SUMO and KSHV Replication

Abstract

Small Ubiquitin-related MOdifier (SUMO) modification was initially identified as a reversible post-translational modification that affects the regulation of diverse cellular processes, including signal transduction, protein trafficking, chromosome segregation, and DNA repair. Increasing evidence suggests that the SUMO system also plays an important role in regulating chromatin organization and transcription. It is thus not surprising that double-stranded DNA viruses, such as Kaposi's sarcoma-associated herpesvirus (KSHV), have exploited SUMO modification as a means of modulating viral chromatin remodeling during the latent-lytic switch. In addition, SUMO regulation allows the disassembly and assembly of promyelocytic leukemia protein-nuclear bodies (PML-NBs), an intrinsic antiviral host defense, during the viral replication cycle. Overcoming PML-NB-mediated cellular intrinsic immunity is essential to allow the initial transcription and replication of the herpesvirus genome after de novo infection. As a consequence, KSHV has evolved a way as to produce multiple SUMO regulatory viral proteins to modulate the cellular SUMO environment in a dynamic way during its life cycle. Remarkably, KSHV encodes one gene product (K-bZIP) with SUMO-ligase activities and one gene product (K-Rta) that exhibits SUMO-targeting ubiquitin ligase (STUbL) activity. In addition, at least two viral products are sumoylated that have functional importance. Furthermore, sumoylation can be modulated by other viral gene products, such as the viral protein kinase Orf36. Interference with the sumoylation of specific viral targets represents a potential therapeutic strategy when treating KSHV, as well as other oncogenic herpesviruses. Here, we summarize the different ways KSHV exploits and manipulates the cellular SUMO system and explore the multi-faceted functions of SUMO during KSHV's life cycle and pathogenesis.

Figure 1

The SUMO conjugation enzyme cascade pathway and its links with KSHV. SUMO precursors are cleaved at the c-terminus by SENP and this reveals a GG motif. Mature SUMO is activated by conjugation to the SUMO E1 activating enzyme (SAE1/SAE2). Activated SUMO is transferred to the SUMO E2 conjugating enzyme (Ubc9). Ubc9 itself is able to catalyze SUMO conjugation to the lysine residue of a target proteins. SUMO E3 ligases are dispensable, but increase target specificity. SUMO is removed from the target by SENP, which cleaves SUMO from the substrate and releases free SUMO; alternatively, STUbL targets the sumoylated protein for degradation. The scheme also illustrates the proteins of KSHV and the corresponding components in SUMO modification system. K-bZIP itself is a SUMO E3 ligase and LANA acts as a SUMO E3 ligase. K-Rta itself is a STUbL and vIRF-3 acts as a STUbL. Detailed information is given in the text.

Figure 2

Schematic representation of KSHV K-Rta (A), K-bZIP (B), LANA (C), vIRF-3 (D) and vPK (E). The functional domains of these proteins are indicated below the schematic. The SIM and the SUMO lysine (L) sites are depicted above the protein structure.

Figure 3

Model outlining epigenetic regulation by K-bZIP. (A) K-bZIP is able to directly compete for TFBS binding with TFs in the promoter of target genes in a sumoylation-dependent (left) or in a sumoylation-independent (right) manner; (B) K-bZIP is able to interact with TFs in a SUMO-dependent (left) or in a SUMO-independent (right) manner, which in turn inhibits gene expression via sumoylation-dependent recruitment of transcriptional repressors; (C) Sumoylation of TFs by K-bZIP in a SIM-dependent manner; (D) K-bZIP is able to directly interact with and compete with histone modification enzyme activity in SUMO-independent manner.

Figure 4

A model of epigenetic regulation by LANA. (A) LANA recruits Ubc9 to specific genomic regions and promotes sumoylation of histone proteins; (B) The SIM-dependent recruitment of SUMO-modified chromatin-remodeling complexes by LANA.

Figure 5

A schematic representation of genome (A) and PML-NB (B) regulation by KSHV viral proteins. (A) K-Rta induces SUMO-dependent ubiquitin-mediated proteasome degradation of SUMO modified proteins. K-bZIP mediates SUMOylation of interacting proteins. LANA elicits a recruitment of SUMO-modified protein. vIRF-3 inhibits the SUMOylation of interacting proteins. Phosphorylation of target protein by vPK causes reduction of SUMO modification; (B) K-bZIP colocalizes with PML in the PML-NB. K-bZIP sumoylates p53 and recruits p53 to the PML-NB. K-Rta functions as a STUbL by ubiquitinating SUMO chains on PML, which then induces the degradation of these sumoylated PML molecules. vIRF-3 promotes a SUMO dependent ubiquitylation of PML and this induces PML degradation and PML-NB disruption in a SIM-dependent manner. Orf75 induces ATRX degradation, which causes relocalization of the PML-NB.