Protein Degradation by Gammaherpesvirus RTAs: More Than Just Viral Transactivators

Abstract

Kaposi's sarcoma-associated herpesvirus (KSHV) is a member of the Gammaherpesvirus subfamily that encodes several viral proteins with intrinsic E3 ubiquitin ligase activity or the ability to hijack host E3 ubiquitin ligases to modulate the host's immune response and to support the viral life cycle. This review focuses specifically on how the immediate-early KSHV protein RTA (replication and transcription activator) hijacks the host's ubiquitin-proteasome pathway (UPP) to target cellular and viral factors for protein degradation to allow for robust lytic reactivation. Notably, RTA's targets are either potent transcription repressors or they are activators of the innate and adaptive immune response, which block the lytic cycle of the virus. This review mainly focuses on what is currently known about the role of the E3 ubiquitin ligase activity of KSHV RTA in the regulation of the KSHV life cycle, but we will also discuss the potential role of other gammaherpesviral RTA homologs in UPP-mediated protein degradation.

Keywords: E3 ubiquitin ligases; KSHV; RTA; gammaherpesviruses; protein degradation; ubiquitination.

Figure 1

The ubiquitin–proteasome pathway is comprised of three enzymes that function in a temporal cascade leading to cell signaling, protein degradation, DNA repair. The enzymes involved in this cascade are the ubiquitin activating enzyme (E1), ubiquitin conjugating enzyme (E2), and ubiquitin ligase enzyme (E3). The amount of each type of enzyme in depicted above with E1 being the most conserved amount vertebrates while there are several E2 proteins that can transfer ubiquitin to a variety of E3 ligases.

Figure 2

The ubiquitin–proteasome pathway. E1 activates and transfers ubiquitin to E2, the ubiquitin-conjugating enzyme then binds to an E3 ligase, where the ubiquitin is transferred onto the substrate protein. When ubiquitin in linked through K48 chain on a substrate protein it can lead to the degradation of the substrate through the proteasome. However, ubiquitin is a reversible post-translational modification that can be removed by deubiquitinating enzymes (DUBs) to prevent the degradation of a substrate through the UPP.

Figure 3

KSHV encodes seven proteins that contain intrinsic E3 ubiquitin ligase activity and/or recruit host E3 ubiquitin ligases to ubiquitinate and induce the substrates degradation through the UPP.

Figure 4

KSHV RTA has an intrinsic E3 ligase activity and the capacity to stabilize and chaperone a host E3 ligase to target its substrate proteins that fit into two distinct categories. The first set of RTA’s substrate proteins are the transcription repressors that suppress the viral lytic cycle such as ID2, Hey1, K-RBP, and K8 or indirectly lead to the repression of RTA’s function and induction of RTA expression like LANA. The second set of proteins targeted by KSHV RTA are cellular and viral factors that mediate innate immune signaling such as vFLIP, TRIF, MyD88, IRF3, and IRF7 or adaptive immune presentation like HLA-DRα. RLD: RING-like domain.

Figure 5

Amino acid sequence alignment between KSHV, EBV, MHV68, RRV, and HVS encoded RTA. Red—High consensus (90%), Blue—low consensus (50%), #—if there is N, D, Q, or E majority consensus, !—if there is a I or V majority consensus. Software used: Multalin.

Figure 6

The predicted RTA protein structure of KSHV (blue), EBV (magenta), and MHV68 (green) are depicted in panels (A,C,E) respectively. Additionally, zoom in views of the three gammaherpesvirus RTAs are respectively shown in panels (B,D,F) to highlight their three amino acids that may possibly be involved in zinc coordination. A superimposition of the three herpesvirus RTAs are shown in panel (G) with a zoom in view in panel (H) highlighting the conserved interior pocket observed in all three herpesvirus RTAs.