The ubiquitin-proteasome system: central modifier of plant signalling

Contents Summary 13 I. Brief history 13 II. Components of the ubiquitinproteasome system 14 III. Ubiquitin-mediated degradation: a recurrent theme in the plant life cycle 18 IV. Conclusion and future prospects 25 Acknowledgements 25 References 25 Summary Ubiquitin is well established as a major modifier of signalling in eukaryotes. However, the extent to which plants rely on ubiquitin for regulating their lifecycle is only recently becoming apparent. This is underlined by the over-representation of genes encoding ubiquitin-metabolizing enzymes in Arabidopsis when compared with other model eukaryotes. The main characteristic of ubiquitination is the conjugation of ubiquitin onto lysine residues of acceptor proteins. In most cases the targeted protein is rapidly degraded by the 26S proteasome, the major proteolysis machinery in eukaryotic cells. The ubiquitinproteasome system is responsible for removing most abnormal peptides and short-lived cellular regulators, which, in turn, control many processes. This allows cells to respond rapidly to intracellular signals and changing environmental conditions. This review maps out the roles of the components of the ubiquitinproteasome system with emphasis on areas where future research is urgently needed. We provide a flavour of the diverse aspects of plant lifecycle where the ubiquitinproteasome system is implicated. We aim to highlight common themes using key examples that reiterate the importance of the ubiquitinproteasome system to plants. The future challenge in plant biology is to define the targets for ubiquitination, their interactors and their molecular function within the regulatory context.