The 20S proteasome α5 subunit of Arabidopsis thaliana carries an RNase activity and interacts in planta with the lettuce mosaic potyvirus HcPro protein.

In plants, the ubiquitin/26S proteasome system (UPS) plays a central role in protein degradation and is involved in many steps of defence mechanisms, regardless of the types of pathogen targeted. In addition to its proteolytic activities, the UPS ribonuclease (RNase) activity, previously detected in 20S proteasome preparations from cauliflower and sunflower (Helianthus annuus), has been shown to specifically target plant viral RNAs in vitro. In this study, we show that recombinant Arabidopsis thaliana proteasomal α(5) subunit expressed in Escherichia coli harbours an RNase activity that degrades Tobacco mosaic virus (TMV, Tobamovirus)- and Lettuce mosaic virus (LMV, Potyvirus)-derived RNAs in vitro.

The ubiquitin/26S proteasome system in plant-pathogen interactions: a never-ending hide-and-seek game.

The ubiquitin/26S proteasome system (UPS) plays a central role in plant protein degradation. Over the past few years, the importance of this pathway in plant-pathogen interactions has been increasingly highlighted. UPS is involved in almost every step of the defence mechanisms in plants, regardless of the type of pathogen.

HcPro, a multifunctional protein encoded by a plant RNA virus, targets the 20S proteasome and affects its enzymic activities.

The proteasome is a multicatalytic complex involved in many cellular processes in eukaryotes, such as protein and RNA turnover, cell division, signal transduction, transcription and translation. Intracellular pathogens are targets of its enzymic activities, and a number of animal viruses are known to interfere with these activities. The first evidence that a plant virus protein, the helper component-proteinase (HcPro) of Lettuce mosaic virus (LMV; genus Potyvirus), interferes with the 20S proteasome ribonuclease is reported here.

Substrate affinity and substrate specificity of proteasomes with RNase activity.

We have partially reconstituted 20S proteasome/RNA complexes using oligonucleotides corresponding to ARE (adenosine- and uridine-rich element) (AUUUA)4 and HIV-TAR (human immunodeficiency virus-Tat transactivation response element), a stem-loop structure in the 5' UTR (untranslated region) of HIV-mRNAs. We demonstrate that these RNAs which associate with proteasomes are degraded by proteasomal endonuclease activity. The formation of these 20S proteasome/RNA substrate complexes is rather specific since 20S proteasomes do not interfere with truncated TAR that is not cleaved by proteasomal endonuclease.

Biochemical identification of proteasome-associated endonuclease activity in sunflower.

Proteasomes have been purified from sunflower hypocotyles. They elute with a molecular mass of 600 kDa from gel filtration columns and two-dimensional gel electrophoresis indicates that the complex contains at least 20 different protein subunits. Peptide microsequencing revealed the presence of four subunits homologous to subunits Beta2, Beta6, Alpha5 and Alpha6 of plant proteasomes.