Plant Virus Infection and the Ubiquitin Proteasome Machinery: Arms Race along the Endoplasmic Reticulum.

Jeanmarie Verchot

doi:10.3390/v8110314

Abstract

The endoplasmic reticulum (ER) is central to plant virus replication, translation, maturation, and egress. Ubiquitin modification of ER associated cellular and viral proteins, alongside the actions of the 26S proteasome, are vital for the regulation of infection. Viruses can arrogate ER associated ubiquitination as well as cytosolic ubiquitin ligases with the purpose of directing the ubiquitin proteasome system (UPS) to new targets. Such targets include necessary modification of viral proteins which may stabilize certain complexes, or modification of Argonaute to suppress gene silencing. The UPS machinery also contributes to the regulation of effector triggered immunity pattern recognition receptor immunity. Combining the results of unrelated studies, many positive strand RNA plant viruses appear to interact with cytosolic Ub-ligases to provide novel avenues for controlling the deleterious consequences of disease. Viral interactions with the UPS serve to regulate virus infection in a manner that promotes replication and movement, but also modulates the levels of RNA accumulation to ensure successful biotrophic interactions. In other instances, the UPS plays a central role in cellular immunity. These opposing roles are made evident by contrasting studies where knockout mutations in the UPS can either hamper viruses or lead to more aggressive diseases. Understanding how viruses manipulate ER associated post-translational machineries to better manage virus–host interactions will provide new targets for crop improvement.

Highlights

The ubiquitin proteasome system (UPS) contributes to all aspects of cell biology and is widely conserved among eukaryotes
The UPS machinery contributes to adaptive cellular responses to environmental stressors, change in endoplasmic reticulum (ER) conditions, and virus infection
Given the plethora of genes that are categorically ascribed to the UPS, we are still uncovering the various contributions of this system to plant cell biology

Summary

Introduction

The ubiquitin proteasome system (UPS) contributes to all aspects of cell biology and is widely conserved among eukaryotes. Adaptor proteins at the dislocation site recruit lumenal factors that help to shuttle ERAD substrates from the folding machinery to the retrotranslocon and aid the hand-off of the substrate to the E2/E3 ubiquitin ligase complex prior to proteasomal degradation (Figure 1A). Such adaptors include SEL1L in mammals, and Hrd in yeast and plants (Figure 1B) [14,15]. The mammalian UBE2g2 coordinates with gp but not HRD1 for ubiquitination of some substrates, suggesting that there are special conditions governing how the E2 and E3 enzymes partner with each other [18,20] Another conserved component of the retrotranslocon is the p97/cdc48A ATPase that acts on the cytoplasmic face of the ER and helps to pull the ubiquitinated protein from the retrotranslocon toward the proteasome (Figure 1B) [12,23]. Ubiquitination, cdc directs substrates to the proteasome [23,25]

Ubiquitin Proteasome System Supporting Virus Replication and Movement

Hijacking the Host E3 Ub-Ligase Using Viral-Encoded F Box Proteins

The Ubiquitin Ligase 26S Proteasome System and the Molecular Arms Race

Conclusion