining cell projections and is associated with an increase in the efficiency of intercellular virus spread (3). US3 is a serine/ threonine kinase that is conserved among all alphaherpesviruses, and US3 orthologs of HSV-2 and Marek's disease virus, a devastating alphaherpesvirus in poultry, have also been shown to induce disassembly of actin stress fibers (7,8). In addition, HSV-1 and VZV have also been reported to induce cell projections that may be involved in intercellular virus spread (9-11).These data on herpesviruses, as well as other data on other viruses, like retroviruses (e.g., HIV) and poxviruses (e.g., vaccina virus), have led to the recent concept that viral reorganizations of the cytoskeleton, including the formation of intercellular cell projections, present a novel and important route of viral transmission (12)(13)(14)(15)(16)(17). Elucidating the mechanism of these virusinduced cytoskeletal rearrangements may lead to novel avenues in the development of antiviral strategies.In the current report, we elucidate the mechanism of the US3-mediated effects on the actin cytoskeleton. We report that US3 induces the actin r...

p21-activated protein kinases (Paks) are cytosolic serine/threonine protein kinases that act as effectors for small (p21) GTPases of the Cdc42 and Rac families. It has long been established that Paks play a major role in a host of vital cellular functions such as proliferation, survival and motility, and abnormal Pak function is associated with a number of human diseases. Here, we discuss emerging evidence that these enzymes also play a major role in the entry, replication and spread of many important pathogenic human viruses, including HIV. Careful assessment of the potential role of Paks in antiviral immunity will be pivotal to evaluate thoroughly the potential of agents that inhibit Pak as a new class of anti-viral therapeutics.

Different viruses exploit the host cytoskeleton to facilitate replication and spread. The conserved US3 protein of the alphaherpesvirus pseudorabies virus induces actin stress fiber disassembly and formation of actin-containing cell projections, which are associated with enhanced intercellular virus spread. Proteins of members of other virus families, notably vaccinia virus F11L protein and human immunodeficiency virus Nef protein, induce actin rearrangements that are very similar to those induced by US3. Interestingly, unlike F11L and Nef, the US3 protein displays serine/threonine kinase activity. Here, we report that the kinase activity of pseudorabies virus US3 is absolutely required for its actin modulating activity. These data show that different viruses have developed independent mechanisms to induce very similar actin rearrangements.

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