The Role of ORF59 in Coupling Viral Transcription and Replication
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KSHV (Kaposi’s sarcoma-associated herpesvirus) can cause multiple malignancies including Kaposi’s Sarcoma, Primary Effusion Lymphoma, and Multicentric Castleman’s Disease in immunocompromised individuals. KSHV, like other herpesviruses, establishes a life-long persistent infection that cannot be cleared either by the immune system or current antiviral therapies. The latent phase may be punctuated by brief periods of lytic reactivation, which can be triggered by a decrease in host immunocompetency or other environmental stimuli. When the virus undergoes lytic reactivation, it seizes control of cellular resources and generates copies of the viral genome as well as the structural components (capsid and tegument layers) of new infectious progeny virions. This stage of the life cycle is a necessary factor in driving the progression of tumorigenesis. Multiple viral factors are expressed during this transition and a viral protein, which is required for the viral DNA replication is ORF59 or DNA processivity factor. Primary role of ORF59 was assigned in translocating viral DNA polymerase from the cytoplasm to nuclei and to the site of viral DNA replication. Our work indicates that ORF59 is actually a multifunctional protein, which performs additional roles in regulating viral gene transcription and cellular DNA replication. ORF59 is capable of upregulating viral gene transcription during lytic reactivation by modulating the delicate viral chromatin structure to facilitate a transcriptionally active conformation. ORF59 modifies the chromatin structure by sterically downregulating the presence of histone modifying enzyme PRMT5, which in turn leads to a loss of H4R3me2s (a transcriptionally repressive histone mark) and upregulates lytic viral gene transcription. Furthermore, ChIP-Seq analysis of ORF59 binding sites on the cellular genome suggest that ORF59 is also capable of binding and modulating the expression of cellular genes. The pathways targeted by ORF59 binding to the cellular gene promoters suggest that ORF59 may participate in impairing host immune responses and apoptosis. Another way in which ORF59 disrupts normal cell functions to the benefit of the virus is by blocking cellular DNA replication. ORF59 disrupts cellular DNA replication by interacting with the minichromosome maintenance proteins (MCMs) – DNA replication licensing factors. In normal cells, cell cycle regulated association of MCM proteins to the cellular chromatin ensures replication of the cellular genome once per cell cycle. During lytic reactivation, however, ORF59 associates with MCM3-6 to disrupt their ability to associate with cellular DNA replication origins. This effectively stalls cellular DNA replication and ensures instead that the virus has ample cellular resources to use in replicating viral genomes. In conclusion, the work presented in these studies demonstrates the elegant efficiency of a herpesviral protein in controlling several different aspects of viral and cellular pathways to ultimately benefit viral replication and propagation.