Research paperStructure, function and regulation of plant proteasomes
Introduction
All fundamental processes, from cell division to cell death, include protein degradation steps. In eukaryotes, the turnover rates of misfolded and damaged proteins as well as numerous regulatory proteins are controlled by the ubiquitin/26S proteasome system (UPS) [1], [2], [3]. The proteolytic component of this system is the proteasome, a multisubunit, multicatalytic protease localized in the cytosol and nucleus.
In plants, UPS-dependent regulation of signaling and metabolic pathways appears to be more complex and prevalent when compared to yeasts and animals [1], [2]. This was suggested by comparisons of fully sequenced genomes that showed that plants assign a large portion of their hereditary information to UPS components [2], [4].
The aim of this review is to summarize the current knowledge of proteasome structure and function in plants. We conclude with a brief discussion of the importance of proteasome activity regulation and provide suggestions for future research into the role of this protease complex in the signaling pathways that control the plant life cycle.
Section snippets
Plant proteasomes
Most of the proteins degraded by the proteasome are first modified by the covalent attachment of a polyubiquitin chain. This conjugation reaction starts with the 76-amino-acid-long peptide ubiquitin (Ub) that binds to a Ub activating enzyme (E1) with a high-energy bond (Fig. 1). Activated Ub is then transferred to a Ub conjugating enzyme (E2) that together with a Ub ligase (E3) catalyzes the conjugation of the Ub monomer to a lysine residue of the target protein. Attachment of one Ub is not a
Ub-dependent proteolysis
There are essentially two classes of proteins that are targeted for Ub-dependent degradation by the proteasome: misfolded or damaged proteins that are detected through their loss of tertiary structures and functional proteins that carry specific degradation signals [2]. Consequently, we recognize two general functions of the UPS. First, cellular quality control, which includes the removal of proteins with translational errors and proteins damaged by stress which can aggregate and become toxic
Regulation of plant proteasome activity
Any decrease in total proteasome activity is expected to cause decreased degradation rates of regulatory proteins and reduced efficiency and accuracy of signal transduction processes. Decreased proteasome activity also leads to the accumulation of damaged proteins and thus stress hypersensitivity. Elevated proteasome activity, at least theoretically, would enhance responsiveness of signal transduction pathways and increase stress resistance levels by accelerating the removal of damaged
Proteasome regulation and plant development
Gene expression analyses combined with proteasome suppression studies are starting to unravel the importance of the 26SP for the regulation of plant development. Given its crucial role in the turnover of regulatory proteins, in cellular housekeeping and in stress tolerance, it is not surprising that loss of function of most proteasome subunits leads to lethal or pleiotropic effects [2]. Analyses of plant proteasome mutants confirm this and also indicate that optimal proteasome activity levels
Conclusions and perspectives
Today, it is generally accepted that protein degradation is essential for cell survival and that regulated proteolysis catalyzed by the proteasome allows cells to fine-tune their responses to changing environments. It is therefore not surprising that cells have mechanisms that control the quantity and specificity of the proteasome itself. Many studies describe significant fluctuations in proteasome abundance and activity during plant development and in response to environmental stress
Acknowledgements
We thank D. Zaitlin for critical reading of the manuscript. Research in the laboratory of J.S. is supported by the National Research Initiative of the USDA Cooperative State Research, Education and Extension Service, grant number 2005-35304-16043, and the Kentucky Science and Engineering Foundation, grant number 148-502-06-189.
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